17 avril 2021

Remarques sur le dépannage et la réparation des fours à micro-ondes

1994-2018
Tous les droits sont réservés

La reproduction de ce document, en tout ou en partie, est autorisée si les deux
les conditions suivantes sont remplies:

1. Cet avis est inclus dans son intégralité au début.
2. Il n'y a pas de frais sauf pour couvrir les frais de copie.

AVERTISSEMENT

Un dépannage imprudent d'un four à micro-ondes peut être fatal ou pire.
Des techniciens expérimentés ont rencontré leur fabricant à la suite d'une erreur momentanée
de jugement tout en testant un four avec le couvercle enlevé. Four à micro-ondes
sont sans aucun doute le type d'équipement électronique grand public le plus meurtrier
largement utilisé.

Les alimentations électriques même pour les plus petits fours à micro-ondes fonctionnent à l'extrême
niveaux de tension et de courant mortels. Ne tentez pas de dépanner, réparer ou
modifier un tel équipement sans comprendre et suivre TOUS les aspects pertinents
consignes de sécurité pour les appareils électriques et haute tension à haute tension et / ou connectés en ligne
systèmes électroniques.

Nous ne serons pas responsables des dommages causés à l'équipement, à votre ego, à l'échelle du comté
pannes de courant, mini-trous noirs générés spontanément (ou plus gros), planétaires
perturbations, ou des blessures ou pire, pouvant résulter de l'utilisation de cette
Matériel.

  • Retour à la table des matières de la réparation des fours à micro-ondes.

    introduction

    Radar Range quelqu'un?

    Rappelez-vous quand vous deviez utiliser le vrai four pour décongeler un téléviseur
    dîner? Repensez – retour – avant les magnétoscopes, avant les PC (et oui, avant
    Ordinateurs Apple également), presque avant les dinosaures, semble-t-il. Là
    était un temps où le terme «arme nucléaire» n'a pas été utilisé pour autre chose que des bombes
    et réacteurs de puissance.

    Pendant longtemps, il y avait une controverse quant à savoir si les fours à micro-ondes étaient
    sans danger – en ce qui concerne les émissions de micro-ondes et les dommages moléculaires causés aux aliments.
    Que ces problèmes soient résolus ou simplement écartés n’est pas totalement
    clair. Néanmoins, le four à micro-ondes a pris sa place dans pratiquement
    toutes les cuisines de la planète. Les connaisseurs de fine cuisine vont arriver
    leurs nez collectifs à l’idée d’utiliser un four à micro-ondes pour beaucoup
    au-delà de l'eau bouillante – si cela. Cependant, il est difficile de nier la
    la commodité et la rapidité de cuisson fournies par ce relativement simple
    appareil.

    Les fours à micro-ondes sont des appareils extrêmement fiables. Il y a une bonne chance
    que votre four fonctionnera pendant 10 ans ou plus sans réparation
    de tout type – et à des niveaux de performance indiscernables de quand il
    a d'abord été sorti de la boîte. Contrairement à d'autres produits électroniques grand public
    où un nouveau modèle est introduit toutes les 20 minutes – certains ont même utile
    améliorations – le four à micro-ondes n'a pas changé de manière substantielle dans le
    20 dernières années. La cuisine est la cuisine. Les touchpads sont maintenant presque universels
    parce qu'ils sont moins chers à fabriquer que des minuteries mécaniques (et aussi
    plus pratique). Cependant, un vieux four à micro-ondes réchauffera les aliments
    ainsi qu'un tout nouveau.

    Ce document fournit des informations sur l’entretien et les réparations applicables à
    la plupart des fours à micro-ondes existants. Cela vous permettra de rapidement
    déterminer la cause probable et estimer le coût des pièces. Vous serez
    capable de prendre une décision éclairée quant à savoir si un nouveau four est le meilleur
    alternative. À quelques exceptions mineures près, certains fabricants et modèles
    ne pas être couvert car il y a tellement de variations qu'un tel traitement serait
    nécessite un texte énorme et très détaillé. Plutôt, les problèmes les plus courants
    seront abordés et suffisamment de principes de fonctionnement de base seront fournis
    pour vous permettre de circonscrire le problème et probablement déterminer le cours du
    action en réparation. Dans de nombreux cas, vous serez capable de faire ce qui est requis
    pour une fraction du coût qui serait facturé par un centre de réparation – ou – soit
    capable de faire revivre quelque chose qui serait autrement allé dans la benne à ordures
    ou a continué dans son occupation actuelle en tant qu’arrêt de porte ou repose-pieds.

    Si vous ne parvenez toujours pas à trouver une solution, vous aurez appris un grand
    traiter et pouvoir poser les questions appropriées et fournir les informations pertinentes
    si vous décidez de poster sur sci.electronics.repair. Dans tous les cas, vous aurez
    la satisfaction de savoir que vous avez fait autant que vous pouviez avant de le prendre dans
    pour réparation professionnelle. Vous serez en mesure de décider si cela vaut le coût
    d'une réparation aussi. Avec vos nouvelles connaissances, vous aurez la plus haute
    main et ne sera pas facilement enneigé par un technicien malhonnête ou incompétent.

    Base de données en ligne sur la réparation de fours à micro-ondes

    Microtech maintient un site Web avec une grande quantité d'informations sur les micro-ondes
    réparation du four, y compris une base de données en ligne Tech Tips avec des centaines de solutions
    problème commun à de nombreux modèles de fours à micro-ondes. Il y a aussi un
    liste complète de liens vers d'autres sites intéressants sur le four à micro-ondes
    (y compris ce document!). Le complet
    Informations de sécurité est un
    doit lire aussi bien. Ce n'est pas tout à fait par hasard, je suppose, certains de ses
    la formulation semble remarquablement familière! Microtech propose également des formations
    vidéos et livres sur la réparation du four à micro-ondes et du magnétoscope.

    Il est fort possible que votre problème soit déjà traité sur le site Microtech.
    Dans ce cas, vous pouvez grandement simplifier votre dépannage ou au moins
    confirmer un diagnostic avant de commander des pièces. Ma seule réservation en ce qui concerne
    aux bases de données de conseils techniques en général – cela n’a rien à voir avec Microtech
    en particulier – est-ce que les symptômes peuvent parfois être trompeurs et une solution
    cela fonctionne dans un cas peut ne pas s'appliquer à votre problème spécifique. Donc,
    une compréhension du comment et du pourquoi de l'équipement ainsi que de bonnes
    tests à l’ancienne est hautement souhaitable pour minimiser le risque de remplacement
    des pièces qui ne se révèlent pas mauvaises.

    Les problèmes les plus simples

  • Mauvais interrupteurs de verrouillage ou mauvais alignement des portes entraînant la destruction des fusibles ou leur absence
     opération lorsque le bouton de démarrage est enfoncé. Localiser et remplacer défectueux
     commutateurs et / ou réaligner la porte.
  • Production d'arc dans la chambre du four: nettoyez soigneusement la chambre du four et le guide d'ondes.
     Remplacez le couvercle de guide d'ondes carbonisé ou endommagé. Lisser les bords en métal rugueux.
     Retoucher la peinture intérieure.
  • Fusible fondu en raison de surtension ou de vieillesse: remplacez le fusible. Sur rare
     Dans certains cas, le fusible principal peut même être intermittent, ce qui entraîne des conséquences très étranges.
     symptômes.
  • Un MOV, probablement sur le contrôleur, peut avoir été court-circuité à cause d'une alimentation
     surtension faisant fondre le fusible du contrôleur. Enlevez les restes de MOV, remplacez
     fusionner et tester, remplacer MOV pour une protection future contre les surtensions.
  • Fonctionnement irrégulier du pavé tactile en raison d'un déversement – laissez le pavé tactile sécher pendant une semaine.
  • Bugs in the works – le circuit imprimé du contrôleur est un agréable coffre-fort bien au chaud
     endroit pour élever une famille …..
  • Des explications plus détaillées sont fournies ailleurs dans ce document.

    Réparer ou remplacer?

    Avec des fours à micro-ondes de taille petite à moyenne allant de 60 à 100 dollars US, cela fait à peine
    sens de dépenser 60 $ pour en avoir un réparé. Même les grands fours à micro-ondes avec
    Un écran tactile complet peut être acheté pour moins de 200 $. Ainsi, le remplacement
    doit être considéré sérieusement avant de couler un investissement important dans un
    four plus ancien.

    Cependant, si vous pouvez réparer vous-même, l'équation change radicalement
    car le coût de vos pièces sera de 1/2 à 1/4 de ce qu'un professionnel facturera
    et bien sûr, votre temps est libre. Les aspects éducatifs peuvent également être
    attirant. Vous apprendrez beaucoup dans le processus. Beaucoup de problèmes peuvent être
    résolu rapidement et à peu de frais. Fixation d'un vieux micro-ondes pour le dortoir
    la pièce peut simplement avoir un sens après tout.

  • Retour à la table des matières de la réparation des fours à micro-ondes.

    Installation et maintenance préventive

    Installation et utilisation du four à micro-ondes

    Pour assurer la sécurité et la commodité, suivez ces recommandations:

  • Lisez le manuel de vos utilisateurs d’un bout à l’autre, surtout s’il s’agit de votre première
     four micro onde. Quel concept! Si rien d'autre, vous pouvez découvrir que votre
     four a des fonctionnalités que vous ne saviez pas étaient même possibles. En tout cas, là
     peuvent être des exigences ou des suggestions qui sont spécifiques à votre modèle et seront
     vous permettent de tirer le meilleur parti de votre nouveau four à micro-ondes.
  • Sélectionnez une unité autonome plutôt que intégrée si possible. Ce sera
     moins cher à acheter, moins cher et plus facile à entretenir, et éventuellement plus fiable
     puisque la ventilation et les appareils produisant de la chaleur adjacents ne seront pas aussi efficaces.
     beaucoup d'un facteur.
  • Choisissez un emplacement pratique – accès facile et ni trop haut ni trop bas.
     Ceci est particulièrement important si la porte du four s'ouvre à la place
     sur le côté gauche (cependant, seuls quelques modèles sont construits de cette façon).
  • Placez le four à micro-ondes sur son propre circuit dédié à la terre à 3 fils.
     L’utilisation temporaire d’un adaptateur à 3 ou 2 broches n’est acceptable que si
     la boîte est correctement mise à la terre pour commencer (BX, Romex ou un conduit avec terre)
     ET le fil de terre ou la borne de l'adaptateur est solidement connecté à la prise
     vis de masse de la boîte.

    Assurez-vous que la prise est en bon état dans les deux cas. Vérifiez que le
     la fiche (ou l'adaptateur) est bien ajustée et qu'il n'y a pas de chauffage appréciable
     de la prise lors de l’utilisation du four à micro-ondes. S'il y en a, écartez le
     bandes métalliques de chacune des branches à part si possible et / ou remplacer le
     sortie.

    Une prise de terre est essentielle pour la sécurité. Les fours à micro-ondes sont hauts
     dispositifs d'alimentation et un circuit séparé permettra d'éliminer les fusibles gênants
     ou le déclenchement du disjoncteur lorsque plusieurs appareils sont utilisés à
     le même temps. Cela minimisera également la possibilité de radiofréquences
     Brouillage (RFI) entre celui-ci et tout équipement électronique susceptible de
     sur le même circuit. Un GFCI n'est pas nécessaire tant que la prise est correctement
     mis à la terre et peut entraîner des déclenchements intempestifs avec certains fours à micro-ondes.

    Des testeurs de sortie bon marché sont disponibles dans les quincailleries, les centres de rénovation,
     et distributeurs de pièces électriques, pour confirmer que la prise est correctement
     câblé et mis à la terre.

  • Permettre une ventilation adéquate – ne pas le pousser contre le mur ou le coin
     sous une armoire murale ajustée (ou à l’intérieur de celle-ci!).
     Laissez au moins 2 pouces de tous les côtés et en haut si possible.
  • Ne laissez pas les enfants utiliser le four à micro-ondes sans une surveillance adéquate. Il
     est très facile de provoquer un incendie en utilisant des temps ou une puissance excessifs
     réglages. Même quelque chose d'aussi simple qu'un pop-corn à micro-ondes peut exploser et / ou
     prendre feu si elle est chauffée trop longtemps – par exemple, 5 minutes au lieu de ma précision
     déterminé 3:41 en haut :-).
  • Entretien du four à micro-ondes

    La plupart des gens ne font rien pour entretenir un four à micro-ondes. Beaucoup iront
    pendant 20 ans ou plus sans baisse notable de la performance.
    Bien que peu d’entretien préventif soit nécessaire, un nettoyage régulier au moins
    évitera des réparations potentiellement coûteuses à l'avenir. La plupart de ces
    implique des choses qui ne nécessitent pas d'aller à l'intérieur et tout le monde peut faire. Un magasin
    qui veut ajouter de la maintenance préventive tout en faisant une autre réparation
    essaie juste de remplir leur portefeuille – tout ce qui était nécessaire pour
    s'assurer que la santé du four aurait dû être incluse. 🙂

  • Nettoyez l’intérieur de la chambre du four après utilisation avec un chiffon humide et un peu
     détergent si nécessaire. Les dépôts de nourriture accumulés peuvent éventuellement se carboniser
     produisant des étincelles, des arcs électriques, un échauffement et des dommages au couvercle en mica
     et la peinture intérieure – ainsi que des dommages potentiellement plus graves à la
     magnétron. S'il y a une chance que des dépôts de nourriture se soient écoulés
     au-dessus du couvercle du guide d'ondes dans le toit de la chambre, retirez le guide d'ondes
     couvrir et nettoyer à l’intérieur du guide d’onde également.
  • Nettoyez l’extérieur du boîtier et du touchpad de la même manière. NE PAS
     utiliser un spray où tout peut trouver son chemin à l'intérieur à travers le loquet de la porte ou
     trous de ventilation ou un chiffon humide. Soyez particulièrement prudent autour
     la zone du pavé tactile, car le liquide peut s'infiltrer dessous, entraînant
     boutons inactifs ou bloqués ou fonctionnement irrégulier. Ne pas utiliser fort
     solvants (cependant, un peu d’alcool isopropylique convient si nécessaire pour éliminer
     résidus collants d’étiquettes indésirables, par exemple).
  • Inspectez le cordon et la fiche pour vous assurer qu'ils ne sont pas endommagés et que le
     la fiche est bien serrée dans la prise – particulièrement si l'appareil est
     installé à l'intérieur d'une armoire. (Oui, je sais que c'est difficile à atteindre mais
     Je vous ai prévenu à ce sujet !.) La chaleur, en particulier d'une combinaison
     micro-ondes / four à convection ou d'autres appareils produisant de la chaleur
     peut endommager la fiche et / ou le cordon. S'il y a des signes de surchauffe à
     la prise elle-même, la prise (et éventuellement la fiche également) doivent être
     remplacé.
  • Vérifiez périodiquement la poussière et la saleté accumulées autour de la ventilation
     des trous ou des grillades. Nettoyez-les et utilisez un aspirateur pour aspirer
     la poussière en vrac. Garder la ventilation libre minimisera le risque de
     surchauffe.
  • Écoutez les sons inhabituels provenant de l'intérieur du four. Alors que ces
     les appareils ne sont pas parfaitement silencieux, ne grincent pas, ne crissent pas, ne grattent pas
     des bruits – surtout s’ils n’étaient pas là quand le four était neuf – peuvent
     indiquer la nécessité d'un entretien plus poussé, comme le remplacement de la courroie
     ou lubrification du moteur. S'occuper de ces problèmes mineurs maintenant peut empêcher
     réparations majeures à l'avenir.
  • Gardez votre cuisine propre. Oui, je sais, ce n'est pas exactement un micro-ondes
     cafards et autres invités non désirés pourraient tout simplement aimer
     installez-vous dans la baie électronique du four sur la belle chaude
     carte de contrôleur ou son voisinage et ils ne sont généralement pas
     les plus petits du monde.

    Si il est trop tard et que vous avez un problème récurrent de cafards se
     à l’intérieur de la baie des appareils électroniques, dites-leur de se perdre, puis mettez un écran de fenêtre
     sur les évents (ou où qu'ils entrent). Un tel maillage ouvert devrait
     pas affecter le refroidissement des composants électroniques de manière significative. cependant,
     le maillage sera probablement obstruer plus rapidement que les persiennes d'origine alors faites
     Bien sûr, il est nettoyé régulièrement. Si possible, nettoyez tout ce qui attire
     locataires indésirables (et tout ce qu’ils ont pu laisser, y compris leur
     des œufs!!). AVERTISSEMENT: Voir la section: SECURITE avant de partir
     à l'intérieur.

  • ATTENTION: Ne vaporisez rien dans les trous où le loquet de la porte est inséré
    ou n'importe où autour du touchpad car cela peut entraîner des courts-circuits internes
    et des dommages coûteux – ou n'importe où ailleurs à l'intérieur, d'ailleurs. Si tu fais ça
    accidentellement, débranchez immédiatement le four et laissez-le sécher pendant un jour ou deux.

    Combien de temps l'énergie micro-ondes traîne-t-elle?

    Ta mère t'a probablement averti: "Attends quelques secondes (ou minutes)
    après le bip pour que tous les micro-ondes disparaissent ". Il n’existe aucun scientifique
    base pour une telle recommandation. Une fois que le bip a retenti (ou que la porte a
    ouvert), il est sûr. Ceci est dû au fait:

  • Le rayonnement micro-ondes résiduel d'un micro-ondes n'existe pas
     four – il est soit produit ou est inexistant.
  • Il y a peu de stockage d'énergie dans le générateur à micro-ondes par rapport à
     la quantité utilisée. Le condensateur haute tension typique – le seul
     composant qui peut stocker de l'énergie – a une capacité inférieure à 15 W-s
     (Watt-secondes) même pour les plus grands fours. La consommation électrique est généralement
     800 à 1500 W selon la taille du four. Par conséquent, le condensateur sera
     complètement drainé en moins de 0,1 seconde, bien avant la fin du bip
     ou la porte a dégagé le panneau avant. (Basé sur les chiffres ci-dessus, pour
     un four de 1500 W avec un condensateur stockant 15 W-s, cela ressemble plus à .01 secondes!)

    AVERTISSEMENT: Ceci s'applique uniquement à un * four * à micro-ondes en fonctionnement! Si il n'y a pas
      chaleur, le magnétron peut ne pas tirer de courant du courant haute tension
      L’alimentation et le condensateur HT peuvent rester chargés pendant une longue période. Dans ce
      cas, il existe un risque très réel de choc électrique potentiellement mortel, même
      après plusieurs minutes ou plus d'être débranché! Voir la section:
      SECURITE si vous voulez dépanner un four à micro-ondes.

  • Retour à la table des matières de la réparation des fours à micro-ondes.

    Dépannage du four à micro-ondes

    SÉCURITÉ

    Ce qui suit s’applique au dépannage du four à micro-ondes – une fois le boîtier
    la couverture est enlevée. Il existe également des informations de sécurité sur le bon usage du produit.
    four dans les sections suivantes, ci-dessous.

    S'il vous plaît voir typique baie micro-ondes électronique
    pour l'identification des pièces.

    ATTENTION! ATTENTION! ATTENTION! ATTENTION! ATTENTION! ATTENTION! ATTENTION! ATTENTION!

    Les fours à micro-ondes sont probablement les appareils électroménagers les plus dangereux
    pour servir. Très hautes tensions (jusqu'à 5000 V) à potentiellement très élevées
    courants (AMP) sont présents lors du fonctionnement – combinaison mortelle. Celles-ci
    les dangers ne disparaissent pas même lorsqu'ils sont débranchés car il y a un stockage d'énergie
    dispositif – un condensateur haute tension – pouvant retenir une charge dangereuse
    pendant longtemps. Si vous avez le moindre doute sur vos connaissances
    et la capacité de faire face à ces dangers, remplacer le four ou l'avoir
    réparé par des professionnels.

    Dépannage insouciant d'un four à micro-ondes peut non seulement vous faire frire
    des tensions élevées à des courants relativement élevés, mais vous pouvez vous irradier au micro-ondes comme
    bien. Lorsque vous retirez le couvercle en métal du four à micro-ondes, vous exposez
    vous-même à des connexions électriques dangereuses – potentiellement mortelles. Vous
    peuvent également être exposés à des niveaux potentiellement nocifs d'émissions de micro-ondes si
    vous faites fonctionner le four avec le couvercle et il y a des dommages ou désalignement à
    le guide d'ondes à la chambre du four.

    Il y a un condensateur haute tension dans le générateur à micro-ondes. Toujours s'assurer
    qu'il est totalement déchargé avant même de penser à toucher ou à sonder
    rien dans les circuits d'alimentation haute tension. Voir les sections de dépannage
    plus tard dans ce document.

    Pour éviter tout risque de choc électrique extrêmement dangereux, débranchez
    le four de la prise secteur avant de retirer le couvercle et ne pas le brancher
    de le faire fonctionner avec le couvercle si possible. Si vous devez sonder
    retirez les connexions au magnétron (voir ci-dessous) pour empêcher la
    génération par inadvertance de micro-ondes sauf lorsque cela est absolument nécessaire
    pendant le dépannage. Décharger le condensateur haute tension (avec le four
    débranché), puis utilisez les fils du clip pour établir des connexions avant de brancher
    et appliquez le pouvoir. Puis, après avoir coupé l'alimentation et débranché le four
    décharger à nouveau le condensateur HT.

    AVERTISSEMENT: des techniciens expérimentés ont été électrocutés plus morts qu'une brique
    de sonder même minutieusement les circuits HT d’un four à micro-ondes alimenté.
    Par conséquent, je recommande fortement d’éviter toute vérification des circuits HT – presque
    tout peut être déterminé par inspection et essais de composants avec le four
    débranché.

    Les circuits du four à micro-ondes sont particulièrement dangereux car le retour de
    la haute tension est le châssis – il n'est pas isolé. De plus, le HV
    peut dépasser 5000 V crête avec un courant nominal continu supérieur à 0,25 AMP à
    50/60 Hz – la puissance nominale continue du transformateur haute tension peut dépasser
    1 500 W avec une disponibilité à court terme d'une puissance bien supérieure. Toujours observer
    protocole haute tension.

    Il y a deux autres problèmes de sécurité non électriques qui sont
    * probablement * pas présent dans les fours à micro-ondes grand public mais il faut tout de même
    à mentionner:

  • Il y a une très faible chance que l'isolant d'antenne à la
     Le sommet du magnétron est constitué d'oxyde de béryllium (BeO), qui
     un matériau extrêmement toxique sous forme de poussière ou de poudre. (Solid BeO est
     pas particulièrement dangereux.) Une céramique en BeO est une excellente chaleur
     conducteur et pour cette raison peuvent être présents dans les parties isolantes de radar
     magnétrons ainsi que des tubes laser à haute puissance et similaires. Si BeO est présent,
     il devrait y avoir au moins une étiquette d'avertissement bien visible. Cependant, il y a
     toujours la possibilité d'un très vieux four à micro-ondes ayant un magnétron
     contenant BeO sans étiquette d'avertissement ou où il est tombé. Donc c'est
     bonne pratique de NE PAS tenter de casser, écraser, broyer, pulvériser ou autrement
     attaquer l'isolant en céramique au sommet du magnétron.
  • Les condensateurs haute tension de très vieux fours à micro-ondes ont peut-être été
     remplis de PCB (PolyChloroBenzenes) qui ont été interdits en 1979 pour être
     cancérogène. Il est peu probable que ces fuites. Il suffit de ne pas les ouvrir!
  • Consignes de sécurité

    Ces consignes ont pour but de vous protéger contre les décharges électriques potentiellement mortelles.
    risques ainsi que l’équipement contre les dommages accidentels.

    Notez que le danger pour vous n’est pas seulement dans votre corps fournissant une conduite
    chemin, particulièrement à travers votre coeur. Toute contraction musculaire involontaire
    causée par un choc, bien que peut-être inoffensif en soi, peut causer des dommages collatéraux
    dommages – il y a beaucoup de bords tranchants à l'intérieur de ce type d'équipement ainsi que
    autres pièces sous tension que vous pourriez toucher accidentellement.

    Le but de cet ensemble de directives n’est pas de vous effrayer, mais plutôt de
    vous faire connaître les précautions appropriées. Réparation de téléviseurs, moniteurs,
    les fours à micro-ondes et autres équipements grand public et industriels peuvent être à la fois
    enrichissant et économique. Assurez-vous simplement que c'est également sûr!

  • Ne travaillez pas seul – en cas d'urgence, présence d'une autre personne
     peut être essentiel.
  • Gardez toujours une main dans votre poche lorsque vous vous trouvez à proximité
     système connecté à la ligne ou haute tension.
  • Portez des chaussures ou des baskets en caoutchouc.
  • Ne portez pas de bijoux ou autres articles qui pourraient accidentellement entrer en contact
     circuits et conduisent le courant, ou se coincer dans les pièces mobiles.
  • Configurez votre zone de travail à l’écart des motifs possibles de collision accidentelle.
     contact.
  • Connaissez votre équipement: les téléviseurs et les moniteurs peuvent utiliser des parties du châssis en métal
     comme retour à la terre mais le châssis peut être électriquement vivre par rapport à la
     la terre de la ligne AC. Les fours à micro-ondes utilisent le châssis comme masse
     revenir pour la haute tension. De plus, ne supposez pas que le châssis
     est un terrain approprié pour votre équipement de test!
  • Si les cartes de circuit imprimé doivent être retirées de leurs supports, mettez des isolants
     matériel entre les planches et tout ce qu'ils peuvent court-circuiter. Tenez-les dans
     placer avec de la ficelle ou du ruban électrique. Soutenez-les avec des bâtons isolants –
     en plastique ou en bois.
  • Si vous avez besoin de sonder, de souder ou de toucher les circuits hors tension,
     décharger (entre) de grands condensateurs de filtrage d’alimentation d’une puissance de 25 W ou
     plus grande résistance de 5 à 50 ohms / V valeur approximative.

    Pour le four à micro-ondes en particulier, utilisez une résistance de 25 K à 100 K conçue pour
     au moins 5 kV et plusieurs watts avec
     un clip sécurisé mène au châssis. Monter la résistance au bout d'un puits
     bâton isolé. Touchez chacune des bornes du condensateur à la non mise à la terre
     fin de la résistance pendant plusieurs secondes. Ensuite, pour être doublement sûr que le
     condensateur s’il est complètement déchargé, court-circuitez ses bornes avec la lame de
     un tournevis bien isolé. Je recommande également de laisser un clip court-circuitant
     à travers les bornes du condensateur tout en travaillant comme une assurance supplémentaire. Au plus,
     vous allez faire sauter un fusible si vous devez oublier de le retirer lors de la mise sous tension du
     four micro onde.

  • Connectez / déconnectez tous les cordons de test avec l’équipement non alimenté et
     débranché. Utilisez des fils à pince ou des fils de soudure temporaires pour atteindre les endroits exigus
     endroits difficiles d'accès.
  • Si vous devez sonder en direct, mettez du ruban isolant sur tout sauf le dernier 1/16 "
     des sondes de test pour éviter la possibilité d'un court-circuit accidentel qui
     pourrait endommager divers composants. Clip la fin de référence de la
     mètre ou portée au retour de masse approprié de sorte que vous devez seulement
     sonde avec une main.
  • Effectuer autant de tests que possible avec mise hors tension et équipement
     débranché.
     Par exemple, les semi-conducteurs de la section d’alimentation d’un téléviseur ou
     moniteur peut être testé pour les courts-circuits avec un ohmmètre.
  • Utilisez un transformateur d'isolement s'il y a un risque de contact avec la ligne
     circuits connectés. Un Variac ™ n'est pas un transformateur d'isolation!
     (Voir la section suivante concernant les transformateurs d’isolation et les
     fours.) L’utilisation d’un disjoncteur de fuite à la terre (GFCI) protégé
     sortie est une bonne idée mais ne vous protégera pas du choc de nombreux points
     dans un téléviseur ou un moniteur connecté en ligne, ou du côté haute tension d'un four à micro-ondes
     four, par exemple. Un disjoncteur est trop lent et insensible pour fournir
     aucune protection pour vous ou, dans de nombreux cas, votre équipement. Un GFCI peut,
     cependant, évitez que votre sonde de champ de fumer fume si vous
     connectez accidentellement une lunette de mise à la terre à un châssis sous tension.
  • N'essayez pas de réparer lorsque vous êtes fatigué. Non seulement serez-vous plus
     négligent, mais votre principal outil de diagnostic – le raisonnement déductif –
     ne pas fonctionner à pleine capacité.
  • Enfin, ne supposez jamais rien sans vérifier par vous-même!
     Ne prenez pas de raccourcis!
  • Comme indiqué, un disjoncteur de fuite à la terre (GFCI) ne vous protégera PAS
    de la haute tension depuis le secondaire du transformateur HT
    fournissant ce courant et tout courant prélevé du secondaire
    à la terre ne sera pas détecté par le GFCI. Cependant, l'utilisation d'un GFCI est
    souhaitable pour minimiser le risque de choc des portions de ligne
    du circuit si vous n'avez pas de transformateur d'isolation.

    Un transformateur d’isolation a même une valeur limitée, car le châssis est
    le retour HV et est un grand endroit très tentant pour toucher, s'appuyer ou brosser
    contre.

    Et, bien sûr, aucun de ces appareils ne protégera les imbéciles d'eux-mêmes!

    Faites très attention lorsque vous travaillez avec le couvercle d'un four à micro-ondes.

    Transformateurs d'isolement et fours à micro-ondes

    Il y a peu d'intérêt à utiliser un transformateur d'isolation avec un micro-ondes
    pour tester les circuits haute tension. Il devrait être énorme en raison de la
    nature haute puissance d'un four à micro-ondes et depuis le retour haute tension est le
    châssis qui est mis à la terre, il ne sera pas terriblement utile comme indiqué ci-dessus.
    Cependant, un transformateur d'isolement peut et doit être utilisé pour tester le primaire
    circuit latéral si nécessaire, y compris les verrouillages, les moteurs, le triac / relais, etc.
    Débranchez le transformateur HT pour éliminer le risque de haute tension
    choquer et réduire la charge.

    En fait, la meilleure politique est de ne JAMAIS JAMAIS essayer de mesurer quoi que ce soit
    dans la section HT lorsque le four est alimenté – il n’est presque jamais nécessaire
    dans tous les cas. Les échecs sont généralement facilement détectés en effectuant un test avec le
    four débranché. Si vous insistez pour effectuer des mesures en direct, connectez le
    mètre avant que le pouvoir est appliqué et déconnecter ou déplacer ses sondes seulement
    après la mise hors tension ET si le capuchon HV a été déchargé (même si le compteur
    prend feu ou explose!). Des techniciens qualifiés ont été électrocutés
    en utilisant un équipement de test approprié sur les fours à micro-ondes!

    Des conseils de dépannage

    Beaucoup de problèmes ont des solutions simples. Ne présumez pas immédiatement que
    votre problème est une combinaison de complexe ésotérique compliqué
    les échecs. Pour un four à micro-ondes, il peut y avoir une porte défectueuse
    interrupteur de verrouillage ou juste un fusible fatigué.

    Si vous êtes coincé, dormez dessus. Parfois, simplement laisser le problème
    rebondir dans votre tête conduira à une autre plus réussie
    approche ou solution. Ne travaillez pas quand vous êtes vraiment fatigué – ce sont les deux
    dangereux (particulièrement avec les fours à micro-ondes) et surtout non productif
    (ou éventuellement destructif – très destructeur).

    Si vous devez retirer le couvercle ou un autre démontage, prenez-en note.
    vis est allé où – ils ne peuvent pas tous être identiques. Plus de notes c'est mieux
    que moins.

    Les flacons de pilules, les boîtes de pellicule et les plateaux à glaçons en plastique sont pratiques pour
    trier et ranger les vis et autres petites pièces après le démontage.

    Sélectionnez une zone de travail bien éclairée où les pièces déposées peuvent
    être situé – pas sur un tapis à poils longs. Quelque chose comme un grand plastique
    plateau avec une légère lèvre peut être utile car il empêche les petites pièces de
    rouler hors de la table de travail. Le meilleur emplacement sera également relativement
    sans poussière et vous permettent de suspendre votre dépannage pour manger ou dormir ou
    penser sans avoir à tout empiler dans une boîte en carton pour le stockage.

    Un ensemble de base d’outils à main de haute qualité suffira à travailler sur un
    four micro-onde. Celles-ci n'ont pas besoin d'être vraiment chères mais de mauvaise qualité
    les outils sont pires qu'inutiles et peuvent causer des dommages. Stanley ou artisan
    sont bien. Les outils nécessaires comprennent une sélection de lames Philips et droites
    tournevis, pinces à bec effilé, pinces coupantes et pinces à dénuder.

    Un fer à souder de moyenne puissance et un noyau de colophane (ne jamais utiliser d'acide
    coeur de soudure ou substance pour la transpiration des tuyaux en cuivre sur les équipements électroniques)
    sera nécessaire si vous devez déconnecter des fils soudés (sur
    ou par accident) ou remplacer les composants soudés.

    Cependant, la plupart des composants de puissance des fours à micro-ondes utilisent des systèmes sans soudure.
    les connecteurs (cosses) et les remplacements viennent généralement avec eux aussi.

    Voir le document: Dépannage et réparation de
    Consumer Electronics Equipment pour des informations supplémentaires sur la soudure et
    techniques de reprise et autres informations générales.

    Un assortiment de connecteurs sans soudure (cosses et wirenuts) est pratique lorsque
    réparer le câblage interne. Un outil de sertissage sera également nécessaire, mais
    la variété à 4 $ convient parfaitement pour une utilisation occasionnelle.

    Les anciens micro-ondes morts peuvent souvent constituer une source précieuse de matériel et parfois
    même des composants tels que des commutateurs de verrouillage et des magnétrons comme ces composants
    sont souvent interchangeables. Tout en ne préconisant pas d'être un rat de meute, cette
    a ses avantages parfois.

    Équipement de test

    Ne commencez pas par l’équipement de test électronique, commencez par quelques analyses
    en pensant. De nombreux problèmes liés aux équipements électroniques grand public
    ne nécessite pas de schéma (même s’il peut être utile). La majorité des
    Les problèmes de four à micro-ondes sont facilement résolus avec au plus un multimètre (DMM
    ou VOM). Vous n’avez pas besoin d’un oscilloscope pour réparer un four à micro-ondes, sauf si
    vous finissez par essayer de réparer la logique dans le contrôleur – extrêmement improbable.

    Un DMM ou un VOM est nécessaire pour vérifier les tensions d’alimentation (PAS
    haute tension, cependant) et le test des interrupteurs de sécurité, des fusibles,
    câblage, et la plupart des composants du générateur de micro-ondes. Cela fait
    pas besoin d'être cher, mais puisque vous serez en fonction de ses lectures,
    la fiabilité est importante. Même un DMM relativement peu coûteux de la radio
    Shack ira pour la plupart des travaux de réparation. Vous vous demanderez comment vous avez jamais
    vécu sans un! Coût: 25-50 $.

    Autres pièces utiles de "matériel d'essai":

  • Un détecteur de fuite à micro-ondes. Types peu coûteux sont facilement disponibles
     dans les centres d'accueil ou par correspondance. Ce ne sont pas super précis ou
     sensible mais vaut mieux que rien. Voir aussi les sections: "Micro-ondes
     compteurs de fuite "et" Détecteurs de fuite à micro-ondes simples ".
  • Un détecteur de puissance à micro-ondes. Ceux-ci peuvent être achetés ou vous pouvez en faire un
     d'une petite ampoule au néon (NE2) ou incandescente avec ses fils conducteurs tordus
     ensemble. Parfois, ces solutions maison ne survivent pas longtemps
     mais va certainement confirmer que le pouvoir micro-ondes est présent à l'intérieur du
     chambre du four. Remarque: ayez toujours une charge à l’intérieur du four lors du test – une
     tasse d'eau est suffisante.
  • Un thermomètre (verre non métallique) pour surveiller la température de l’eau pendant
     tests de puissance.
  • Sonde haute tension (professionnel, pas fait maison!). Cependant, c'est
     ce n'est que rarement nécessaire. Basse tension, résistance ou continuité
     les contrôles permettront d'identifier la plupart des problèmes. ATTENTION: la haute tension dans un
     Le four à micro-ondes est NÉGATIF ​​(-) par rapport au châssis. Devrait
     vous utilisez accidentellement la mauvaise polarité de la sonde de test avec votre lecteur,
     n'échangez pas simplement les sondes = c'est peut-être la dernière chose que vous fassiez.
     Débranchez le four, déchargez le condensateur HT, puis changez le
     les liaisons.
  • Il existe des instruments de test magnétron et à micro-ondes spéciaux, mais à moins que vous ne le soyez
    dans l'entreprise, ce sont des extravagances inutiles.

    Décharge sécurisée du condensateur haute tension

    Il est essentiel – pour votre sécurité et pour éviter d’endommager l’appareil sous
    tester ainsi que votre équipement de test – que le grand condensateur haute tension
    dans le générateur de micro-ondes soit complètement déchargée avant de toucher quoi que ce soit
    ou faire des mesures. Alors que ceux-ci sont supposés inclure des
    résistances de purge, celles-ci peuvent échouer. Dans tous les cas, plusieurs minutes peuvent être
    nécessaire pour que la tension chute à des niveaux négligeables.

    La technique que je recommande consiste à utiliser une résistance de forte puissance d'environ 5 à
    50 ohms / V de la tension de travail du condensateur. Cela empêchera le
    soudage à l'arc associé à la décharge de tournevis, mais aura une assez courte
    constante de temps de sorte que le condensateur va tomber à une basse tension dans au plus un
    few seconds (dependent of course on the RC time constant and its original
    voltage).

  • For the high voltage capacitor in a microwave oven, use a 100K ohm
     resistor rated at least 5 kilovolts and several watts for your
     discharge widget, with a clip lead to the chassis. As a practical
     matter, a single resistor like this will be hard to find.
     So, make one up from a series string of 10 to 20 1/2 W or 1 W normal
     resistors.

    The reason for specifying the resistor in this way is for voltage hold-off.
     Common resistors only are rated for 200 to 500 V, but there may be as much
     as 5 kV on the HV cap. You don't want the HV zapping across the terminals
     of the resistor. Special high voltage resistors are available but they are
     expensive and not readily available from common electronics distributors.

  • Clip the ground wire to an unpainted spot on the chassis. Utilisez le
     discharge probe on each side of the capacitor in turn for a second or two.
     Since the time constant RC is about .1 second, this should drain the charge
     quickly and safely.
  • Then, confirm with a WELL INSULATED screwdriver across the capacitor
     terminals. If there is a big spark, you will know that somehow, your
     original attempt was less than entirely successful. There is a very slight
     chance the capacitor could be damaged by the uncontrolled discharge but at
     least there will be no danger.
  • Finally, it is a good idea to put a clip lead across the capacitor
     terminals just to be sure it stays fully discharged while you are working
     in the area. Yes, capacitors have been known to spontaneously regain some
     charge. At worst, you will blow the fuse upon powering up if you forget to
     remove it.
  • WARNING: DO NOT use a DMM for checking voltage on the capacitor unless you
    have a proper high voltage probe. If your discharging did not work, you may
    blow everything – including yourself.

    A suitable discharge tool can be made as follows:

  • Solder one end of the appropriate size resistor (100K ohms, 25W in this
     case, or a series string of smaller resistors) to a well insulated clip
     lead about 2 to 3 feet long. Don't just wrap it around – this connection
     must be secure for safety reasons.
  • Solder the other end of the resistor to a well insulated contact point
     such as a 2 inch length of bare #14 copper wire mounted on the end of a
     2 foot piece of PVC or Plexiglas rod which will act as an extension handle.
  • Secure the resistor to the insulating rod with some plastic electrical
     tape.
  • This discharge tool will keep you safely clear of the danger area. le
    capacitor discharge indicator circuit described in the document:
    Capacitor Testing, Safe Discharging and Other Related
    Information can be built into the discharge tool if desired.

    Again, always double check with a reliable high voltage meter or by shorting
    with an insulated screwdriver!

    Reasons to use a resistor and not a screwdriver to discharge capacitors:

  • It will not destroy screwdrivers and capacitor terminals.
  • It will not damage the capacitor (due to the current pulse).
  • It will reduce your spouse's stress level in not having to hear those
     scary snaps and crackles.
  • Getting inside a microwave oven

    You will void the warranty – at least in principle. There are usually no
    warranty seals on a microwave so unless you cause visible damage or mangle the
    screws or plastic, it is unlikely that this would be detected. You need to
    decide. A microwave still under warranty should probably be returned for
    warranty service for any covered problems except those with the most obvious
    and easy solutions.

    Unplug the unit! Usually, the sheet metal cover over the top and sides
    is easily removed after unscrewing 8-16 philips head or hex head sheet
    metal screws. Most of these are on the back but a few may screw into the
    sides. They are not usually all the same! At least one of these includes
    a lockwasher to securely ground the cover to the case.

    Note that on some ovens (I've heard that some Sharp models do this), there
    may also be one screw that is slightly longer than the others to engage a
    safety case interlock switch and prevent the oven from getting power if it
    is not present or one of the shorter screws is used in its place.
    So, with the cover removed, nothing is powered inside (which is a good
    thing for safety!). But when the cover is
    replaced with the screws in random locations, there's a high probability
    that the oven no longer works at all. Kind of like Russian Roulette.
    And, if it's then taken to a service center, they will know someone has
    been inside. If less than entirely honest, they can make any sort of
    claim they want as to what might have been damaged even if all you did
    was remove and replace the cover without touching anything inside.
    "The repair will be $195 because you blew out the touch panel by removing the
    cover."

    Therefore, it is essential to make note of any differences
    in screw types so they can be put back in the same place. The cover will
    then lift up and off. Note how fingers on the cover interlock with
    the main cabinet – these are critical to ensure prevention of microwave
    leakage after reassembly.

    Please see Typical Microwave Oven Electronics Bay
    for parts identification. Not all ovens are this wide open. If yours is a
    compact unit, everything may be really squeezed together. 🙂 Details will
    vary depending on manufacturer and model but most of the major components will
    look fairly similar to those depicted in the photo. Note that for this model,
    the oven lamp is actually inside the electronics bay right next to the high
    voltage on the magnetron filament – light bulb changing here is really best
    left to a professional if you would otherwise not go inside!

    Discharge the high voltage capacitor as described in the section:
    Safe discharging of the high voltage capacitor
    before even thinking about touching anything.

    A schematic showing all of the power generation components is usually
    glued to the inside of the cover. How much of the controller is included
    varies but is usually minimal.

    Fortunately, all the parts in a microwave can be easily replaced and most of
    the parts for the microwave generator are readily available from places
    like MCM Electronics, Dalbani, and Premium Parts.

    Reassemble in reverse order. Take particular care to avoid pinching any
    wires when reinstalling the cover. Fortunately, the inside of a microwave
    is wide open and this is not difficult. Make sure ALL of the metal fingers
    around the front edge engage properly with the front panel lip. This is
    critical to avoid microwave emissions should the waveguide or magnetron
    become physically damaged in any way. Confirm that the screws you removed
    go back in the proper locations, particularly the one that grounds the
    cover to the chassis.

  • Back to Microwave Oven Repair FAQ Table of Contents.

    Principles of Operation

    Instant (2 minutes on HIGH) microwave oven theory

    Please see Typical Microwave Oven Electronics Bay
    for parts identification.

    A typical microwave oven uses between 500 and 1000 W of microwave energy
    at 2.45 GHz to heat the food. This heating is caused mainly by the vibration
    of the water molecules. Thus plastic, glass, or even paper containers will
    heat only through conduction from the hot food. There is little transfer of
    energy directly to these materials. This also means that the food does not
    need to be a conductor of electricity (try heating a cup of distilled water)
    and that electromagnetic induction (used elsewhere for high frequency
    non-contact heating) is not involved.

    What is significant about 2.45 GHz? Not that much. Water molecules are not
    resonant at this frequency. A wide range of frequencies will work to heat
    water efficiently. 2.45 GHz was probably chosen for a number of other reasons
    including not interfering with existing EM spectrum assignments and convenience
    in implementation. In addition, the wavelength (about 5 inches) results in
    reasonable penetration of the microwave energy into the food. The 3 dB (half
    power) point is about 1 inch for liquid water – half the power is absorbed in
    the outer 1 inch of depth, another 1/4 of the power in the next inch, and so
    en avant.

    From: Barry L. Ornitz (ornitz@tricon.net).)

    "Industrial ovens still often operate at 915 MHz and other frequencies near 6
      GHz are also used.

    Water has numerous resonances over the entire spectra range, but the lowest
      frequency resonance is the rotational resonance is around 24 GHz. Autre
      resonances occur in the millimeter wave range through the infrared.

    For references, check books on microwave spectroscopy by Townes and Gordy."

    Since the oven chamber cavity is a good reflector of microwaves, nearly all
    the energy generated by the oven is available to heat the food and heating
    speed is thus only dependent on the available power and how much food is being
    cooked. Ignoring losses through convection, the time to heat food is roughly
    proportional to its weight. Thus two cups of water will take around twice as
    long to bring to a boil as one.

    Heating is not (as popularly assumed) from the inside out. The penetration
    depth of the microwave energy is a few cm so that the outside is cooked faster
    than the inside. However, unlike a conventional oven, the microwave energy
    does penetrate these few cm rather than being totally applied to the exterior
    of the food. The misconception may arise when sampling something like
    a pie filling just out of the microwave (or conventional oven for that
    matter). Since the pie can only cool from the outside, the interior filling
    will appear to be much hotter than the crust and will remain that way for a
    long time.

    One very real effect that may occur with liquids is superheating. Il est
    possible to heat a pure liquid like water to above its boiling point
    if there are no centers for bubbles to form such as dust specks or container
    imperfections. Such a superheated liquid may boil suddenly and violently
    upon removal from the oven with dangerous consequences. This can take place
    in a microwave since the heating is relatively uniform throughout the liquid.
    With a stovetop, heating is via conduction from the burner or coil and there
    will be ample opportunity for small bubbles to form on the bottom long before
    the entire volume has reached the boiling point.

    Most metal objects should be excluded from a microwave oven as any sharp
    edges (areas of high electric field gradient) may create sparking
    or arcing which at the very least is a fire hazard. Microwave safe metal
    shelves will have nicely rounded corners.

    A microwave oven should never be operated without anything inside as the
    microwave generator then has no load – all the energy bounces around
    inside an a great deal is reflected back to the source. This may cause
    expensive damage to the magnetron and other components.

    Why don't microwaves leak out from through the glass?

    "I am trying to find out what the glass on a microwave consists of
     exactly. i have not been able to get a better answer than
     'a wire mesh'. if you can help, i would greatly appreciate it."

    There *is* a wire mesh embedded in the glass panel. Since the holes
    in the mesh are much much smaller than the wavelength of the 2.45 GHz
    microwaves (about 5 inches or 12.5 cm), it is essentially opaque to
    microwaves and essentially all the energy is reflected back into the
    oven cavity.

    (From: Filip (I'll buy a vowel) Gieszczykiewicz (filipg@repairfaq.org).)

    Salutations. Did you ever see a "mesh" satellite disk up close? Vous serez
    note that it looks much like it's made out of simple wire mesh that
    you can get in a hardware store (in the USA, it's called "chicken fence"
    :-). The reason this works is that the wave that the dish picks up
    is longer than the hole in the mesh. Consider bouncing a tennis
    ball on the "wire mesh" in the microwave – it WOULD work because
    the ball is bigger than the holes. The wave in the microwave is
    about 2.5cm "long" … as long as the holes are smaller than that
    (actually, you want them as small as possible – without affecting the
    "watching the food" – to minimize any stray and harmonic waves
    from escaping… like bouncing tennis and golf and ping-pong balls and
    marbles off the mesh – you want to catch all the possible sizes – yet
    still be able to see through it) they will not let anything out of the
    oven.

    BTW, it's not really "glass" but rather a 'sandwich' of glass, from
    the outside, wire mesh (usually a sheet of metal which is either stamped
    or drilled with a hole pattern – like a color TV CRT mask!), and followed
    by a sheet of glass or plastic to make sure that food splatters and
    vapor condensation are easy to clean – imagine scraping the mesh!

    How a microwave oven works

    The operation of a microwave oven is really very simple. It consists
    of two parts: the controller and the microwave generator.

    A schematic diagram of the microwave generating circuitry and portions of
    the controller is usually glued to the inside of the cover.

    The controller is what times the cooking by turning the microwave energy
    allumé et éteint. Power level is determined by the ratio of on time to off time
    in a 10-30 second cycle.

    The microwave generator takes AC line power. steps it up to a high voltage,
    and applies this to a special type of vacuum tube called a magnetron – little
    changed from its invention during World War II (for Radar).

    Manette

    The controller usually includes a microcomputer, though very inexpensive
    units may simply have a mechanical timer (which ironically, is probably
    more expensive to manufacture!). The controller runs the digital clock
    and cook timer; sets microwave power levels; runs the display; and in high
    performance ovens, monitors the moisture or temperature sensors.

    Power level in most microwave ovens is set by pulse width control of the
    microwave generator usually with a cycle that lasts 10-30 seconds. Pour
    example, HIGH will be continuous on, MEDIUM may be 10 seconds on, 10
    seconds off, and LOW may be 5 seconds on, 15 seconds off. The power
    ratios are not quite linear as there is a 1 to 3 second warmup period
    after microwave power is switched on.

    However, some models use finer control, even to the point of a continuous
    range of power. These are typically "inverter" models which use a more
    sophisticated type of power supply than the simple high voltage transformer,
    capacitor, rectifier, system described below. However, there have been
    some back in the 1970s that did this with a 1 second or so pulse width
    modulated cycle, fast enough to have the same effect as continuous control
    for all practical purposes.

    The operating voltages for the controller usually are derived from a stepdown
    transformateur. The controller activates the microwave generating circuitry
    using either a relay or triac.

    Capteurs

    More sophisticated ovens may include various sensors. Most common are
    probes for temperature and moisture. A convection oven will include a
    temperature sensor above the oven chamber.

    Since these sensors are exposed to the food or its vapors, failures of the
    sensor probes themselves are common.

    Cooling fans

    Since 30 to 50 percent of the power into a microwave oven is dissipated as
    heat in the Magnetron, cooling is extremely important. Always inspect the
    cooling fan/motor for dust and dirt and lubricate if necessary. A couple of
    drops of electric motor oil or 3-in-One will go a long way. If there are any
    belts, inspect for deterioration and replace if necessary.

    An oven that shuts off after a few minutes of operation could have a cooling
    problem, a defective overtemperature thermostat, a bad magnetron, or is being
    operated from very high AC line voltage increasing power to the oven.

    One interesting note: Since 30 to 50 percent of the power goes out the vents
    in the back as heat, a microwave oven is really only more efficient than
    conventional means such as a stovetop or gas or electric oven for heating
    small quantities of anything. With a normal oven or stovetop, wasted energy
    goes into heating the pot or oven, the air, and so on. However, this is
    relatively independent of the quantity of food and may be considered to be a
    fixed overhead. Therefore, there is a crossover point beyond which it is more
    efficient to use conventional heat than high tech microwaves.

    Microwave generator

    This is the subsystem that converts AC line power into microwave energy.
    The majority of microwave ovens use a brute force approach which
    consists of 5 parts: high voltage (HV) transformer running off the AC line,
    HV rectifier diode, HV capacitor, magnetron, waveguide to oven chamber.
    (A few employ solid state inverter in place of the simple HV transformer.
    These will be discussed later.)

    The most common microwave generator consists of the following:

  • High Voltage Transformer. Typically has a secondary of around 2,000 VRMS
     at 0.5 to 1 amp – more or less depending on the power rating of the oven.
     There will also be a low voltage winding for the Magnetron filament (3.3 V
     at 10 A is typical).

    You cannot miss this as it is the largest and heaviest component visible
     once the cover is removed. There will be a pair of quick-connect terminals
     for the AC input, a pair of leads for the Magnetron filament. and a single
     connection for the HV output. The HV return will be fastened directly to
     the transformer frame and thus the chassis.

    These transformers are designed with as little copper as possible. le
     primary for 115 VAC is typically only 120 turns of thick wire – thus about 1
     turn per volt input and output (this is about 1/4th as many turns as in a
     "normal" power transformer. (It's usually possible to count the primary
     turns by examining how it is wound – no disassembly required!) So there
     would be about 3 turns for the magnetron filament and 2080 turns for the
     high voltage winding for the transformer mentioned above. The reason they
     can get away with so few turns is that it operates fully loaded about 90
     percent of the time but is still on the hairy edge of core saturation.
     The HV components are actually matched to the HV transformer characteristics.
     Performance will suffer if the uF value of a replacement HV capacitor is not
     close to that of the original.

    There is also generally a "magnetic shunt" in the core of the transformer.
     This provides some current limiting, possibly to compensate for various
     magnetron load conditions. However, it's not enough to provide any reduction
     in the likelihood of electrocution should you come in contact with the
     HV winding!

  • Rectifier – usually rated 12,000 to 15,000 PRV at around 0.5 amp. Most
     commonly, this will be rectangular or cylindrical, about 0.5 inch long
     with wire leads. Sometimes, it is a box bolted to the chassis. Un
     end will be electrically connected to the chassis.
  • Capacitor – 0.65 to 1.2 uF at a working voltage of around 2,000 VAC. Remarque
     that this use of 'working voltage' may be deceiving as the actual voltage
     on the capacitor may exceed this value during operation. The capacitor
     is metal cased with quick-connect terminals on top (one end). Toujours
     discharge the capacitor as described below before touching anything inside
     once the cover is removed.
  • Magnetron – the microwave producing tube includes a heated filament
     cathode, multiple resonant cavities with a pair of permanent ceramic ring
     magnets to force the electron beams into helical orbits, and output antenna.
     The magnetron is most often box shaped with cooling fins in its midsection,
     the filament/HV connections on the bottom section, and the antenna (hidden
     by the waveguide) on top. Sometimes, it is cylindrical in shape but this is
     less common. The frequency of the microwaves is usually 2.45 GHz.
  • When salvaging parts from dead microwave ovens, save the HV components
    (transformer, capacitor, and diode) as a group (assuming all are still
    good). Then, if a repair is needed to another oven it may be better to
    replace all 3 both because this eliminates uncertainty if more than 1
    part failed or is marginal, and they will have been designed to have
    the best compatibility.

    High voltage transformer

    (From: John De Armond.)

    The transformer goes by several names, depending on where you are. Variable
    reluctance, leakage flux, stray flux, etc. It is exactly the same
    construction and operating principle as a neon transformer, some kinds of HID
    light ballasts and some series streetlight constant current transformers.

    The core is an almost standard "E" core (or "H" core if you prefer) with one
    exception. The center leg has an air gap. The windings are on the end legs
    of the "E" instead of the center leg.

    There are two magnetic paths around the core for the field set up by the
    primary to travel. Around the periphery and across the secondary and around
    the center leg and across the air gap. The field that travels along the center
    leg does not cross the secondary and induces no voltage.

    With no load applied, the bulk of the field travels the peripheral, very much
    lower reluctance solid iron path, inducing full secondary voltage proportional
    to the turns ratio. As current flows in the secondary, counter-MMF raises the
    reluctance of the peripheral path so that some of the flux travels through the
    center leg. With less flux traveling around the periphery and cutting across
    the secondary, the secondary voltage drops as the current remains about the
    same. At the limit, if the secondary is shorted, the peripheral path has so
    much reluctance that most of the flux travels the center leg and across the
    air gap. The same current as before flows through the secondary but at zero
    volts.

    When the dimensions of the core and gap are set up correctly, the transformer
    behaves as an almost perfect constant current device. That is, the secondary
    voltage varies as necessary to keep the same current flowing through a varying
    load. Just what the doctor ordered to keep the magnetron happy.

    The secondary current can be increased by opening up the air gap. This raises
    the reluctance of that path and forces more field through the secondary leg.
    Closing the gap has the opposite effect.

    The center leg is often called the magnetic shunt and frequently it is a
    separate piece of laminated iron stuck between the coils and TIG welded in
    endroit. It is a common trick for Tesla Coilers to open up a neon transformer
    and either knock out the shunt entirely or grind it down to open the air gap.
    This modification causes the transformer to output much more current than it
    is designed for – for a little while, at least 🙂 The same thing works with
    microwave oven transformers (MOT).

    This design in a microwave oven is a vital part of keeping the magnetron anode
    current within spec. The magnetron is electrically a diode. A diode that
    isn't emission-limited would draw destructive current if not externally
    limited. With this design, the filament can be heated good and hot for long
    life and not have the tube run away. The design also is vital for protecting
    the magnetron from potentially damaging conditions such as operating the oven
    empty, arcing, etc.

    It's popular to use several MOTs to build an arc welder. This works quite
    well specifically because these transformers are constant-current devices –
    exactly the characteristic stick welding needs. If they were conventional
    transformers, the first time the rod touched the work and shorted the
    secondary, fault current would flow and the breaker would trip or blue smoke
    would leak out.

    Along similar lines, one can cut off the high voltage secondary and replace it
    with a suitable number of turns of heavy wire, connect a bridge rectifier and
    have a nice constant current battery charger. Select the turns carefully and
    it'll do the bulk/absorption stages of the smart 3 stage charging algorithm.

    Magnetron construction and operation

    The cavity magnetron was invented by the British before World War II. Il est
    considered by many to be the invention most critical to the Allied victory
    in Europe.

    The story goes that shortly after the War, a researcher at the Raytheon
    Corporation, Dr. Percy Spencer, was standing near one of the high power radar
    units and noticed that a candy bar in his shirt pocket had softened. dans le
    typical 'I have to know why this happened' mentality of a true scientist, he
    decided to investigate further. The Amana Radarange and the entire future
    microwave oven industry were the result.

    Here are two descriptions of magnetron construction. The first is what you
    will likely find if you go to a library and read about radar. (Some really old
    microwave ovens may use the classic design as well.) This is followed by my
    autopsy of a dead magnetron of the type that is probably in the microwave oven
    in your kitchen. (Items (1) to (6) in the following sections apply to each
    type while items (7) to (9) apply to both types.)

    For more detailed information with some nice diagrams, see the articles at the
    Microtech Web Site. Topics include basic microwave theory as well as a
    complete discussion of microwave oven magnetron construction and principles of
    operation.

    Magnetron construction – basic textbook
     la description

    This is the description you will find in any textbook on radar or microwave
    ingénierie. The original Amana Radarange and other early microwave ovens
    likely used this design as well.

  • A centrally located cylindrical electron emitting cathode. This is
     supplied with pulsed or continuous power of many thousands of volts (negative
     with respect to the anode.
  • A cylindrical anode block surrounding but separate and well insulated from
     the cathode.
  • Multiple cylindrical resonator cavities at a fixed radius from the cathode
     bored in the anode block. Channels link the cavities to the central area
     in which the cathode is located.

    The wavelength of the microwave energy is approximately 7.94 times the
     diameter of the cavities. (For the frequency of 2.45 GHz (12.4 cm) used
     in a microwave oven this would result in a cavity diameter of approximately
     .62" (15.7 mm).

  • An antenna pickup in one of the cylindrical cavities which couples the
     microwave energy to the waveguide.
  • The entire assembly is placed in a powerful magnetic field (several
     thousand Gauss compared to the Earth's magnetic field of about .5 Gauss).
     This is usually supplied by a permanent magnet though electromagnets have
     been also used. The original designs used huge somewhat horseshoe shaped
     permanent magnets which were among the most powerful of the day.
  • Cooling of the anode block must be provided by forced air, water, or oil
     since the microwave generation process is only about 60 to 75 percent
     efficient and these are often high power tubes (many kilowatts).
  • Magnetron construction – modern microwave
     four

    This description is specifically for the 2M214 (which I disassembled) or
    similar types used in the majority of medium-to-high power units. cependant,
    nearly all other magnetrons used in modern domestic microwave ovens should be
    very similar.

    The item numbers are referenced to the diagram in the section:
    Cross section diagram of typical magnetron.

    Also see this photo of the Typical Magnetron Anode and
    Resonant Structure. This is a view looking up through the anode cylinder
    from the filament end of the tube. See the text below for parts names and
    dimensions.

  • The filament and cathode are one in the same and made of solid tungsten
     wire, about .020" (.5 mm) diameter, formed in a helix with about 8 to 12
     turns, 5/32" (4 mm) diameter and just over 3/8" (9.5 mm) in length. le
     cathode is coated with a material which is good for electron emission.

    Note: this coating is the only material contained in the microwave oven
     magnetron that might be at all hazardous. Beryllium, a toxic metal, may
     be used in the form of a ceramic of beryllium oxide (BeO) in large radar
     magnetrons due to its excellent heat conductivity. But should not be
     present in modern domestic microwave ovens. However, see the section:
     SAFETY.

    The filament gets its power via a pair of high current RF chokes – a dozen
     or so turns of heavy wire on a ferrite core – to prevent microwave leakage
     back into the filament circuit and electronics bay of the oven. Typical
     filament power is 3.3 VAC at 10 A.

    The cathode is supplied with a pulsating negative voltage with a peak value
     of up to 5,000 V.

  • The anode is a cylinder made from .062" (1.5 mm) thick copper with an
     inside diameter of 1-3/8" (35 mm) and a length of about 1" (25.4 mm).

    Steel plates (which probably help to shape the magnetic field, see below)
     and thin steel covers (to which the filament and antenna insulators are
     sealed) are welded to the ends of the cylinder.

    The filament leads/supports enter through a cylindrical ceramic insulator
     sealed to the bottom cover and then pass through a hole in the bottom end
     assiette.

  • Rather than cylindrical cavities (as you would find in most descriptions
     of radar magnetrons), there are a set of 10 copper vanes .062" (1.5 mm)
     thick and approximately 1/2" (12.7 mm) long by 3/8" (9.5 mm) wide. Celles-ci
     are brazed or silver soldered to the inside wall of the cylinder facing
     inward leaving a 5/16" (8 mm) central area clear for the filament/cathode.

    Surrounding this space are the .062" (1.5 mm) thick edges of the 10 vanes
     with gaps of approximately .04" (1 mm) between them.

    Copper shorting rings at both ends near the center join alternating vanes.
     Thus, all the even numbered vanes are shorted to each other and all the odd
     numbered vanes are shorted to each other. Of course, all the rings are
     also all shorted at the outside where they are joined to the inner wall
     of the cylinder.

    This structure results in multiple resonant cavities which behave like
     sets of very high quality low loss L-C tuned circuits with a sharp peak
     at 2.45 GHz. At this high frequency, individual inductors and capacitors
     are not used. The inductance and capacitance are provided by the precise
     configuration and spacing of the copper vanes, shorting rings, and anode
     cylinder.

  • A connection is made near the middle of a single vane to act as the output
     power takeoff. It passes through a hole in the top end plate, exits the
     tube via a cylindrical ceramic insulator sealed to the top cover, and
     attaches to the pressed-on bu?ll-nose antenna cap.
  • The entire assembly is placed in a powerful magnetic field (several
     thousand Gauss compared to the Earth's magnetic field of about .5 Gauss).
     This is provided by a pair of ceramic ring magnets placed against the top
     and bottom covers of the anode cylinder. For the 2M214, these are about
     2-1/8" (54 mm) OD, 1-13/16" (46 mm) ID, 1/2" (12.7 mm) thick.
  • A set of thin aluminum fins act as a heat sink for removing the significant
     amount of wasted heat produced by the microwave generation process since
     it is only about 60 to 75 percent efficient. These are press fit on the
     magnetron anode and also in contact with the magnetron case. There will
     always be a cooling fan to blow air through this assembly.

    The anode and magnetron case are at ground potential and connected to the
     châssis.

  • Magnetron construction – common features

    The following items apply to all types of magnetrons.

  • The gap between the cathode and anode, and the resonant cavities, are all
     in a vacuum.
  • When powered, electrons stream from the cathode to the anode. The magnetic
     field forces them to travel in curved paths in bunches like the spokes of
     a wheel. The simplest way to describe what happens is that the electron
     bunches brush against the openings of the resonating cavities in the anode
     and excite microwave production in a way analogous to what happens when you
     blow across the top of a Coke bottle or through a whistle.
  • The frequency/wavelength of the microwaves is mostly determined by the size
     and shape of the resonating cavities – not by the magnetic field as is
     popularly thought. However, the strength of the magnetic field does affect
     the threshold voltage (the minimum anode voltage required for the magnetron
     to generate any microwaves), power output, and efficiency.
  • Cross section diagram of typical magnetron

    The really extraordinary ASCII art below represents (or is supposed to
    represent) a cross section of the 2M214 type magnetron (not to scale) through
    the center as viewed from the side.

                                    ________
                                   | ____ |
                                   |_| |_| Antenna cap
                                   / |____|
                                  | | || | | Antenna insulator
                                  | | || | |
                          xxxxxxxx|__| || |__|xxxxxxxx RF sealing gasket
              ____________________| || |____________________
             | | (5)|| || || (5)| |
             | | Top || || || Top | |
             | | Magnet || || || Magnet | | Outer case
             | |__________|| || ||__________| |
             | ______| \ |______ |
             | /____ (7) \ ____ |
             |____________|| __ ______ \ / ||____________|
             | ||_______ |__ __| _\ ___|| |
             |____________|| | o || o | ||(4)||____________|
             | || | o || o | || (6) | Heat sink fins
             |____________|| Vane | o || o | Vane ||____________|
             | || (3) | o || o | (3) || |
             |____________|| | o || o | ||____________| o: Filament
             | ||_______|(1)|| o |_______|| | hélix
             |____________|| __ |_||||_| __ ||____________|
             | ||____/ || || ____||
    Microwave generator circuit diagram

    Nearly all microwave ovens use basically the same design for the microwave
    generator. This has resulted in a relatively simple system manufactured at
    low cost.

    The typical circuit is shown below. This is the sort of diagram you are
    likely to find pasted inside the metal cover. Only the power circuits
    are likely included (not the controller unless it is a simple motor driven
    timer) but since most problems will be in the microwave generator, this
    schematic may be all you need.

    || +————————+
                                          ||( 3.3 VAC, 10 A, typical |
                            TP Relay or || +————+——+FA F| Magnetron
           _ I I __ Triac || | +-|—-|-+
       o— _—/ –+—/ — —-/ —-+ || +——||—-+ | |_ _| |
                    | )||( HV Cap | | / |
      AC I I=Interlock )||( __|__ | ___ |
     Line | TP=Thermal Prot. )||( 2,000 VAC __/_ +—-|:–+
       o————+——————-+ ||( 0.5 A | HV |'–> Micro-
                                          ||( typical | Diode | waves
           (Controller not shown) || +————+———+
                                                         _|_
                                                          – Chassis ground

    Note the unusual circuit configuration – the magnetron is across the diode,
    not the capacitor as in a 'normal' power supply. What this means is that the
    peak voltage across the magnetron is the transformer secondary + the voltage
    across the capacitor, so the peaks will approach the peak-peak value of the
    transformer or nearly 5000 V in the example above. This is a half wave voltage
    doubler. The output waveform looks like a sinusoid with a p-p voltage equal to
    the p-p voltage of the transformer secondary with its positive peaks at chassis
    ground (no load). The peaks are negative with respect to the chassis. le
    negative peaks will get squashed somewhat under load. Take extreme care – up
    to 5000 V at AMPs available! WARNING: Never attempt to view this waveform on
    an oscilloscope unless you have a commercial high voltage probe and know how
    to use it safely!

    The easiest way to analyze the half wave doubler operation is with the
    magnetron (temporarily) removed from the circuit. Then, it becomes a simple
    half wave rectifier/filter so far as the voltage acrtoss the capacitor is
    concerned – which will be approximately V(peak) = V(RMS) * 1.414 where V(RMS)
    is the output of the high voltage transformer. The voltage across the HV
    rectifier will then be: V(peak) + V where V is the waveform out of the
    transformateur. The magnetron load, being across the HV diode, reduces the peak
    value of this somewhat – where most of its conduction takes place.

    Note that there is a difference in the labels on the filament connections of
    the magnetron. Functionally, it probably doesn't matter which way they are
    connected. However, the typical schematic (as above) shows FA going to
    the node attached to the Anode of the HV diode, while F goes to the lone
    Filament terminal on the HV transformer.

    WARNING: What this implies is that if the magnetron is not present or is not
    drawing power for some reason – like an open filament – up to V(peak) will
    still be present across the capacitor when power is removed. At the end of
    normal operation, some of this will likely be discharged immediately but will
    not likely go below about 2,000 V due to the load since the magnetron does not
    conduct at low voltages.

    Other types of power supplies have been used in a few models – including high
    frequency inverters – but it is hard to beat the simplicity, low cost, and
    reliability of the half wave doubler configuration. See the section:
    High frequency inverter type HV power supplies.

    There is also usually a bleeder resistor as part of the capacitor, not shown.
    HOWEVER: DO NOT ASSUME THAT THIS IS SUFFICIENT TO DISCHARGE THE CAPACITOR –
    ALWAYS DO THIS IF YOU NEED TO TOUCH ANYTHING IN THE MICROWAVE GENERATOR AFTER
    THE OVEN HAS BEEN POWERED. The bleeder may be defective and open as this does
    not effect operation of oven and/or the time constant may be long – minutes.
    Some ovens may not have a bleeder at all.

    In addition, there will likely be an over-temperature thermostat – thermal
    protector – somewhere in the primary circuit, often bolted to the magnetron
    Cas. There may also be a thermal fuse or other protector physically
    elsewhere but in series with the primary to the high voltage transformer.

    Other parts of the switched primary circuit include the oven interlock
    switches, cooling fan, turntable motor (if any), oven light, etc.

    Interlock switches

    Various door interlock switches prevent inadvertent generation of microwaves
    unless the door is closed completely. At least one of these will be directly
    in series with the transformer primary so that a short in the relay or triac
    cannot accidentally turn on the microwaves with the door open. The interlocks
    must be activated in the correct sequence when the door is closed or opened.

    Interestingly, another interlock is set up to directly short the power line
    if it is activated in an incorrect sequence. The interlocks are designed
    so that if the door is correctly aligned, they will sequence correctly.
    Otherwise, a short will be put across the power line causing the fuse
    to blow forcing the oven to be serviced. This makes it more difficult for
    an ignorant consumer to just bypass the door interlocks should they fail or
    to run the oven with an open door as a room heater – and protects the
    manufacturer from lawsuits. (That interlock may be known as a "dummy switch"
    for obvious reasons and is often not even mentioned in the schematic/parts
    manifest.) Of course, should that switch ever actually be used, not only will
    the fuse blow, but the switch contacts will likely be damaged by the high
    initial current! This also means it probably wouldn't be a bad idea to
    replace the interlock switch which might have been affected if your oven
    fails with a blown fuse due to a door problem.

    Failed door interlocks account for the majority of microwave oven problems –
    perhaps as high as 75 percent. This is not surprising considering that two
    of the three switches carry the full oven current – any deterioration of the
    contacts results in increased resistance leading to their heating and further
    deterioration. And, opening the door to interrupt a cook cycle results in
    arcing at the contacts. Complete meltdowns are not unusual! If any defective
    door switches are found, it is probably a good idea to replace all of them as
    long as the oven is already apart.

    The typical door switches and their function:

  • Door Sensing: Input to the microcontroller to indicate the state of
     la porte.
  • Interlock Monitor: Shorts out the AC line (and blows the main fuse)
     should the Primary Interlock not open due to incorrect sequencing of the
     door switches or a failed switch.
  • Primary Interlock: In series with the high voltage (magnetron)
     power supply so cuts power when the door is open.
  • Note that if the Door Sensing switch should malfunction, peculiar behavior
    may occur (like the fan or turntable operating at the wrong time) but should
    never result in microwaves being generated with the door open.

  • Back to Microwave Oven Repair FAQ Table of Contents.

    Troubleshooting Guide

    Instant troubleshooting chart – most common problems and possible causes

    The following chart lists a variety of common problems and nearly all possible
    causes. Diagnostic procedures will then be needed to determine which actually
    apply. The 'possible causes' are listed in *approximate* order of likelihood.
    Most of these problems are covered in more detail elsewhere in this document.

    While this chart lists many problems, it is does not cover everything that can
    se tromper. However, it can be a starting point for guiding your thinking in
    the proper direction. Even if not listed here, your particular problem may
    still be dealt with elsewhere in this document.

  • Problem: Totally dead oven.Possible causes:
  • No power to outlet (blown fuse or tripped breaker or GFCI).
  • Blown main fuse – likely due to other problems.
  • Open thermal protector or thermal fuse.
  • Defective controller or its power supply.
  • Clock needs to be set before other functions will operate (some models).
  • Problem: Totally dead oven after repair.Possible causes:
  • Cabinet screws replaced in incorrect location (safety interlock not
      engaged).
  • Any number of screwups. 🙂
  • Problem: No response to any buttons on touchpad.Possible causes:
  • Door is not closed (some models).
  • You waited too long (open and close door to wake it up).
  • Controller is confused (pull plug for a minute or two to reset).
  • Defective interlock switches.
  • Faulty controller or its power supply.
  • Touchpad or controller board contaminated by overenthusiastic cleaning.
  • Defective/damaged touchpad.
  • Problem: Oven runs when door is still open.Possible causes:
  • Damaged interlock assembly.
  • Cooling fans (only) running due to bad sensor or still warm.
  • Problem: Oven starts on its own as soon as door is closed.Possible causes:
  • Defective triac or relay.
  • Controller is confused (pull plug for a minute or two to reset).
  • Defective controller or its power supply.
  • Touchpad or controller board contaminated by overenthusiastic cleaning.
  • Defective/damaged touchpad.
  • Problem: Oven works but display is blank.Possible causes:
  • Defective controller or its power supply.
  • Broken display panel.
  • Oven needs to be reset (pull plug for a minute or two to reset).
  • Problem: Whacked out controller or incorrect operation.Possible causes:
  • Previous or multipart cook cycle not complete.
  • Controller is confused (pull plug for a minute or two to reset).
  • Defective controller or its power supply.
  • Touchpad or controller board contaminated by overenthusiastic cleaning.
  • Defective/damaged touchpad.
  • Defective sensor (particulalry covection/mirowave combos).
  • Problem: Erratic behavior.Possible causes:
  • Previous or multipart cook cycle not complete.
  • Bad connections in controller or microwave generator.
  • Faulty relay – primary (or HV side, much less commonly used).
  • Defective controller or its power supply.
  • Bad contacts/connections on mechanical timers. Intermittent fuse.
  • Power surge at start of cook cycle confusing controller.
  • Microwave (RF) leakage into electronics bay.
  • Problem: Some keys on the touchpad do not function or perform the wrong
      action.Possible causes:

  • Touchpad or controller board contaminated by overenthusiastic cleaning.
  • Defective/damaged touchpad.
  • Controller is confused (pull plug for a minute or two to reset).
  • Faulty controller.
  • Problem: Microwave oven does not respond to START button.Possible causes:
  • Defective START button.
  • Faulty interlock switches.
  • Door is not securely closed.
  • Faulty controller.
  • You waited too long – open and close door to wake it up!
  • Problem: No heat but otherwise normal operation.Possible causes:
  • Blown fuse in HV transformer primary circuit or HV fuse (if used).
  • Bad connections (particularly to magnetron filament).
  • Open thermal protector or thermal fuse.
  • Open HV capacitor, HV diode, HV transformer, or magnetron filament.
  • Shorted HV diode, HV capacitor (will blow a fuse), or magnetron.
  • Damaged protective VDR from filament to chassis (not commonly used).
  • Defective HV relay (not commonly used).
  • Problem: Timer and light work but no heat, cooling fan, or turntable
      rotation.Possible causes:

  • Defective (lower) door interlock switch or door not closing fully.
  • Faulty relay or triac.
  • Problem: Fuse blows when closing or opening door:Possible causes:
  • Defective door interlock switch(s).
  • Interlock switch knocked out of position.
  • Misaligned door.
  • Problem: Loud hum and/or burning smell when attempting to cook.Possible causes:
  • Shorted HV diode, magnetron.
  • Burnt carbonized food in or above oven chamber.
  • Shorted winding in HV transformer.
  • Frayed insulation on HV wiring.
  • Problem: Arcing in or above oven chamber.Possible causes:
  • Burnt carbonized food deposits.
  • Exposed sharp metal edges.
  • Problem: Fuse blows when initiating cook cycle.Possible causes:
  • Defective interlock switches or misaligned door.
  • Shorted HV capacitor.
  • Shorted HV diode.
  • Shorted magnetron (probably won't blow main fuse but HV fuse if used).
  • Defective triac.
  • Old age or power surges.
  • Defective HV transformer.
  • Short in wiring due to vibration or poor manufacturing.
  • Problem: Fuse blows when microwave shuts off (during or at end of cook
      cycle).Possible causes:

  • Defective triac (doesn't turn off properly).
  • Defective relay.
  • Shorting wires.
  • Problem: Oven heats on high setting regardless of power setting.Possible causes:
  • Faulty primary relay or triac or HV relay (not commonly used).
  • Faulty controller.
  • Problem: Oven immediately starts to cook when door is closed.Possible causes:
  • Shorted relay or triac.
  • Faulty controller.
  • Problem: Oven heats but power seems low or erratic.Possible causes:
  • Low line voltage.
  • Magnetron with low emission.
  • Faulty controller or set for wrong mode.
  • Stirrer (or turntable) not working.
  • Intermittent connections to magnetron filament or elsewhere.
  • Faulty primary relay or triac or HV relay (not commonly used).
  • Damaged protective VDR from filament to chassis (not commonly used).
  • Problem: Oven heats but shuts off randomly.Possible causes:
  • Overheating due to blocked air vents or inoperative cooling fan.
  • Overheating due to bad magnetron.
  • Bad connections in controller or microwave generator.
  • Faulty interlock switch or marginal door alignment.
  • Faulty controller.
  • Overheating due to extremely high line voltage.
  • Stuck stirrer fan resulting hot spots detected by sensors.
  • Problem: Oven makes (possibly erratic) buzzing noise when heating.Possible causes:
  • Fan blades hitting support or shroud.
  • Vibrating sheet metal.
  • Vibrating transformer laminations.
  • Turntable or stirrer hitting some debris.
  • Problem: Oven light does not work.Possible causes:
  • Burnt out bulb :-).
  • Bad connections.
  • Problem: Fans or turntables that do not work.Possible causes:
  • Gummed up lubrication or bad motor bearing(s).
  • Loose or broken belt.
  • Bad motor.
  • Bad thermostat.
  • Bad connections.
  • What can go wrong

    The most common problems occur in the microwave generating portion of the
    system, though the controller can be blown by a lightning strike or other power
    surge. Bad interlock switches probably account for the majority of microwave
    oven problems. Also, since the touchpad is exposed, there is a chance that it
    can get wet or damaged. If wet, a week or so of non-use may cure keys that
    don't work. If damaged, it will probably need to be replaced – this is
    straightforward if the part can be obtained, usually direct from the
    fabricant. Unfortunately, it is an expensive part ($20-50 typical).

    The interlock switches, being electromechanical can fail to complete the
    primary circuit on an oven which appears to operate normally with no blown
    fuses but no heat as well. Faulty interlocks or a misaligned door may result
    in the fuse blowing as described above due to the incorrect sequencing of the
    door interlock switches. Failed interlocks are considered to be the most
    common problems with microwave ovens, perhaps as high as 75% of all failures.
    See the section: Testing and replacing of interlock
    switches.

    No adjustments should ever be required for a microwave oven and there are no
    screws to turn so don't look for any!

    General system problems

    The following problems are likely power or controller related and not in
    the microwave generator unless due to a blown fuse or bad/intermittent
    connections:

  • Totally dead oven.
  • No response to any buttons on touchpad
  • Oven runs when door is still open.
  • Oven starts on its own as soon as door is closed.
  • Oven works but display is blank.
  • Whacked out controller or incorrect operation.
  • Erratic behavior.
  • Some keys on the touchpad do not function or perform the wrong action.
  • Microwave oven does not respond to START button.
  • First, unplug the microwave oven for a couple of minutes. Sometimes, the
    microcontroller will get into a whacko mode for some unknown reason – perhaps
    a power surge – and simply needs to be reset. The problem may never reoccur.

    Note: when working on controller related problems, unplug the connection
    to the microwave generator (HV transformer primary) from the power relay
    or triac – it is often a separate connector. This will prevent any possible
    accidental generation of microwave energy as well as eliminating the high
    voltage (but not the AC line) shock hazard during servicing.

    If this does not help, there is likely a problem with the controller circuitry
    or its power and you will have to get inside the oven.

    Uninvited guests

    Some cockroaches (or other lower life forms) may have taken up residence on
    the controller circuit board. It is warm, cozy, safe, and from their point of
    view makes an ideal habitat. If you got the microwave oven from a flea market,
    garage sale, the curb, a relative, or friend, or if your kitchen isn't the
    cleanest in the world, such visitors are quite possible. Creatures with six
    or more legs (well, some two legged varieties as well) are not known for their
    skills in the areas of housekeeping and personal hygiene.

    Clean the circuit board and connectors thoroughly with water and then isopropyl
    de l'alcool. Dry completely. Inspect the circuit traces for corrosion or other
    dommage. If there are any actual breaks, these will have be be jumpered with
    fine wire and then soldered. Hopefully, no electronic components were affected
    though there is always a slight possibility of other problems.

    Totally dead oven

    First, check power to the outlet using a lamp or radio you know works. le
    fuse or circuit breaker at your service panel may have blown/tripped due to
    an overload or fault in the microwave oven or some other appliance. Tu peux
    just have too many appliances plugged into this circuit – microwave ovens are
    high current appliances and should be on a dedicated circuit if possible. Si
    you attempt to run a heating appliance like a toaster or fryer at the same
    time, you *will* blow the fuse or trip the circuit breaker. A refrigerator
    should never be plugged into the same circuit for this reason as well – you
    really don't want it to be without power because of your popcorn!

    If you find the fuse blown or circuit breaker tripped, unplug everything from
    the circuit to which the microwave is connected (keep in mind that other
    outlets may be fed from the same circuit). Replace the fuse or reset the
    circuit breaker. If the same thing happens again, you have a problem with
    the outlet or other wiring on the same branch circuit. If plugging in the
    microwave causes the fuse to blow or circuit breaker to trip immediately,
    there is a short circuit in the power cord or elsewhere.

    The microwave oven may be powered from a GFCI outlet or downstream of one and
    the GFCI may have tripped. (Removing a broken oven lamp has been known to
    happen.) The GFCI outlet may not be in an obvious location but first check
    the countertop outlets. The tripped GFCI could be in the garage or almost
    anywhere else! Pushing the RESET button may be all that's needed.

    Next, try to set the clock. With some ovens the screen will be totally blank
    following a power outage – there may be nothing wrong with it. En outre,
    some ovens will not allow you perform any cooking related actions until the
    clock is set to a valid time.

    Assuming these are not your problems, a fuse has probably blown although
    a dead controller is a possibility.

    If the main fuse is upstream of the controller, then any short circuit
    in the microwave generator will also disable the controller and display.
    If this is the case, then putting in a new fuse will enable the
    touchpad/display to function but may blow again as soon as a cook cycle
    is initiated if there is an actual fault in the microwave circuits.

    Therefore, try a new fuse. If this blows immediately, there may be a
    short very near the line cord, in the controller, or a defective triac
    (if your oven uses a triac). Or, even a shorted oven lamp – remove and
    inspect the light bulb and socket.

    If it does not blow, initiate a cook cycle (with a cup of water inside). Si
    the oven now works, the fuse may simply have been tired of living. This is
    common.

    If the fuse still blows immediately, confirm that the controller is
    operational by unplugging the microwave generator, power relay, and/or
    triac from the controller. If a new fuse does not now blow when a cook
    cycle is initiated – and it appears to operate normally – then one of
    the components in the microwave generator is defective (shorted). Voir
    the section: Microwave generator problems.

    Some models have a thermal fuse as well and this may have failed for no
    reason or a cooling fan may not be working and the oven overheated (in
    which case it probably would have died while you were cooking something
    for an important guest – assuming you would use a microwave oven for such
    a thing!).

    Other possible causes: bad controller power supply or bad controller chip.

    Totally dead oven after repair

    On some microwave ovens, there is at least one cabinet screw that is slightly
    longer than all the others. This engages a safety interlock which prevents
    the oven from receiving power if the correct screw is missing or in the wrong
    hole. Check the length of all the screws and locate the interlock switch
    behind one of the screw holes. I don't know how common this practice is
    but have heard of it on some Sharp models.
    Also see the section: Getting inside a
    microwave oven.

    Of course, any number of other pre-existing or induced problems can result
    in the oven playing dead after it has been "repaired". :

    Dead controller

    The most common way that the controller circuitry can be harmed is by a power
    surge such as from a lightning strike. Hopefully, only components on the
    primary side of the power transformer will be affected.

  • Check the primary of the power transformer – if it is open, there may be a
     fuse/thermal fuse underits outer insulation. If not, the transformer will
     need to be replaced. There is a good chance that the surge didn't propagate
     beyond the transformer and thus the rest of the controlled should be
     unaffected.
  • In some cases, circuit board traces may have been vaporized (but repair may
     still be possible by simply jumpering across the crater). Some of these
     thin traces may be there specifically to act as fuses – and there may even
     be spares to use for just this situation!
  • Assuming that the main fuse and power transformer primary checks out, then
     check the power supply for the controller next.
  • As always, also check for bad solder connections.
  • If the controller power supply is working and there is still no sign of life
    (dead display and no response to buttons) the microcontroller chip or some
    other part may be bad. It could be a simple part like a capacitor or diode,
    but they would all need to be tested. At this point, a schematic of the
    controller board will be needed – often impossible to get – and replacement
    controller or even just the main chip may be nearly as expensive as a complete
    new oven.

    No response to any buttons on touchpad

    There can be many causes for this behavior (or lack of behavior):

  • Door is not closed – on many ovens, there will be no response to any
     buttons – even setting the clock – unless the door is securely closed.
  • You waited too long – some models (like Sharp) have a timeout. Si vous
     close the door but don't proceed to activate any functions with a couple
     of minutes, they will require you to open and close the door to reset their
     pathetic brains.
  • Controller is confused – a power surge or random non-reproducible action of
     the universe may have resulted in the controller's program ending up in an
     infinite loop. Pull the plug for a minute or two to reset it.
  • Defective interlock switches – this can result in the controller thinking
     the door is open and ignoring you.
  • Faulty controller or its power supply – a power surge may have damaged
     the electronics. Other than checking for bad connections and obviously
     bad power supply components, diagnosing this will be tough without a
     schematic (and possibly much more).
  • Touchpad or controller board contaminated by overenthusiastic cleaning – if
     you recently power washed the oven (or even if you only use some spray
     cleaner), some may have gotten inside and shorted out the touchpad or
     manette.
  • Defective or damage touchpad – physical abuse is not a recommended
     technique for getting a microwave oven to cooperate. If there is any
     visible damage to the touchpad – the outer film is broken – it will probably
     need to be replaced.
  • Also see the section: Some of the keys on the touchpad do
    not function or perform the wrong action.

    Oven runs when door is still open

    WARNING: Needless to say, DO NOT operate the oven with the door open! While
    extremely unlikely, the microwave be generator could be running!

    For microwaves to actually be generated with the door still open would require
    the failure of all 3 interlock switches. The only way this could really
    happen would be for the 'fingers' from the door that engage the interlocks to
    break off inside the oven keeping the interlocks engaged. In this case, the
    controller would think the door was always closed.

    Where no such damage is evident, a failure of this type is extremely unlikely
    since power to the microwave generator passes through 2 of the 3 interlock
    switches. If both of these failed in the closed position, the third switch
    would have blown the fuse the last time the door was opened.

    Another more benign possibility is that one or more fans are running as a
    result of either a defective sensor or normal operation to maintain air flow
    until all parts have cooled off.

    Oven starts on its own as soon as door is closed

    If the oven starts up as soon as the door is closed – regardless of whether a
    cook cycle has been selected, the cause could be a shorted triac or relay or
    a problem with the controller or touchpad.

    First, unplug the oven for a couple of minutes to try to reset the controller.

    If this doesn't help, put a cup of water into the oven and let it run for a
    minute to check for heating. (You could also note the normal sound change or
    slight dimming of lights that accompanies operation of the magnetron.)
    Much more must be enabled to actually power the magnetron so this might point
    more to the controller as being faulty but not always.

    Also see the section: Whacked out controller or incorrect
    operation.

    Oven works but totally dead display

    If all functions work normally including heating but the display is blank
    (assuming you can issue them without being able to see the display),
    the problem is almost certainly in the controller or its power supply.

    Try pulling the plug for a minute or two – for some reason the display portion
    of the controller may have been sent out to lunch by a power surge or alpha
    particle. It woudn't be the first time.

    Check for bad connections between the display panel and the power supply
    and solder joints on the controller board.

    With everything else operational, a bad microcontroller chip is not that
    likely but is still a possibility. If the oven was physically abused,
    the display panel may have fractured though it would take quite a bit
    of violence. In this case, more serious damage to the door seals may
    have resulted as well which would be a definite hazard.

    Whacked out controller or incorrect operation

    The following are some of the possible symptoms:

  • All the display digits may have come on, EEEE or FFFF, or be displaying in
     Greek.
  • The end-of-cooking cycle or keypress tone may be wailing away continuously.
     (By 'tone' I mean from the controller (not a low buzzing or humming when
     attempting to cook which would indicate a microwave generator power problem
     like a shorted magnetron).
  • Pressing a button on the touchpad may result in a totally incorrect action
     such as entering the time resulting in the oven starting to cook. cependant,
     for the special case where pressing START results in erratic behavios, see
     the section: Erratic behavior.
  • The oven may start cooking (or at least appear to) as soon as the door is
     closed. Pressing buttons on the touchpad may or may not have any effect.
     (This could also be a shorted triac or power relay).
  • First, try unplugging the oven for a couple of minutes – perhaps the controller
    is just confused due to a power surge, lightning strike or the EMP from a
    nearby nuclear detonation because it wanted attention.

    If you recently cleaned the oven, some liquid may have accidentally gotten
    inside the touchpad or even the controller circuitry (though this is less
    likely). See the section: Some of the keys on the
    touchpad do not function or perform the wrong action.

    If the oven seems to have a mind of its own – running a cycle you didn't
    think you programmed, are you sure a previous cook cycle was not interrupted
    and forgotten? Try to recreate the problem using a cup of water as a load.

    Assuming this does not apply, it sounds like a controller problem – possibly
    in its power supply. First check the controller PCB for obvious problems
    like burnt components and bad solder connections. Look for bulging or leaking
    electrolytic capacitors. Check for AC across them – there should be little or
    none. (But make sure your multimeter has an internal capacitor to block DC,
    else it will not read AC correctly.) Bad electrolytic capacitors resulting
    in a large amount of ripple on one or more DC power supplies
    are particularly likely if there is a flickering display
    or chattering relay. There have been reports of bad capacitors in late
    model GE ovens but of course GE will want to sell you a $200+ controller
    board, not a 50 cent cap so don't expect this advice should you call them!
     but could also be the controller chip. My guess is that
    unless you were to find some simple bad connections or an obvious problem
    with the controller's power supply, the cost to repair would be very high
    as the custom parts are likely only available from the manufacturer.

    The controller's program may be corrupted (unlikely) but we have no real way
    of diagnosing this except by exclusion of all other possibilities. Depending
    on the model, some or all operations – even setting the clock – may be
    conditional on the door interlocks being closed, so these should be checked.
    Some ovens will not allow any actions to be performed if the door has been
    closed for more than a few minutes – open and close the door to reset.

    A controller failure does little to predict the reliability of the rest
    of the oven. The microwave generator circuits could last a long time
    or fail tomorrow. The output of the magnetron tube may decrease slightly
    with use but there is no particular reason to expect it to fail any time
    bientôt. This and the other parts are easily replaceable.

    However, unless this oven has a lot of fancy features, you can buy a
    replacement (depending on size) for $100-200 so it is probably not worth
    fixing unless it is something relatively simple and inexpensive.

    Erratic behavior

    There are three different situation:

  • Whenever the oven performs unexpectedly both during setup and the cook
     cycle, suspect the controller power supply or bad connections.
  • Where problems only occur when entering or during the cook cycle, suspect a
     power relay or mechanical timer (if used) with dirty or worn contacts, or
     (less likely) the power surge from energizing the microwave generator or
     microwave (RF) leakage into the electronics bay affecting the controller.
  • However, if erratic simply means that it doesn't heat consistently, see
     the section: Oven heats but power seems low or
     erratic.
  • The filter capacitor(s) in the controller's power supply may be dried
    up or faulty. Check with a capacitor meter or substitute known good ones.
    Prod the logic board to see if the problem comes and goes. Reseat the
    flex cable connector to the touchpad.

    For mechanical timers, the timing motor could be defective or require
    lubrication. The contacts could be dirty or worn. There may be bad
    connections or loose lugs.

    The primary relay may have dirty or burnt contacts resulting in erratic
    operation. If the oven uses a HV relay for power control, this may be
    defective.

    If the times and power levels appear on the display reliably but then become
    scrambled when entering the cook cycle or the oven behaves strangely in some
    other way when entering the cook cycle, there are several possibilies:

  • The power surge caused by the cook cycle starting is resulting in changes
     to the settings or else the microcontroller is not interpreting them
     properly. This may be due to a faulty part of bad connections in the
     controller or elsewhere. As with intermittent problems, a thorough search
     for loose ground and other connections and bad solder joints may locate
     the source of the difficulty.
  • Microwave (RF) leakage into the electronics bay due to an faulty joint
     between the magnetron and the waveguide or structure failure of the
     magnetron may be interfering with the operation of the microcontroller.
     Unless the oven was dropped or 'repaired' by an butcher, this sort of
     failure is unlikely. If you suspect either of these, inspect the integrety
     of the magnetron-waveguide joint and make sure the RF gasket is in place.
     Unfortunately, this is sometimes difficult to pinpoint because unless there
     is obvious mechanical damage, the 'problem' may disappear once the cover
     is removed for testing. See the section: Problems with
     internal microwave leakage.
  • On rare occasions, the main fuse may become intermittent rather than
     failing completely. The surge or vibration of starting can jiggle the
     element open or closed. It is easy to try replacing it!
  • Problems with internal microwave leakage

    (From: Charles Godard (cgodard@iamerica.net).)

    I only service Amana's, but have serviced lot's of them over the years. J'ai
    only found a few that leaked with my expensive leak detector. The most
    memorable was the one with the leak that was due to the copper gasket that's
    between the magnetron tube and the cavity. I just reformed the gasket and
    reseated the magnetron and that fixed the leak.

    The symptom was that the Touch Pad timer lights and indicators would change
    while the unit was cooking. I thought I had a timer problem. I took it apart
    and checked for loose solder joints and even cleaned the glass touch pad
    contacts.

    For some reason that I don't remember now, I checked for radiation with the
    cover off the unit and found it extremely high.

    It turned out that the radiation was affecting the controller.

    From the outside, with the cover on, the unit didn't leak.

    Long ago, I tried one of the cheapie detectors because one of my parts supply
    houses suggested it, and it detected leaks on everything. After that I
    shelled out the bucks and bought a real detector.

    (From: Matthew Sekulic (goatboy@telusplanet.net).)

    I have had a similar experience with a Sanyo, similar symptoms, but with the
    leakage from the spot welded waveguide inside the unit. Our calibration meter
    showed a two watt leakage, with none escaping the outer case when attached.

    (My worst case of actual external leakage was from a misaligned door at
    .75 watts with the probe's styrofoam spacer placed against the door, of course
    dropping off to near zero a few inches away. My clue in was a spark between
    the waveguide and the case, when I was messing with the Controller PCB.)

    Some of the keys on the touchpad do not function or perform the wrong action

    Touchpads are normally quite reliable in the grand scheme of things but can
    fail as a result of physical damage (your spouse threw the roast at the oven),
    liquid contamination (from overzealous cleaning, for example), or for no
    reason at all.

    Look carefully for any visible signs of damage or spills. The touchpads
    often use pressure sensitive resistive elements which are supposed to be
    sealed. However, any damage or just old age may permit spilled liquid
    to enter and short the sensors. A week or so of drying may cure these
    problèmes. If there is actual visible damage, it may be necessary to
    replace the touchpad unit, usually only available from the original
    fabricant. Also, check the snap type connector where the touchpad
    flex-cable plugs into the controller board. Reseating this cable may cur
    a some keys dead problem.

    Some people have reported at least temporary improvement by simple peeling
    the touch pad off of the front panel and flexing it back and forth a few
    fois. Presumably, this dislodges some bit of contamination. I am skeptical
    as this could just be a side effect of a bad connection elsewhere.

    With a little bit of effort (or perhaps a lot of effort), the internal
    circuitry of the touchpad can be determined. This may require peeling it
    off of the front panel). Then, use resistors to jumper the proper contacts
    on the flex cable connector to simulate key presses. This should permit
    the functions to be verified before a new touchpad is ordered.

    Caution: unplug the microwave generator from the controller when doing
    this sort of experiment!

    If the problem was the result of a spill into the touchpad, replacement will
    probably be needed.

    However, if you have nothing to lose, and would dump it otherwise, remove the
    touchpad entirely and wash it in clean water in an effort to clear out any
    contamination, then do the same using high purity alcohol to drive out the
    water, and then dry it out thoroughly. This is a long shot but might work.

    Microwave oven does not respond to START button

    While all other functions operate normally including clock, cook time, and
    power setting, pressing START does nothing, including no relay action and
    the timer digits do not count down. It is as though the START button is
    being totally ignored. (However, if there is a momentary response but then
    the oven shuts off, see the section: Erratic behavior.

    If there is an alternate way of activating the cook cycle, try it. Pour
    example, Sharp Carousel IIs have a 'Minute Plus' button which will cook
    for one minute on HIGH. Use this to confirm the basic controller logic and
    interlock circuitry. If it works, then the problem may indeed be a faulty
    START button. If it is also ignored, then there may be a bad interlock
    or some other problem with the controller.

    Check for bad interlocks or interlocks that are not being properly activated.

    Next confirm if possible that the START touch pad button is not itself faulty.
    If you can locate the matrix connections for this button, the resistance should
    go down dramatically (similar to the other buttons). See the section:
    Some of the keys on the touchpad do not function or
    perform the wrong action. The START button does, after all, sees quite
    a lot of action!

    Assuming it is not the touch pad, it sounds like the controller is either not
    sensing the start command or refusing to cooperate for some reason – perhaps
    it thinks an interlock is open. Otherwise, the timer would start counting.
    Testing the relay or triac control signal will likely show that it is not
    Là. Check that there are no missing power supply voltages for the
    controller and bad connection.

    Microwave generator problems

    Failures in the microwave generator can cause various symptoms including:

  • No heat but otherwise normal operations.
  • Fuse blows when closing or opening door.
  • Loud hum and/or burning smell when attempting to cook.
  • Arcing in or above oven chamber.
  • Fuse blows when initiating cook cycle.
  • Fuse blows when microwave shuts off (during or at end of cook cycle).
  • Oven heats on high setting regardless of power setting.
  • Oven immediately starts to cook when door is closed.
  • Oven heats but power seems low or erratic.
  • Oven heats but shuts off randomly.
  • Most of these are easy to diagnose and the required parts are readily
    available at reasonable prices.

    No heat but otherwise normal operation

    If the main power fuse is located in the primary of the high voltage
    transformer rather then at the line input, the clock and touchpad will
    work but the fuse will blow upon initiating a cook cycle. Or, if the
    fuse has already blown there will simply be no heating action once the
    cook cycle is started. There are other variations depending on whether the
    cooling fan, oven light, and so forth are located down stream of the fuse.

    Some models may have a separate high voltage fuse. If this is blown, there
    will be no heating but no other symptoms. However, high voltage fuses are
    somewhat rare on domestic ovens.

    A number of failures can result in the fuse NOT blowing but still no heat:

  • Bad connections – these may be almost anywhere in the microwave generator
     or the primary circuit of the HV transformer. A common location is at the
     crimp connections to the magnetron filament as they are high current and
     can overheat and result in no or intermittent contact. See the section:
     See the section: Testing the magnetron.
  • Open thermal protector – usually located on magnetron case. Test for
     continuity. It should read as a dead short – near zero ohms. Voir le
     section: Testing thermal protectors and thermal
     fuses.
  • Open thermal fuse – some ovens have one of these in the primary circuit.
     It may be in either connection to the HV transformer or elsewhere. Test
     for continuity. It should read as a dead short – near zero ohms.
  • Open HV capacitor – see the section: Testing the
     high voltage capacitor. A shorted HV capacitor would likely immediately
     blow the fuse.
  • Open HV diode – see the section: Testing the high
     voltage diode.
  • Open magnetron filament – This failure may also be due to loose, burnt,
     or deteriorated press (Fast-on) lugs for the filament connections and not
     an actual magnetron problem. See the section: Testing
     the magnetron.
  • Open winding in HV transformer. See the section:
     Testing the high voltage transformer.
  • Defective HV relay. A few models use a relay in the actual high voltage
     circuitry (rather than the primary) to regulate cooking power. This may
     have dirty or burnt contacts, a defective coil, or bad connections
  • Shorted HV diode – see the section: Testing the high
     voltage diode.
  • Short or other fault in the magnetron – see the section:
     Testing the magnetron.
  • Short in certain portions of the HV wiring. See the section:
     Testing and repairing the wiring and connections.
  • A shorted HV diode, magnetron, or certain parts of the HV wiring would
    probably result in a loud hum from the HV transformer but will likely not
    blow the main fuse. (However, the HV fuse – not present on most domestic
    ovens – might blow.)

    Depending on design, a number of other component failures could result in
    no heat as well including a defective relay or triac, interlock switch(s),
    and controller.

    Timer and light work but no heat, cooling fan, or turntable rotation

    This means the controller thinks the oven is working but the microwave
    generator AND motors aren't being powered. Note that these symptoms are
    subtly different than just having no heat and eliminates the actual components
    of the microwave generator from suspicion in most cases.

    (From: Bonita Lee Geniac (bgen@wdl.net).)

    When the timer counts down but nothing else works, 99% of the time the lower
    door switch is bad or else the door is not closing fully and the latch hooks
    are not depressing the upper and lower switches. There is also a slight
    possibility that the relay or triac on the control board is not closing but
    those usually do not result in these particular symptoms. Most of the
    microswitches used in recent production microwaves are very poor quality
    and the silicone lubrication used by some of the manufacturers migrates
    into the switch contact area and makes the switch fail even faster than
    it should.

    Fuse blows when closing or opening door

    This means that the main fuse in the microwave (or less commonly, the fuse or
    circuit breaker for the power outlet) pops when the microwave oven door is
    closed or opened. This may be erratic, occurring only 1 out of 10 times, for
    example.

    The cause is almost certainly related to either the door interlock switches
    or the door itself. Marginal door alignment, broken 'fingers' which operate
    the switches, dislocated parts in the interlock mechanism, or a defective
    interlock switch may result in either consistent or erratic behavior of this
    type.

    On some ovens, this can happen at any time regardless of the control panel
    settings or whether the oven is in the cook cycle or not. On others, it can
    only happen when interrupting the cook cycle by opening the door or when
    initiating the cook cycle from the front panel (if the switches are in the
    wrong state).

    The rational for this basic design – some form of which is used in virtually
    all microwave ovens – is that a defect in the interlock switches or door
    alignment, which might result in dangerous microwave radiation leakage, will
    produce a hard permanent failure. This will prevent the oven from being used
    until it is inspected and repaired.

    Loud hum and/or burning smell when attempting to cook

    A loud abnormal hum is an indication of a short somewhere. The sound may
    originate from the HV transformer vibrating and/or from within the magnetron
    depending on cause. There may be a burnt odor associated with this behavior:

  • Shorted HV diode – see the section: Testing the high
     voltage diode.
  • Shorted magnetron (filament to anode) or other internal fault in the
     magnetron – see the section: Testing the magnetron.
     Arcing within the Magnetron case (visible through ventilation holes in the
     bottom section) is usually an indication of a bad magnetron.
  • Note that a short on the load side of the HV capacitor will likely result
    in the actual wattage drawn from the power line being
    much lower than under normal conditions. Bien que
    there will be a high current flowing in the HV transformer secondary
    through the HV capacitor (which is what causes the hum or buz), the
    real power consumed will be reduced since the
    current and voltage will be out of phase (due to the series capacitor)
    and the power factor will be low.
    A reading on an AC line wattmeter of 300 W compared to the normal 1,200 to
    1,500 W would be reasonable.

  • Other short resulting from frayed insulation or wires touching in the
     microwave generator.
  • Shorted HV transformer – see the section: Testing
     the high voltage transformer.
  • Short resulting from burnt on food (usually) in or around the waveguide.
     If the odor is coming from the oven chamber, see the section:
     Arcing in or above oven chamber.
  • The following procedure will quickly identify the most likely component if
    the problem is not food/spills/carbon related:

    (Usually a loud hum that doesn't result in a blown main fuse is caused by a
    short in the HV diode, magnetron, or wiring on the load side of the HV
    condensateur. The other items listed below would likely blow the main fuse but
    possibly not always.)

    (Portions from: Tony (tonyb@ramhb.co.nz).)

  • Discharge HV capacitor! (If there is a short it is doubtful if it has any
     charge but never hurts to be safe).
  • Remove one end of the lead from the HV capacitor to the transformer.
  • Start the oven.
  • Hum gone? If so, it is the HV circuitry, go to step 4.
  • If it still hums you probably have a faulty HV Transformer. (Not
      uncommon.)
  • Discharge the HV capacitor again, reconnect wire and disconnect the 2 wires
     to the magnetron.
  • Restart oven.
  • Hum Gone? If so, magnetron is shorted. Replace or get a new oven.
  • Hum still there? If so, go to step 6.
  • You have either
  • Shorted HV capacitor,
  • Shorted HV Diode,
  • Shorted clamp diode across the HV Cap terminals (if one is present, about
      30% of microwave ovens use these). (The oven will run 100% without this
      protection for the HV capacitor but it should be replaced if possible.)
  • Some older Panasonic ovens have a HV reed switch which can also short,
       but these ovens are rare now because of their age.
  • Arcing in or above oven chamber

    There is often a simple cause:

  • Arcing in the oven chamber with a normal load (a cup of water, for
     example), often just indicates that a thorough cleaning of the oven chamber
     is needed, particularly around and inside/above the waveguide cover. Tout
     food that gets trapped here will eventually burn and carbonize resulting in
     a focal point for further arcing. Usually, the waveguide cover is designed
     to be removable without taking the (cabinet) cover off of the oven.
     However, burnt food and carbon often make this difficult so that some
     disassembly will be required. See the sections: "SAFETY" and "Getting
     inside a microwave oven". Clean the waveguide cover and clean inside the
     waveguide as well. If the waveguide cover is broken or damaged seriously,
     a sheet of replacement material is available from places like MCM
     Electronics. Trim to fit with a pair of heavy duty scissors, metal snips,
     or a paper cutter. The oven will work fine without it but replacement will
     prevent contamination of the waveguide with food vapors or splatters which
     can lead to more expensive damage. Take extra care to cover all food (which
     you should do anyhow) until the waveguide cover is replaced.
  • Arcing at the roof of the oven chamber on the waveguide cover may be
     due to carbonized food there. Or, if the cover is missing, check for
     pieces inside the waveguide that can be arcing. How this would happen
     is a mystery but apparently it can. 🙂
  • Any sharp metal edges may also result in arcing or sparking. Cependant, le
     only way such damage could occur as part of the oven (not added knives or
     forks!) would be through physical abuse.
  • If your oven uses a stirrer above the oven chamber (no turntable), it may
     be stuck. The result will be an uneven distribution of microwave energy and
     localized heating, arcing, and possibly melting plastic or metal.
  • Flashing and sparking may also result from the stirrer/fan blades
     contacting the metal surrounding it due to the motor/bearings becoming
     loose or dislodged.
  • More on the waveguide cover and cleaning

    That cover is made of an insulator transparent to microwaves, usually mica,
    not a metal. The material can be obtained from places like MCM Electronics
    which you then cut to size with a pair of scissors or a paper cutter.

    First, completely clean below, above, inside, and whatever of the cover
    material is remaining. All traces of carbon and burnt on food must be
    enlevé. In particular, you need to clean inside the waveguide above the
    inside top of the oven as well.

    Then run the oven (with the waveguide cover removed, if necessary) to verify
    that there are no other problems (there probably are none).

    Sometimes, you need to remove the outside metal cover in order to remove the
    waveguide cover. There may be little plastic pins or snaps which tend to get
    gummed up with burnt food and may be difficult to pry off from inside the
    oven. If you do need to remove the metal cover, jot down the locations of
    each of the screws (they are not always all alike) and stay away from
    everything but the waveguide cover itself (especially the high voltage
    components!).

    That waveguide cover is not essential to the operation of the oven but it
    does prevent food from entering the waveguide and getting trapped there.

    Fuse blows when initiating cook cycle

    The fuse may only blow when actually attempting to cook but depending on
    design, triacs and/or door switches may always be live and may result in a
    blown fuse at any time when plugged in or when the door is opened or closed.

    The following can cause the fuse to blow (in approximate order of likelihood):

    Note that a shorted magnetron or shorted HV diode – which you would think
    should blow the fuse – probably will not do so because current will be limited
    by the impedance of the HV capacitor (assuming it is not shorted as well).
    However, there will likely be a loud hum from the HV transformer as it strains
    under the excess load. Such a sound in conjunction with no heat is a likely
    symptom of a shorted magnetron or HV diode. If your oven has a separate
    high voltage fuse – somewhat rare in domestic ovens – it may certainly blow
    due to a fault in any of the HV components.

    Fuses also die of old age. The types of fuses used in microwave ovens are
    subjected to a heavy load and you may find that all that is needed is to
    replace the fuse with one with equivalent ratings. (but check for shorts
    first). There could be an intermittent problem as well which will only show
    up at some random time in the future. A poorly timed power surge (as opposed
    to the well timed variety) could also weaken the fuse element resulting in
    eventual failure.

    The fuses used in microwave ovens are usually ceramic 1-1/4" x 1/4" 15 or
    20 A 250 V fast blow type. Replace with exactly the same type and rating.

    Another possible cause of a blown fuse is a partially bad triac. Some ovens
    use a triac rather than a relay to control the main power to the high voltage
    transformateur. One type of failure of a triac is for it to be totally shorted
    causing the oven to come on whenever the door is closed. Alternatively, the
    gate may be defective preventing the triac from ever turning on. A third, and
    most interesting possibility, is that one half of the triac is bad – shorted
    or open, or doesn't turn on or turn off reliably. Recall that a triac is in
    effect a pair of SCRs in parallel in opposite directions. If one side is
    defective, the main fuse will blow due to transformer core saturation since
    the triac will act as a rectifier and transformers really do not like DC.

    See the chapter: "Testing and Replacement of Components" for more information
    on this and similar problems.

    Fuse blows when microwave shuts off (during or at end of cook cycle)

    This could be due to a number of faults including shorting wires or defective
    relay. However, a common cause that might not be obvious is that the triac
    used to switch power to the high voltage transformer is faulty. Quel est
    probably happening is that only one half of the triac (recall that a triac
    is controlled for both polarities of the line voltage/current) is turning off
    completely resulting in DC to the HV transformer, core saturation, and
    excessive current which blows the fuse. Drive to the triac could also be
    marginal but the bad triac is more likely.

    Exactly how a bad relay could result in these symptoms unless it was actually
    arcing and shorting is unclear. However, there is anecdotal evidence to
    suggest that inspecting the relay contacts and cleaning them if necessary may
    cure it in some cases.

    The following description applies directly to some GE and Hotpoint models.
    Modify it accordingly for your oven. Depending on model, the triac may
    be located on the control board or mounted directly on the chassis.

    (From: John Gallawa (john@microtechfactoryservice.com).)

    I have seen exactly this problem; and I've seen it baffle many a repair shop.
    It is likely that the triac on the 'Power Control Board' is breaking down.
    This is a fairly common problem in GE and Hotpoint models that use this board.

    You can usually confirm the problem by setting the oven to a lower power level,
    say "medium," and heat a cup of water. You will probably hear a 'thump!' chaque
    time the magnetron cycles on. This is an indication of a weakened triac.

    Replace the triac (Q1) with either of the following: ECG 56010, or SK 10265.
    Finally, replace the line fuse, install the outer cover, and test the oven
    for proper operation.

    The only other alternative is to replace the board. The cost used to be pretty
    reasonable, but now it's gotten expensive – probably about $80.00.

    The triac is probably located beneath a red plastic guard on the power control
    board. Its designation is usually Q1.

    (From: John Montalbano (jrmont@iquest.net).)

    The microwave oven in my General Electric JHP65G002AD cooking center
    blew its 15 AMP fuse each time the timing cycle expired. Remplacement du
    triac GE Part number WB27X5085 ($65.00 from GE) with a new NTE56014
    ($13.00) solved the problem.

    (From: Les Bartel lbartel@veribest.com).)

    I had the exact same symptoms on my GE microwave. I replaced the triac with a
    $3 15 amp off-the-shelf triac and it has been working for several years since.

    See the chapter: "Testing and Replacement of Components" for more information
    on triac testing though replacement is probably the only sure test.

    Oven heats on high setting regardless of power setting

    Power levels in a microwave oven are controlled by cycling the microwave
    generator on and off with a variable duty cycle – kind of like slow pulse
    width modulation. For 'HIGH', it runs continuously; for low, it may run
    10% on and 90% off; other settings are in between.

    When the oven always seems to be stuck at high power, it is likely to be
    due to one of two possible causes – a faulty relay or Triac, or controller.
    The relay or triac may have failed in the on state. This will probably
    show up with ohmmeter tests (with the oven unplugged!) but not always.

    Replacements should be readily available. If the problem is is the
    controller, it will be more difficult to diagnose as schematics for the
    controller are usually not readily available. However, it could be something
    simple like a bad connection or dirty connector.

    Oven heats but power seems low or erratic

    Some considerations are how old the oven is and did the problem happen
    suddenly or did it just gradually weaken over the years.

    First, are you sure the problem is real? Perhaps you are just a little
    less patient than you used to be. Perform a water heating test or try
    to pop a bag of popcorn using you usual time setting. See the section:
    Testing the oven – the water heating test.

  • If you are subject to brownouts or are running on your own generator,
     the line voltage may be low. Power output is quite sensitive to the
     AC input – there is no regulation. A 10% drop in line voltage is likely
     to reduce microwave power output by more than 20%.
  • Magnetrons, like other vacuum tubes, can weaken with age and use. An oven
     that sees daily use may indeed weaken over the course of several years. Il
     is unlikely that any other electronic components could change value in such a
     way as to significantly affect power output. However, a failure of the
     controller or sensor (if you have one) could result in short cycling.

    Testing on HIGH will eliminate this possibility. Make sure the magnetron is
     powered continuously and it is not cycling. You can often tell by listening
     for the relay clicks and/or by observing the oven light/other lights dimming
     as the magnetron kicks in. 50% power should result in approximately equal
     on and off times.

  • If you run the oven on HIGH, can you tell if it is actually heating
     continuously or rather it thinks you want LOW? Many microwave ovens make a
     clicking sound as they use a relay to switch microwave power on and off –
     check if you can hear this. Alternatively, lights on the same circuit or
     the oven light may dim slightly when the magnetron kicks in. There should
     not be any cycling on HIGH – the microwave power should stay on continuously
     while it is cooking. If it is cycling, there may be a problem with the
     controller or you may unknowingly be in a low power mode – check it.
  • Mechanical problems are also possible. Where a spinning paddle wheel is
     used to 'stir' the microwave energy (often where there is no turntable), its
     failure to rotate can result in hot and cold spots. Thus, you may see an
     unexplained variation in cooking times. The paddle is often accessible by
     unclipping a plastic cover above the oven cavity. Check for bearing failure,
     binding, broken or lose belt if direct driven, etc. Note that some are
     rotated by air flow from the cooling fan and require that cover to be in
     place to rotate. Therefore, it is not really possible to inspect for correct
     operation with the cover removed. However, you can put a microwave power
     indicator (NE2 neon light bulb with its leads twisted together) in the oven
     (with a cup of water for a load) and observe it through the window. Vous
     should see a periodic variation in intensity as the paddles do their job.
  • There could be intermittent connections to the magnetron filament, thermal
     protector, or elsewhere. But, these would likely show up as erratic
     operation – no heat at all sometimes – not just a weak oven.

    Inspect and clean and tighten (if necessary) all connections in the microwave
     generator including the magnetron filament, HV transformer, HV Diode, HV
     capacitor, and thermal protector. Be sure to unplug the unit first and
     discharge the HV capacitor before touching anything!

  • The thermal protector may be intermittent. Test by clipping a light bulb
     across it or monitoring with a multimeter on AC voltage. See the section:
     Testing thermal protectors and thermal fuses.
  • Oven heats but shuts off randomly

    Everything operates normally, but the oven shuts off after varying amounts of
    temps. This could be a faulty magnetron, bad cooling fan (or just built up dust
    and grime block ventilation grilles), bad thermal protector, faulty controller,
    some other intermittent component, or bad connections.

  • If resetting it allows cooking to resume immediately, if even for a few
     seconds, I would not suspect the magnetron or thermal problem as no cool
     down time is required. It could be bad connections in the controller or
     elasewhere, a marginal door interlock switch, or a controller problem.
     Jiggle the door to see if this will cause it to shut off.
  • If the magnetron was overheating, you would not be able to resume cooking
     until it cooled and the thermal protector reset. If it just stopped working
     (i.e., the filament opened), everything would appear normal but there would
     be no heating. If the magnetron were shorting, there would likely be a loud
     hum associated with the periods where there was no heat.
  • If it is not possible to resume cooking for a few minutes indicating that
     something needs time to cool off, then the magnetron could be faulty but
     check for the obvious cooling problems first: blocked or dirty ventilation
     grill. Determine if the magnetron cooling fan is operating by listening for
     its sound or looking through the ventilation opening in the back of the oven.
     If it is not, there could be a broken or weak belt, gummed up or lack of
     lubrication, other mechanical problems, a bad motor, or bad connections.
  • Extremely high power line voltage may also result in overheating on a
     poorly designed or oven where the components are marginal.
  • Make sure the stirrer fan is turning normally. Should it gets stuck,
     some models may sense this and shut down/restart.
  • Oven makes (possibly erratic) buzzing noise when heating

    Assuming operation is normal otherwise, this is most likely either a fan or
    other motor vibrating on its mounts, fan blades hitting something, or some
    sheet metal or the high voltage power transformer laminations vibrating.
    There may be something stuck under the turntable or above the waveguide cover
    interfering with the stirrer.

    Something may have loosened up with age and use.

    If the noise is caused be simple vibrations, no damage is likely to result.
    However, if the main cooling fan is on its way out and it stops or gets stuck,
    parts will overheat quite quickly at which point the oven will shut down
    (hopefully) and there could be damage to the magnetron or other components.
    Therefore, at least identifying the cause is probably a good idea.

    The solution may be as simple as tightening a screw or weging a shim
    between two pieces of vibrating sheet metal.

    Oven light does not work

    If the oven light no longer works, believe it or not, a burned out
    light bulb is likely.

    You would think that something like replacing a light bulb would be
    trivial and self evident. Unfortunately, not always so with microwave
    ovens. Light bulbs may be typically located in any of 3 places:

  • Oven chamber – it may be behind a mesh grill requiring a screw or snap
     to be removed. This is the easiest.
  • Rear – the bulb may be in a recessed compartment accessible by removing
     a screw or two on the back of the oven.
  • Inside – it may be behind a non-removable grille requiring the removal
     of the cover.
  • These are typically not your usual vanilla flavored appliance bulbs either.

    Bad connections are also possible but not that likely.

    Fans or turntables that do not work

    There are up to 4 motors in a microwave oven:

  • Magnetron cooling fan – always present.
  • Mechanical timer (on inexpensive non-touchpanel or older units).
  • Turntable.
  • Convection air circulation (combo units only).
  • When any of these do not operate properly, the most likely causes are:

  • Gummed up lubrication/dry bearings. Check for free rotation of the
     affected part(s). Clean and lubrication as needed. Also confirm that
     there are no other mechanical problems (e.g., turntable improperly
     installed).
  • Loose or broken belt. Confirm that belt is properly installed. Test to
     determine if it is worn and flabby – stretch it by about 25%. It should
     return to its relaxed length instantly. Clean and/or replace if needed.
  • Bad motor. Disconnect one wire and check for continuity with an ohmmeter.
     If open, winding is bad but check for break at terminal which you can
     resolder.
  • Bad thermostat. Where a fan only runs when the oven is hot as in a
     microwave/convection oven, the thermostat or controller could also be
     at fault. Locate the thermostat and jumper across its terminals with
     power off. Plug the oven in and see if the fan now runs all the time
     or at least when the appropriate mode(s) are entered.
  • Bad connections – trace wiring and check continuity (unplugged, capacitor
     discharge) to motor terminals.
  • Note that the opposite problem – a turntable and/or fan that runs after
    the cook cycle is completed may be normal for your oven. This is a "cool-down"
    function designed to allow the heat to equalize or possibly added by the
    company's legal department to reduce the number of lawsuits due to
    stupidity. 🙂

    What to do if the door handle breaks off

    Usually this happens at the places where the handle is screwed to the door.

    I would NOT recommend making the repair in any manner that compromises
    the shielding properties of the door. (I have visions of someone using
    1/2" stove bolts through the door and handle which would definitely be a
    bad idea). Anything that penetrates the door seal is a potential hazard –
    likely a very small one but it is not worth the risk.

    Therefore, I would recommend staying with repairs that can be made totally
    externally unless there is no possibility of a change to the integrity of
    la porte. For example, replacing the screws with similar sized screws that
    gripped better or using filler to reconstruct or strengthen the threaded
    holes would be acceptable.

    Plastic is generally tough to glue where a strong bond is needed and where
    the joint is subject to abuse. However, depending on the type of plastic,
    one or more of the following may work: semiflexible adhesive like windshield
    sealer, plastic cement (the kind that fuses the plastic, not model cement),
    Duco cement, PVC (pipe) cement, or even superglue (though it seems not all
    brands are equally effective). Make sure the surfaces to be glued are
    perfectly clean (remove any residual library paste if you tried that!) and
    provide a means of clamping the pieces until the bond sets up (adhesive
    tape and/or rubber bands may be all you need). Consider providing some
    reinforcements around the joint (i.e., plastic splints or sisters depending
    on your profession) for added durability.

    Replacement door handles and/or entire doors may be available from the
    manufacturer of the oven. Replacements for a few Panasonic models are
    even stocked by MCM Electronics (and no doubt other places as well).

    (From: John Gallawa (john@microtechfactoryservice.com).)

    Here are the door disassembly instructions from the Amana service manual.
    Many others are similar:

  • Pry out the inner door trim with a small screwdriver on the latch side of
     la porte.
  • Remove two screws securing the latch assembly and door handle to the outer
     panel (this may be all that's needed to replace the handle).
  • Remove six screws and release 4 spring fingers that secure the choke to the
     outer panel.
  • WARNING: A microwave leakage test must be performed any time a door is
    removed, replaced, disassembled, or adjusted for any reason.

    Crack or other damage to door window

    "My microwave oven has a crack in the glass of its door. Is this safe to
     continue using or should I get it fixed? Will there be any radiation leakage?"

    So you were throwing roasts at the oven again, huh? 🙂

    If the metal screen/mesh is behind and separate from the glass, there is no
    danger. In this case, the function of the glass is mostly cosmetic and a
    small crack should not be a problem.

    However, if the screen is inside the glass and now broken as well, there
    could be microwave leakage. Even if it is not actually broken at this
    time, future failure is possible. Therefore, the glass panel or entire
    door should be replaced.

    Also, any break large enough to allow something to touch the metal screen
    is a hazard because during cooking, there could be shock hazard due to
    microwaves inducing current in the screen. And, poking something metallic
    through the screen would make is susceptible to microwave pickup as well.

    However, damage to the inner plastic is probably not a cause for concern
    as that is only there to keep the screen and inside of the door glass clean.

    Repairing damage to the oven interior

    If spilled food – solid or liquid – is not cleaned up soon after the
    oven is used, it will tend to harden and carbonize. Not only will this
    be much more difficult to remove, but hot spots may develop and result in
    possible sparking, arcing, and damage to the interior paint.

    If this happens in the vicinity of the mica waveguide cover, it may be
    damaged as well. In addition, sometimes splatters may find their way
    above the waveguide cover and cause problems above the roof of the oven
    chamber in the waveguide.

    Needless to say, clean up spills and food explosions as soon as possible.
    Not only will it be easier, the chance of future expensive problems will
    be minimized.

    To prevent arcing and sparking, the interior needs to be smooth. Sharp
    edges and hard carbon in particular creates places where electric field
    gradients can become great enough to cause problems. Thus the warning
    not to use any metal utensils in a microwave.

    Once damage occurs – paint blisters and peels, or totally hardened impossible
    to remove carbon deposits – more drastic action is called for:

  • Assuming cleaning does not work on the carbon – even after repeated
     attempts, carefully scrape it off with a blunt knife or other suitable tool.
     This will probably damage the paint. Use fine sandpaper to completely smooth
     out the metal and feather the edges of the paint in the immediate area.
     Until you can obtain paint, the oven will work fine but since the chamber
     is made of sheet steel, rust will set in eventually. So, do paint it.

    Special microwave oven cavity paint is available but any common gloss enamel
     will work just as well (and costs about 1/10th as much). Unplug the oven
     as paint solvent is generally flammable. Use touch-up paint
     with a small brush (recommended) or spray paint (be careful to mask off
     all but the immediate area). Allow at least 24 hours to dry with the
     microwave oven door OPEN so all the solvent has evaporatedf. The typical
     color is beige, almond, or some other form of off-white – just match it
     to your oven (if you care). While I have never heard of problems caused
     by these non-approved paints, it's always a good idea to test first in
     an inconspicuous location to be sure there are no surprises when power is
     applied. Test by putting a cup of water in as a load and running for a
     minute or so on HIGH. The area where the new paint has been applied
     should not be any warmer than other areas. Of course, there should be no
     smoke or six foot flames. 🙂 But the odor from petro-chemical solvent-based
     paints may linger for some time and could be quite objectionable in the
     vicinity of food. Once the paint is dry to the touch, a blow-dryer on low
     heat (NOT a heat gun!) applied to the newly painted areas may be used to
     speed this along. Running the oven on the lowest setting should help as
     well, as the fan will circulate air throughout. Make sure there is a
     water or other load in the oven when doing this! Also, putting a container
     of used coffee grounds in the oven overnight for several nights should help
     clear the odor.

    As noted, damaged paint is often a symptom of other problems, most likely
     due to debris causing hot spots. If around the waveguide cover, there
     may be gummed up food trapped under the cover. If it occurred along the
     turntable track, the turntable wheels themselves may be full of carbonized
     food causing heating and/or arcing as they rotate on the bottom paint.
     Any of this will destroy the new paint if not thoroughly cleaned first.

  • If the waveguide cover is damaged seriously – such that it no longer
     will prevent splatters from entering the waveguide, obtain replacement
     material, cut to fit. Leaving it larger than necessary is fine as well.
     Use a suitable bit in a hand drill to make holes in the mica for the
     mounting screws or plastic snaps.

    Alternatives to mica which can stand the elevated temperatures in a microwave
     oven may also be acceptable. Possible choices include plastic or fiberglass
     laminate but not all materials will allow microwaves to pass without some
     heating – check it out. Heat a cup of water and the candidate material on
     high for a couple of minutes. If the material doesn't heat up, it should be
     fine. Of course, it must also not have any metal coating (don't use a piece
     of one of those 'browning disks' :-). Mica is also non-flammable which is
     may not be the case with other materials.

  • If the interior of the door is damaged seriously such that either it will
     not longer seal around the edge properly or that the mesh screening is
     breeched, a replacement will be required to assure continued safety with
     respect to minimizing microwave emissions.
  • Microwave oven cavity paint, waveguide cover mica sheets, and even some
    replacement doors are available from the parts suppliers listed at the
    end of this document. For most ovens, parts like doors will need to
    be obtained direct from the manufacturer, however.

    Microwave/convection oven problems

    In addition to the microwave components, these ovens also include an air
    circulating fan and an electric heating element as well as a temperature
    sensing themister. Any of these can fail.

  • A convection oven which shuts down after a couple of minutes during the
     preheat cycle with the temperature display (if any) stuck at LOW (even though
     the oven is hot when opened) may have a bad thermistor temperature sensor.
  • The overtemperature protection sensor (rather than the normal temperature
     sensor) is shutting the oven down. The termister will usually be accessible
     after removing the oven cover. It will be located centrally just above the
     oven ceiling duct or elsewhere in the convection air flow. It is a two
     terminal device that may look like a tiny resistor or diode and may be
     mounted on a metal header fastened with a couple of screws. Remove and test
     with an ohmmeter. An infinite reading means it is bad. As a test, jumper
     a 50 K ohm potentiometer in place of the thermistor. During preheat, as
     you lower the resistance of the pot you should see the temperature readout
     climb. The oven will then indicate READY when the simulated temperature
     exceeds the setpoint. Replacement thermistors are available from the oven
     manufacturer – about $20. Cheaper alternatives may be possible but you
     would need to know the exact specifications and it is probably impossible to
     obtain this information.

    Also see the section:

  • Sensor problems, below.
  • If the convection preheat cycle never completes and the oven is cool when
     opened, then either the heating element is bad (test with an ohmmeter) or
     the relay controlling the heating element or the controller itself is bad.
     If the circulating fan runs off of the same relay and it is operating, then
     the problem must be the heating element.
  • The heating element will be either a Calrod type (GE trade name?) which is
     a steel tube enclosing a Nichrome wire coil embedded in ceramic filler or
     a coiled Nichrome element strung between ceramic insulators. L'ancien
     is probably only available from the oven manufacture, though it is worth
     trying an appliance parts distributor or a place like MCM electronics
     premier. It may be possible to find a replacement Nichrome coil and form
     it to fit. Make sure the wire gauge and length are identical.
  • The circulating fan is probably driven by a belt, which may break or
     deteriorate. Inspect the belt. If it is loose, cracked, or does not
     return to its normal length instantly after being stretched by 25% replace
     il. Check the fan motor and fan itself for adequate lubrication. Vérifier
     the fan blades for corrosion and damage.
  • Sensor problems

    Fancier microwave or microwave/convection ovens include various probes that
    can be used to shut off the oven when the food is supposedly done or maintain
    it at a preset temperature.

    A problem with a sensor, controller, or wiring, may result in incorrect
    operation (never getting past 'preheat' or not terminating a cook cycle) or
    in a display of 'EEEE', 'FFFF', ERROR, or something similar:

    (From: Wilton Itamoto (witam40231@aol.com).)

    "The 'FFFF' display is a common problem in older Panasonic convection ovens.
      The problem is the temperature sensor thermostat located on the top rear of
      the oven. This is the convection temp. sensor for the correct oven
      temperature. Replacing this open sensor will correct the problem."

    When problems develop with these automatic features, the sensor and the probe
    cable are the primary suspects. However, it is possible that the electronic
    circuitry could also be affected by a damaged or defective probe unit.

  • Check for bad connections where the probe plugs in as well as broken wires
     inside the cable particularly near the ends where it gets flexed.
  • Temperature probes may use a thermistor similar to one that controls the
     convection portion of a microwave/convection oven. Steam/humidity probes
     may also behave similarly.
  • If you have never tried the probe before, check your users manual. Cela pourrait
     only be active in certain modes, etc.
  • The best test of the probe unit is to substitute a known good one. Of course,
    this is generally not convenient.

  • There should be some resistance when measuring between the signal
     conductors of the probe cable. It may be high (hundreds of K ohms) but
     probably should not be open. A very low value (a few ohms or less) might
     indicate a short in the cable or sensor.
  • See the section: Microwave/convection oven
     problems for a discussion of thermistors. Testing to determine if the
     controller is responding to the input from the sensor can be done in a
     similar manner except that access must be from inside the electronics bay
     while the oven is running (the probe normally plugs in inside the oven
     chamber). Substitute a fixed or variable resistor and see if you can get
     the oven to shut off (or stay on) as a function of resistance. MISE EN GARDE:
     Don't forget to put a cup of water in as a load if you are testing microwave
     operation.
  • If the resistor test determines that the controller is responding, than a
    bad probe unit is likely.

    If the probe checks out or substituting a known good one makes no difference
    in behavior, look for corrosion or other deterioration of the socket in the
    oven chamber as well as bad connections. Faulty circuitry in the controller
    is also possible.

  • Back to Microwave Oven Repair FAQ Table of Contents.

    Testing and Replacement of Components

    Please see Typical Microwave Oven Electronics Bay
    for parts identification.

    Testing the oven – the water heating test

    The precise number of degrees a known quantity of water increases in
    temperature for a known time and power level is a very accurate test of
    the actual useful microwave power. A couple of minutes with a cup of
    water and a thermometer will conclusively determine if your microwave
    oven is weak or you are just less patient (or the manufacturer of your
    frozen dinners has increased their weight – sure, fat chance of that!)

    You can skip the heavy math below and jump right to the final result
    if you like. However, for those who are interested:

  • 1 Calorie (C) will raise the temperature of 1 gram (g) of liquid water
        exactly 1 degree Centigrade (DegC) or 9/5 degree Fahrenheit (DegF).
  • 1 Calorie is equal to 4.184 Joules (J) or 1 J = 0.239 C.
  • 1 Watt (W) of power is 1 J/s or 1 kW is 1000 J/s.
  • 1 cup is 8 fluid ounces (fl.oz.) which is 8 x 29.57 g/fl.oz. = 236.6 g.
     (For Avoirdupois ounces, use 28.35 g.)
  • 1 minute equals 60 s (but you know this!).
  • Therefore, in one minute, a 1 kW microwave oven will raise the temperature
    of 1 cup of water by:

    T(rise) = (60 s * 1000 J/s * 0.239C/J * (g * DegC)/C)/(236.6 g) = 60.6 °C.

    Or, if your prefer Fahrenheit: T(rise) = 109.8 °F.

    To account for estimated losses due to conduction, convection, and imperfect
    power transfer, I suggest using temperature rises of 57 DegC and 135 DegF.

    Therefore, a very simple test is to place a measured cup of water in the
    microwave from the tap and measure its temperature before and after heating
    for exactly 1 minute on HIGH. Scale the expected temperature rise by the
    ratio of the microwave (not AC line) power of your oven compared to a 1 kW
    unit.

    Or, from a Litton microwave handbook:

  • Heat one Liter (L) of water on HIGH for 1 minute.
  • Oven power = temperature rise in DegC multiplied by 70.
  • Use a plastic container rather than a glass one to minimize the needed
    energy loss to raise its temperature by conduction from the hot water.
    There will be some losses due to convection but this should not be that
    significant for these short tests. For the ultimate in accuracy (as these
    things go), put the water in a styrofoam cup, invert another styrofoam cup
    over it, and poke your thermometer through it.

    (Note: if the water is boiling when it comes out – at 100 DegC or 212 DegF,
    then the test is invalid – use colder water or a shorter time.)

    The intermediate power levels can be tested as well. The heating effect of
    a microwave oven is nearly linear. Thus, a cup of water should take nearly
    roughly twice as long to heat a specific number of degrees on 50% power or
    3.3 times as long on 30% power as on full power. However, for low power
    tests, increasing the time to 2 minutes with 2 cups of water will result
    in more accurate measurements due to the long period pulse width power
    control use by microwave ovens which may have a cycle of up to 30 seconds.

    Any significant discrepancy between your measurements and the specified
    microwave power levels – say more than 10 % on HIGH – may indicate a problem.
    (Due to conduction and convection losses as well as the time required to
    heat the filament of the magnetron for each on-cycle, the accuracies of
    the intermediate power level measurements may be slightly lower).

    See the section: Oven heats but power seems low or
    erratic.

    Testing the main fuse

    Where the oven is dead or mostly dead, the main fuse is the place to start:

  • UNPLUG THE OVEN and locate and remove the main fuse. It will usually be a
     1" x 1-1/4" ABC ceramic type directly in-line with the Hot (black wire) of
     the power cord.
  • Test it with an ohmmeter – the reading should be zero ohms.
  • If it is blown, suspect problems with the interlock switches, high voltage
      capacitor, or high voltage wiring.
  • If it is good but the oven makes a loud humming sound when you attempt to
      cook, suspect the magnetron or high voltage diode.
  • Testing and replacing of interlock switches

    With the oven unplugged, put an ohmmeter across the AC input just before the
    interlocks (but beyond the power relay or triac if it precedes these). Ouvrir
    and close the door slowly several times – there should be no significant
    change in resistance and it should be more than a few ohms. If it approaches
    zero while opening or closing the door, the interlock switches and door
    alignment should be checked. (You may need to disconnect one side of the
    transformer primary since its resistance is a fraction of an ohm. Refer to
    the schematic pasted inside the cover.)

    Replace with switches having a precisely identical fit and equal or better
    electrical specifications (terminal configuration, current rating). Quand
    removing the old switch make a note as to where each wire goes. Vérifier
    the embossed marking on the old switch – don't depend on location as your
    replacement might just have a different arrangement. Make sure the new
    switch aligns correctly with the actuating mechanism and then check for
    correct electrical operation with an ohmmeter before applying power.

    Even slamming the door really hard has been known to knock an interlock
    switch out of position, resulting in breaker tripping at the electrical
    service panel whenever the microwave oven door was closed. (Another reason
    to stay calm after accidentally nuking that bagel for 5 minutes on HIGH!)
    So if there was some kind of "event" after which the microwave failed,
    check the interlock mechanism first – a switch may just need to be popped
    back into place.

    Making measurements inside microwave ovens

    WARNING: In general, I DO NOT recommend making any sorts of measurements on
    the high voltage components of a live microwave oven. I only include this
    section for those who really want to know the details.

    You may be temped to break out your Radio Shack DMM and start poking away
    inside a live microwave oven. DON'T! This isn't like a CD player! Most of
    the time, no measurements of any kind on the oven while it is operating will
    be needed to identify and correct the problem. However, where this is not the
    case, here are some guidelines to a long life:

    WARNING: ALWAYS pull the plug and discharge the HV capacitor BEFORE doing
    anything inside! Never be tempted to make any changes of any kind while
    the oven is on – not even if your meter is being consumed by 5 foot flames!
    First, pull the plug and discharge the HV capacitor!

  • High voltage – DON'T even think about this unless you have a proper high
     voltage probe or meter, or a proper microwave oven tester – AND KNOW HOW TO
     USE IT SAFELY. Even professionals have been killed performing measurements
     of this type using proper equipment! Luckily, current measurements can
     provide enough information to help make a diagnosis.

    WARNING: The high voltage components inside a microwave oven are at a
     NEGATIVE potential with respect to the chassis. DO NOT be tempted to
     interchange the probe and ground wire if you are using a high voltage
     probe on a meter with a POSITIVE input (e.g., for testing CRT HV) and no
     polarity switch! The ground cable doesn't have anywhere near the required
     insulation. Get the proper equipment!

    One thing you can do relatively safely is to connect a Variac directly to
     the primary of the HV transformer. With this set at a MAXIMUM of 10
     percent, the voltage on the filament terminals of the magnetron should read
     from -150 to -250 V with respect to the chassis. A scope can also be used
     if it has a proper 10:1 probe as long as you aren't tempted to turn up the
     Variac any higher! The scope waveform should be close to a sinusoid with
     its positive tips at 0 V. Such reduced voltage tests won't identify
     problems that only occur at full voltage, however.

  • Magnetron current – Place a 10 ohm 10 watt resistor in series with the HV
     diode cathode and ground. Measure the voltage drop across this resistor.
     Sensitivity will be 10 V/A. Normal anode current is around 300 to 400 mA
     for a typical oven. This will be -3 to -4 VDC across the 10 ohm resistor
     with respect to chassis ground. SET EVERYTHING UP AND THEN STAND BACK and
     don't forget to DISCHARGE the HV capacitor after making the measurement:

  • If it is around this range, the magnetron is probably fine.
  • If it is very low or 0, magnetron is bad or HV is not working. Note that
      a shorted as well as open magnetron also results in no current. Si la
      magnetron is shorted, it bypasses all current to ground. If the magnetron
      is open, the HV capacitor charges up and then there is no more current
      through the HV diode (but there will be an initial transient).
  • If it is much too high (whether fuse blows or not), capacitor is shorted.
  • (From: Michael Caplan (cy173@freenet.carleton.ca).)

    A properly conducting magnetron will load down the HV power supply. Si la
    magnetron is non-conducting, the voltage remains high.

    The power supply will produce 3,500 to 4,000 volts DC, or more, open circuit
    (as when the oven is first turned on and the magnetron filament/cathode is not
    fully heated). With full conduction by the magnetron, the HV drops to between
    1,800 and 2,100 V. Weak magnetrons conduct somewhat, but the HV remains
    well above the 2,100 V. (The voltages vary with design and model, but the
    magnitude of the change is the key.)

    I check the HV using my 30 kV HV probe with a DMM, measuring between the
    magnetron filament connectors (either one) or at another equivalent point, and
    case ground. (Again, depends on the circuit, but I think this is a common
    configuration.) The HV at the magnetron filament is negative to ground.

    Testing the high voltage components

    WARNING: First, with power disconnected, discharge the high voltage capacitor.
    See the section Safe discharging of the high voltage
    condensateur.

    Assuming the oven passes the above test for interlocks and door alignment, the
    triac (if used) may be defective. There could also be a wire shorting to the
    châssis. However, the most likely problems are in the microwave generator.

    An ohmmeter can be safely used to quickly determine if the capacitor, HV diode,
    or magnetron are a dead short (as well as for an open magnetron filament).

    Use an ohmmeter to test the diode and capacitor. While connected in circuit,
    the resistance in at least one direction should be several M ohms. (Try it in
    both directions, use the higher reading). Test the magnetron from the filament
    to chassis – it should be high in at least one direction. Test the filament
    for continuity – the resistance of a good filament is close to 0 (less than 1
    ohm).

    Where the capacitor and diode are combined into one unit, it should be possible
    to test each component individually. In some cases, it may also be possible
    to replace only the one that is found to be defective or make up a substitute
    HV cap/diode assembly from individual components if the combined unit is
    excessively expensive or no longer available.

    These may be considered to fail/no conclusion tests – they can definitively
    identify parts that are bad but will not guarantee that they are good. Parts
    may test ok with no voltage applied but then fail once operated in-circuit.
    Connections may open up when they heat up. The magnetron may short out when
    full voltage is applied.

    Don't overlook the wiring as no heat or erratic operation can result from
    simple bad connections!

    An alternative way of determining if the problem is in the control circuits
    (triac, relay, wiring) or microwave generator (HV transformer, HV capacitor,
    HV diode, magnetron, wiring, etc.) is to connect the HV transformer primary
    directly to a line cord and plug. Tape the removed wire lugs to prevent
    shorts.

    Plug the transformer cord into a switched outlet strip which includes a fuse
    or circuit breaker.

    Put a cup of water into the oven cavity to act as a load.

  • Power the oven via its line cord. Initiate a cook cycle. It should go
     through the normal cycle (of course no heat) without blowing the fuse or any
     unusual sounds. If there is a problem in this case, something in the
     controller or its wiring is shorted.
  • Now, initiate a 1 minute cook cycle on HIGH and with the oven running,
     switch on the HV transformer.

  • If the transformer or other HV components are faulty, the outlet strip
      fuse will blow or circuit breaker will trip. Or, if a lamp is plugged
      into the outlet strip at the same time, it will likely dim significantly
      due to the heavy load before the fuse or breaker cuts out.
  • If the problem is with the triac or its drive, the oven will now heat
      normally. When the cook cycle is near its end, switch off the outlet
      strip. Check the water's temperature.
  • More complete information on testing and replacing the individual components
    is provided in the next few sections.

    Testing the high voltage diode

    WARNING: First, with power disconnected, discharge the high voltage capacitor.
    See the section Safe discharging of the high voltage
    condensateur

    The HV diode can fail shorted (most likely) or open. It is not likely for
    there to be anything in between as so much heat would result that the diode
    would not remain that way for long.

  • A shorted HV diode will likely result in a loud hum from the HV transformer
     when a cook cycle is initiated. The main fuse will probably not blow.
     However, note that the actual wattage drawn from the power line will
     probably be much lower than under normal conditions. Bien que
     there will be a high current flowing in the HV transformer secondary
     through the HV capacitor (likely causing a loud hum or buzz),
     the real power consumed will be reduced since the
     current and voltage will be out of phase (due to the series capacitor)
     and the power factor will be low.
     A reading on an AC line wattmeter of 300 W compared to the normal 1,200 to
     1,500 W would be reasonable.
  • An open HV diode will result in AC instead of DC across the magnetron with
     a peak negative value (the only one that matters) about 1/2 of what it should
     être. The result will likely be little or no detectable heat but no other
     symptoms.
  • The resistance measured across the leads of the HV diode should be greater
    than 10 M ohm in at least one direction when disconnected from the circuit.
    However, the HV diode is composed of multiple silicon diodes in series to
    get the voltage rating. Its forward voltage drop will therefore be too great
    (6 V or more) for a DMM to produce a definitive answer as to whether it
    actually works as a rectifier.

    The HV diode can be tested with a DC power supply (even a wall adapter of
    at least 12 or 15 V output), series resistor (to limit current), and your
    multimeter. This will determine proper behavior, at least at low voltages.

    The following is the schematic of a simple HV diode tester:

    240 ohms, 1 W
           + o———–//———+————o +
                                      |
                                    __|__ HV Good: 6 to 10 V
         15 VDC __/_ diode Shorted: 0 to 2 V
                                      | Open or reversed: 15 V
                                      |
           – o————————+————o –

    The voltage drop in the forward direction should be at least 6 V with a few
    mA of current but may be somewhat higher (8 V or more) with a few hundred mA.
    If your DMM or VOM has a resistance scale operated off a battery of at least
    6 V, you may get a reading in one direction (but only one) without the need
    for an external power supply.

    Or, assume for now that the diode is good if it is not shorted – which is
    likely.

    Although a shorted HV diode is usually an isolated event, it is possible for
    failures elsewhere to have caused the diode to blow. Possible causes include
    a shorted HV cap, arcing between windings in the HV transformer, and possibly
    even a defective magnetron or damaged waveguide. These may only occur with
    full voltage so unless there is obvious physical damage (e.g., charring
    between the HV transformer windings or hole burned in the waveguide), it may
    be necessary to eliminate the other components one by one.

    Replacing the HV diode

    WARNING: First, with power disconnected, discharge the high voltage capacitor.
    See the section Safe discharging of the high voltage
    condensateur.

    Most HV diodes have press fit (Fast-On) or ring lugs so replacement is very
    straightforward. Discharge the high voltage capacitor. Make sure you get
    the polarity correct if your replacement can be installed either way. Putting
    the diode in backwards will result in positive instead of negative high
    voltage and, needless to say, no heat, but no other symptoms either.

    Note: the lugs on your new HV diode may just be crimped onto the wire leads
    and not welded or soldered. If this is the case, take care not to stress them
    excessively which might result in bad connections now or in the future. Il
    may be a good idea to solder the lugs to the wires as well (though this may be
    overkill).

    Where the diode is part of the capacitor assembly, it may be possible to
    just replace the diode leaving the old one unconnected (at one end) as long
    as the original diode isn't tied to ground inside the case. Cette volonté
    probably be much much cheaper than replacing the entire assembly.

    HV diode ratings

    Most replacement microwave oven diodes are rated 12 to 15 kV PRV at .5 A. A
    PRV of around 8 kV is actually required even for a small oven. Voici pourquoi:
    Until the magnetron heats up and starts conducting in its forward direction,
    what you have is a half wave rectifier/filter formed by the HV transformer
    secondary, the HV diode, and the HV capacitor. The reverse voltage across the
    HV diode will be equal to: 2 * 1.414 * (VRMS of the HV transformer). Cela peut
    easily be 6 or 7 kV or more! Once the magnetron start conducting, the reverse
    voltage goes down somewhat.

    HV diodes rated at .5 A are adequate for most domestic microwave ovens. Pour
    example, the largest of these will have a nameplate rating of around 1,800 W
    power line input and a HV transformer secondary of 2,500 VAC. Alors qu'il y a
    some losses in the HV transformer, and some power is used by the magnetron
    filament, controller, motors, and light, this still leaves, perhaps, 1,600 W
    into the HV generator. However, due to the design of the half wave doubler
    circuit, not all the power flows through the HV diode (as would be the case
    with a regular power supply. Thus, even though calculations using Ohms law
    (I = P/V = 1,600/2,500 or .64 A) would suggest that .5 A is not enough, closer
    to 1/2 of the total current actually flows through the HV diode.

    To be doubly sure that your new HV diode is happy, run the oven on full power
    (high) for 10 minutes with two quarts of water as a load (or a roast). Unplug
    the oven (while your spouse prepares the veggies), quickly DISCHARGE THE HV
    CAPACITOR, and then check the HV diode for overheating. It might be warm but
    should not be too hot to touch. Unless you have the largest oven on earth,
    this test is probably not needed.

    Testing the high voltage capacitor

    WARNING: First, with power disconnected, discharge the high voltage capacitor.
    See the section Safe discharging of the high voltage
    condensateur.

  • A shorted HV capacitor will blow the fuse instantly.
  • An open HV capacitor will result in no heat but no other symptoms.
  • (The following assumes no internal rectifier or other circuitry except of
    a bleeder resistor. Adjust procedures accordingly if your oven is different.)

    The resistance measured across the terminals of the high voltage capacitor
    should be very high – several M ohms for bleeder resistor. If it is less
    than 1 M ohms, the capacitor is definitely shorted. Yes, if you measure
    0.00 ohms across the terminals (and they are not bussed together on the
    case), then the capacitor is positively, without a shadow of a doubt, bad!

    A high resistance does not prove that the capacitor is actually functional,
    just not shorted with no voltage across it. If you have a capacitance meter,
    check it for proper value (should be printed on the case). Even this does
    not prove that it will not short when full voltage is applied. Substitution
    is the only sure test beyond this.

    Replacing the high voltage capacitor

    WARNING: First, with power disconnected, discharge the high voltage capacitor.
    See the section Safe discharging of the high voltage
    condensateur.

    Make a diagram of the precise wiring as multiple connections are often made to
    the capacitor terminals. The capacitor is usually mounted with a clamp which
    is easily loosened. Sometimes, the capacitor is jammed into a location that
    requires moving some other components to extract it.

    Replace in reverse order. Tighten the clamp securely but not so much as to
    distort the case.

    Where the capacitor assembly also includes the HV diode, it is possible to
    just replace the capacitor if space permits leaving the old one unconnected
    (at one end). However, the cost of a generic replacement diode is small
    (around $3) so replacing both at the same time is usually best. cependant,
    you don't need to use the exact combined part – which may be very expensive
    or difficult to obtain. Just make sure the ratings of the capacitor and
    diode are correct (use a generic replacement microwave oven HV diode and a
    microwave HV capacitor with a uF rating within 10% or so of the old one and
    at least equal working voltage).

    What if the HV diode or capacitor are leaky?

    An (electrically) leaky HV diode or cap would likely fail totally in short
    order since it would be dissipating a lot of power. However, until this
    happened, the oven might continue to operate and not blow a fuse. The effect
    on performance in both cases would be to reduce the effective voltage across
    the magnetron and thus the output power.

    I consider these sorts of failures somewhat unlikely as the HV diode and
    capacitor do not generally fail half-way!

    Testing the magnetron

    WARNING: First, with power disconnected, discharge the high voltage capacitor.
    See the section Safe discharging of the high voltage
    condensateur.

  • A magnetron with an open filament will result in no heat but no other
     symptoms. The bad connection may be internal (in which case the magnetron
     will need to be replaced) or external at the filament terminals (which may
     be repairable).
  • A magnetron with with a short between the filament/cathode and anode will
     likely result in a loud hum from the HV transformer and/or magnetron when
     the cook cycle is initiated but the main fuse will probably not blow.
     However, note that the actual wattage drawn from the power line will
     probably be much lower than under normal conditions. Bien que
     there will be a high current flowing in the HV transformer secondary
     through the HV capacitor (likely causing a loud hum or buzz),
     the real power consumed will be reduced since the
     current and voltage will be out of phase (due to the series capacitor)
     and the power factor will be low.
     A reading on an AC line wattmeter of 300 W compared to the normal 1,200 to
     1,500 W would be reasonable.
  • A magnetron with other faults may result in a variety of symptoms including
     erratic or low output power or intermittent operation. See the section:
     Comprehensive list of magnetron failure modes.
  • There is no totally definitive way to determine if a magnetron is good without
    actually powering it under operating conditions but the following tests will
    catch most problems:

  • Magnetron filament. The resistance should be infinite from the filament
     connections to the case and a fraction of an ohm between the filament
     terminals with the wiring disconnected from the magnetron.

    While measuring resistance from filament chassis, gently tap the magnetron
     to determine if there is an intermittent short. However, such problems may
     only show up once the filament heats up and parts expand.

    It may be possible to determine if the magnetron filament is actually
     working by connecting just the filament connections to a low voltage
     high current supply on a Variac (e.g., a microwave oven transformer but just
     the filament connections). Most ceramic insulators are translucent and should
     show a glow with a working filament. The one at the antenna may be visible
     if the magnetron is removed from the oven or with a dental mirror looking
     into the waveguide. WARNING: Make sure you ONLY have the filament connected!

    I tried powering the filaments of a few magnetrons. On those that had
     white or pink ceramic insulators between the antenna cap and body of
     the magnetron, the glow was very bright. Even on one with a dark
     red insulator, the glow could be seen with the lights out.

  • Evidence of arcing (visible blackening around ventilation holes in base or
     burnt odor) usually indicates a bad magnetron.
  • Melting or other damage to the antenna cover ('bull-nose' or 'bullet') may
     be the result of arcing due to problems in the oven cavity or waveguide
     (perhaps operating with nothing in the oven) or a defective magnetron.

    (This part is only visible with the magnetron removed from the oven). Si
     a problem elsewhere has been corrected, the damaged antenna cover can be
     pulled off and replaced from a magnetron that died of other causes – try
     your local appliance repair shop. (The shape doesn't matter as long as
     it fits tightly – there are several diameters, however.) Your magnetron
     may still be good.

    Note: Since the antenna is attached directly to one of the vanes which is
     part of the anode assembly, it will test as a dead short to the case on your
     multimeter using DC and is normal. At 2.45 GHz, this won't be the case! 🙂

  • Most common magnetron failure modes:

  • Filament could be shorted to case – check with ohmmeter. Anything less
     than infinity means the tube is bad though it could be charring due to arcing
     outside the vacuum in the box with the filament connections. Tap the tube
     while measuring to check for intermittents.
  • Filament could be shorted to itself – tough to test since it is such a low
     resistance to start. Compare with good magnetron. (Yeh, right. If you had
     one, this wouldn't be an issue!) Tap the tube while measuring to check for
     intermittents. This fault isn't really likely.
  • Filament could be open – check with ohmmeter. Tap the tube while measuring
     to check for intermittents. However, loose filament connectors (Fast-Ons)
     are more likely than a broken filament. Therefore, check directly at the
     magnetron terminals with both lugs pulled off.
  • Magnetron could be gassy (or up to air) and arcover internally once power
     is applied. The filament could expand, shift position, and short once heated.
     There is no easy way to test for these possibilities other than substituting
     a known good magnetron.
  • Internal or external arcing resulting in physical damage. External arcing
     could be at the antenna or inside the filament box. Internal arcing will
     not leave any visible evidence but the damage will result in the magnetron
     failing totally or running with reduced output.
  • Overheating might result from a broken or cracked magnet (reduced magentic
     field) or other internal problems. While there may be some output power,
     the thermal protector will shut down the oven prematurely.
  • Comprehensive list of magnetron failure modes

    (Portions from: John Gallawa (john@microtechfactoryservice.com).)

    Here is a list of typical magnetron failure modes. The percentage of each type
    of failure varies. Currently, internal shorts and loose filament connectors
    are probably at the top of the list. An internal plate-cathode short may only
    manifest itself under the stress of high voltage during operation.

  • Shorts. (a) Internal plate-cathode/filament short or (b) Internal arcing.

    Symptoms: No heat, loud hum when entering cook cycle, possible blown HV
     fuse (but will not likely blow the main fuse).

    In ovens equipped with fuses that monitor the high voltage system, such
     as some commercial Sharp models and most commercial and domestic Amana
     models, the high voltage fuse would probably blow. But, rarely will a
     shorted magnetron cause the main line fuse to blow. (I suppose the
     transformer absorbs most of the current surge.) In fact, with reference
     to the other symptoms below, there are almost no failures where the
     magnetron causes the line fuse to blow.

  • Loose filament connectors (these may be repairable). There will often
     also be visual symptoms at the magnetron: Signs of overheating, such as
     discoloration; and evidence of carbon tracks or pits on magnetron terminals
     when the connectors are removed. An intermittent filament (internal) is
     also possible (but not repairable).

    Symptoms: No heat or erratic heat.

    The slip-on connectors can loosen, overheat, build up resistance and
     eventually loose contact. If the the magnetron terminal(s) have not been
     burned too severely, the connection(s) can usually be repaired. We prefer
     cleaning up the terminal, then soldering the filament wires directly to
     the terminal.

    Note: when discharging HV capacitor, since there is no load, it may end
     up being charged to a much higher voltage than is normal. Être préparé
     for a larger spark if you use a screwdriver to discharge it!

  • Open filament.

    Symptoms: No heat.

    See note about HV capacitor in (2) above.

  • In the older glass-dome models, the vacuum envelope can rupture.

    Symptoms: No heat, loud buzz due to arcing when entering cook cycle,
     possible blown HV fuse.

    See comments about fuses in (1) above.

  • Filament breakdown. Usually occurs after a few minutes of normal operation,
     possible blown HV fuse.

    Symptoms: No heat, loud hum once it occurs.

    See comments about fuses in (1) above.

  • Low output. Occurs as cathode emission decreases from long use.

    Symptoms: Reduced cooking power.

  • Moding. Occurs when magnetron oscillates in one or more undesirable
     frequencies.

    Symptoms: (a) Reduced or no cooking power, (b) RF interference. cependant,
     some food products (with high water content) may cook normally, whereas
     the result with other foods is very unsatisfactory. RF interference is
     possible but usually only occurs if there is actual structural damage to
     either the magnetron, its RF gasket or waveguide flange, or its RF
     (feed-through) capacitors.

  • Off frequency. Physical characteristics can change and cause magnetron to
     oscillate at frequencies slightly higher or lower than 2.45 GHz.

    Same as (7a) above.

  • RF leakage. Structural failure can cause leakage from magnetron housing.

    Symptoms: Microwave leakage into electronics bay, erratic control panel
     behavior. It can be very frustrating because the symptoms disappear when
     the oven's outer cover is removed. With the cover in place, the escaping
     RF energy is confined, and eventually builds up around the control panel
     circuitry causing unusual symptoms.

  • Insulation breakdown of the internal leads or at magnetron insulators
     or antenna terminal.

    Symptoms: Arcing, burning smell from magnetron, loud hum, no heat.

  • Cracked magnet(s).

    Symptoms: Reduced or no cooking power, magnetron overheating, occasional
     'snapping' sound.

  • Where to obtain replacement magnetrons

    Depending on the age of your oven the magnetron may still be under warranty.
    Check the original paperwork that came with the oven – either the users
    manual or a separate warranty document. Contact the manufacturer if specific
    instructions on how to file claims are not provided. Full coverage on the
    magnetron of several years is common. If you have not sent in the warranty
    registration card (right, who actually does this?!), a copy of the sales
    receipt or other proof of date of purchase may be required.

    Both original and generic replacement magnetrons are available. Going direct
    to the oven manufacturer will guarantee a compatible magnetron but is by far
    the most expensive option. For a typical oven, one without the gold-plated
    trim :-), such a replacement may be more than half the cost of a similar
    new oven. In some cases (like Sears), you may need to convince their service
    department that you are qualified to be poking around inside one of *their*
    appliances before they will consider selling one to you (too many lawyers).

    In some cases, original magnetrons may also be available from parts suppliers
    like MCM Electronics – at somewhat less rediculous prices. They will be
    identified as 'original' or 'genuine' along with the manufacturer and their
    part number.

    Generic replacement magnetrons are available for the majority of microwave
    ovens. These will almost certainly be much less expensive than original
    parts. Essentially, there is only one type 'tube' (at least for any similar
    power range). The differences are mostly mechanical – which side the filament
    connections are on, the location of screw holes and whether they are tapped,
    and so forth. Sometimes, it's possible to make the wrong style fit but
    this should be avoided, especially if it requires forcible changes to the
    magnetron structure. However, quality may
    varier. In some cases, the generic variety may actually be better than the
    original. See the section: Comments on replacement
    magnetron quality for some recommendations.

    However, it turns out that eBay can be an excellent source of genuine
    "new" magnetrons. These may be removed from cosmeticly damaged or
    otherwise un-saleable ovens. It is often possible to find the
    exact original make and model with a simple search. The cost is likely
    to be as low or lower than for a generic replacement from a repair parts
    distributor. Of course, as with anything else on eBay, checkout the
    reputation of the seller via the Feedback rating and associated comments.

    Comments on replacement magnetron quality

    (From John Gallawa (john@microtechfactoryservice.com).)

    In my experience, mags purchased from after-market suppliers may or may
    not be OEM parts (there are not that many manufacturers of magnetrons in
    the world). Here's the interesting thing, though: In many cases, these
    after-market tubes are actually higher in quality than the original
    tube, as in the case of the OEM Sanyo magnetrons, which tend to fail
    prematurely. Of course, the opposite can also be true, depending on the
    after-market supplier. Some manufacturers, such as Toshiba and Hitachi,
    produce both high and low end magnetrons. They sell these under a
    variety of specialty names, as well as under manufacturer brand names. je
    have seen the low-end tubes in many brand-new microwave ovens.

    When buying magnetrons from other than the manufacturer, I have found it
    best to go to a supplier who specializes in microwave oven parts (i.e.
    AMI, Global Micro-parts, QB products). These sales people are usually
    more knowledgeable about the magnetrons they sell, and they can help you
    with proper choice and application.

    Replacing the magnetron

    WARNING: First, with power disconnected, discharge the high voltage capacitor.
    See the section Safe discharging of the high voltage
    condensateur.

    When you receive the replacement, compare it with the original. It is critical
    that the replacement magnetron be mechanically identical: this means that the
    mounting configuration (studs or holes and their location), waveguide seating
    surface, and the orientation of the filament connections and cooling fins are
    the same. The studs may be removable so that the same assembly can be used
    with or without them. The cooling fins are particularly important as there
    must be adequate airflow from the fan for removal of the substantial waste
    heat – up to half of the input power to the magnetron ends up as heat. le
    shape of the antenna terminal – cone, bull nose, or square – doesn't matter.

    Magnetron replacement is generally straightforward but other assemblies like
    the cooling fan may need to be removed to gain access. Make careful notes
    of both the wiring and mechanical relationships. Usually, the magnetron is
    fastened to the waveguide with 4 nuts on studs. When removing it from its
    mounting, do not lose the RF gasket – a metal mesh ring which seals the
    connection against microwave leakage. Reuse it unless your replacement
    magnetron comes with a new one. Transfer any thermal protector to the new
    unit. Replace other components in reverse order and then reattach the
    filament and HV wires.

    Although the magnetron is a vacuum tube, there is probably no glass in yours
    (unless it is quite old) so it isn't really very fragile. However, a sharp
    blow or fall (during shipping as well if not properly packed) could shatter
    the filament. Do keep it (the magnets) away from your diskettes unless you
    want them bulk erased!

    As for the old one, see the section: The magnets in dead
    magnetrons. 🙂

    Testing the high voltage transformer

    WARNING: First, with power disconnected, discharge the high voltage capacitor.
    See the section Safe discharging of the high voltage
    condensateur.

  • A shorted winding or short between a winding and the core/chassis in the HV
     transformer may result in a blown fuse, loud hum, overheating, audible
     arcing, a burnt aroma, or simply no heat.
  • An open winding will likely result in no heat but no other symptoms.
  • The typical schematic is shown below:

    +——————-o White wire
                                ||( Filament winding
                                || +——————-o White wire
                                ||
                                || +——————-o Red Wire
               AC H o———+ ||(
                               )||(
                               )||( HV Winding
                      .1 to .5 )||( 1.5 to 2.5 KV RMS
                          ohms )||( .5 A or MORE
                               )||( 25 to 150 ohms
                               )||(
               AC N o———+ ||(
                                 | +-+ HV return connected to frame
                                 | |
               AC G o————+—+

    Disconnect terminals as required to make the following tests:

  • The resistances of the primary should be .1 to .5 ohms (.2 ohms typical).
  • The resistance of the filament winding will likely be so low as not to be
     detectable with your multimeter. The only measurement easily made would
     be that there is no short to the chassis.
  • Typical resistance readings for the transformer HV secondary are in the 25
     to 150 ohms range (depending on the power rating of the oven) from HV
     connection to chassis. A typical midsize might be 65 ohms. An open would
     be an obvious failure. However, based on the way these are wound, a
     winding-to-winding short would not cause enough of a resistance change to
     be detected with an ohmmeter unless you could compare with an identical
     model transformer from the same lot number.
  • Check the resistance between all windings (and to the core):
  • Filament to primary, high voltage, and core, should be infinite.

    It may be possible to repair a filament winding which is shorted to the
      core (the only likely place) as it is only 2 or 3 turns of heavy wire.
      However, it must be insulated for 5,000 V, may get quite hot with normal
      use, and similar fire resistant materials must be used for the repair as
      were present original. However, if the filament winding is adjacent to
      the HV winding (in the same channel), the arcing may have been taking
      place to the HV winding rather than the core. Therefore, you need to make
      sure that it hasn't been damaged as well.

  • Primary to high voltage and core should be infinite.
  • High voltage to core should be between 25 and 150 ohms as discussed above.
  • If you have a clamp-on ammeter, you can measure the primary current with
     all secondaries disconnected. See the section:
     Testing the HV transformer using an AC current
     meter.
  • Testing the high voltage transformer more fully is difficult without fancy
    équipement. Only major short circuits can be identified in the transformer
    with an ohmmeter since the nominal resistance of the windings is unknown.
    However, open windings (not very likely) can be located and other faults
    can be identified by the process of elimination.

    Note: in the discussion below, it is assumed that the fuse is blowing due to
    a possible short in the HV transformer. Alternatively, there may be a loud
    hum as the HV transformer struggles due to a fault in the HV transformer or
    a shorted HV diode, magnetron, or a short in the HV wiring. Also note that
    depending on the severity of the fault, the fuse may not actually blow (at
    least not immediately) but there will likely be a loud hum when the HV
    transformer is powered.

    Testing the HV transformer using an AC current meter

    Where the HV transformer doesn't blow a fuse but overheats or produces
    insufficient output, this test may be useful. If you have a clamp-on AC
    ammeter, the transformer can be powered up to see if the primary current it
    draws is reasonable with no load.

    WARNING: Up to 3,000 VAC on HV terminal – AND possibly other windings if there
    is a short in the transformer somewhere. Use a 3 prong cord with H and N
    connected to the primary and G firmly screwed to the transformer core/mounting
    structure. Or, just remove the 3 secondary connections and power it through
    the existing wiring using the normal oven controls. The meter's clamp needs
    to go around H or N but not both. Stand well clear when you apply power!

    Use of a Variac is recommended but not essential. However, here are the input
    current readings at various input voltages for the HV transformer from a
    typical mid-size microwave oven:

    Input VAC Input Amps
     ————————
         80 .3
         90 .6
        100 1.1
        110 2.0
        115 3.0
        120 >4.0

    Above about 100 VAC, there was also a noticeable hum (though not nearly as
    great as with a secondary short).

    No, these readings do not indicate a problem. Microwave oven transformers are
    designed with as little copper as possible. And, yes, the non-linear increase
    in current indicates that the core is saturating with no load.

    If your readings are similar to these, the transformer is likely good.
    Shorted turns would result in much higher current at all input voltages.

    Replacing the high voltage transformer

    WARNING: First, with power disconnected, discharge the high voltage capacitor.
    See the section Safe discharging of the high voltage
    condensateur.

    Replacement of a HV transformer is straightforward but other assemblies may be
    using the transformer bolts for their mounting and/or may block your way.

    Label the wires before pulling off the Fast-Ons if there is any doubt as to
    where they go.

    If the replacement transformer is not mechanically identical, you may need
    to use some creativity in anchoring it and any structures that are attached
    to its frame. However, the transformer must be secure – don't just sit it
    en place.

    Try not to drop either the old or new transformer on your foot!

    Testing and repairing the wiring and connections

    WARNING: First, with power disconnected, discharge the high voltage capacitor.
    See the section Safe discharging of the high voltage
    condensateur.

    Inspect the wiring – especially between the magnetron, HV transformer, and
    other components of the high voltage circuits for signs of arcing and excessive
    heating or burning. Arcing may be the result of the wire scraping against a
    sharp sheet metal edge due to poor placement and or vibration. A bit of
    electrical tape may be all that is needed.

    Since the magnetron filament in particular uses high current, any resistance
    at the press (Fast-On) connections will result in heating, weakening of the
    lug, more heating, and eventual failure or erratic operation. Try to pull off
    each of the lugs. They should not be loose – you should have to work at
    removing them. However, note that some lugs are of the locking variety and
    require that you push a little tab to release them.

    Check for loose, burnt, or deteriorated lugs in the filament circuit (not just
    the magnetron). If you find evidence of this:

    Also check for bad solder connections between the terminals on the high voltage
    transformer and the enameled wire used for its windings. If you find anything
    suspect, scrape away the enamel and surface corrosion and resolder with a high
    wattage soldering iron or soldering gun.

    Testing thermal protectors and thermal fuses

    There may be two types of devices present in your oven:

  • Thermal protectors are thermostats that open a set of high current contacts
     at a preset temperature. They should reset when they cool off. cependant,
     like a relay or switch, the contacts sometimes deteriorate.
  • Thermal fuses will open at a preset temperature but do not reset. Ils
     blow and need to be replaced.
  • At room temperature, both types should read as a dead short with an ohmmeter
    (disconnect one terminal as there may be low resistance components or wiring
    which may confuse your readings). If the resistance is more than a small
    fraction of an ohm, the device is bad. Replacements are somewhat readily
    disponible. You must match both the temperature and current ratings.

    If you suspect a bad thermal protector in the HV transformer primary, clip a
    100 W light bulb or AC voltmeter across it and operate the oven. Si la
    thermal protector is functioning properly, there should never be any voltage
    across it unless there is actual overheating. If the bulb lights up or
    the meter indicates approximately line voltage – and there is no sign of
    overheating – the thermal protector is defective and will need to be replaced.

    An overheating condition would generally be obvious as the mounting surface
    on which the thermal protector is located would be scorching hot when it
    tripped – too hot to touch (but discharge the HV capacitor first – a burn from
    the heat will be nothing compared to the potential shock!).

    Replacement of a thermal protector is very straightforward as it is almost
    always screwed in place with push-on lug terminals. The new thermal fuse will
    probably come with lugs attached.

    Testing and replacing the triac

    A triac may fail in a variety of ways:

  • A shorted triac would result in the oven coming on as soon as the door is
     closed or the power being stuck on high no matter what the touchpad setting.
  • An open triac or one that didn't respond to the gate would result in no
     heat and possibly other things like the fan and turntable not working as well.
  • A triac that didn't turn off would result in the parts of the oven
     continuing to run even after the timer counted to zero.
  • A triac where one half was shorted would result in a blown fuse due to it
     acting as a rectifier pumping DC through the HV transformer.
  • A triac where one half doesn't properly turn off would result in the main
     fuse blowing when the cook cycle completed.
  • Nearly all triac failures will be shorts. Thus, measuring across the
    MT1 and MT2 terminals of the triac (the power connections) should read
    as a high resistance with a multimeter. A few ohms means a bad triac.

    As noted above, triacs can fail in other – possibly peculiar ways – so
    substitution or bypassing may be necessary to rule out all possibilities.

    Replacement is very straightforward – just don't get the wires mixed up.

    Testing and replacing the power relay

    A defective relay can result in a variety of symptoms:

  • A relay with its contacts welded (stuck) closed would result in the oven
     coming on as soon as the door is closed or the power being stuck on high
     no matter what the touchpad setting.
  • A relay that doesn't close (due to defective contacts or a bad coil) would
     result in no heat and possibly other things like the fan and turntable not
     working as well.
  • If the relay is totally inoperative, test for voltage to the coil. Si la
    voltage is correct, the relay may have an open coil. If the voltage is low
    or zero, the coil may be shorted or the driving circuit may be defective.
    If the relay makes a normal switching sound but does not correctly control
    its output connections, the contacts may be corroded, dirty, worn, welded
    closed, binding, or there may be other mechanical problems.

    Remove the relay from the circuit (if possible) and measure the coil
    resistance. Compare your reading with the marked or specified value
    and/or compare with a known working relay of the same type. An open
    coil is obviously defective but sometimes the break is right at the
    terminal connections and can be repaired easily. If you can gain access
    by removing the cover, a visual examination will confirm this. Si la
    resistance is too low, some of the windings are probably shorted. Ce
    will result in overheating as well as no or erratic operation. Replacement
    will be required.

    The resistance of closed contacts on a relay that is in good condition
    should be very low – probably below the measurable limits on a typical
    multimeter – a few milliohms. If you measure significant or erratic
    resistance for the closed contacts as the relay is switched or if very
    gentle tapping results in erratic resistance changes, the contacts are
    probably dirty, corroded, or worn. If you can get at the contacts, the
    use of contact cleaner first and a piece of paper pulled back and forth
    through the closed contacts may help. Superfine sandpaper may be used as
    a last resort but this is only a short term fix. The relay will most likely
    need to be replaced if as in this case the contacts are switching any
    substantial power.

  • Back to Microwave Oven Repair FAQ Table of Contents.

    Items of Interest

    Not your typical home microwave oven

    (From: Daniel Armstrong.)

    I own an Amana Commercial Radarange RC22LW. Les specs
    are: 4 kW input power, 2.2 kW output power, 3 magnetrons each on its own HV
    transformer, etc., and a roughly $3,000 price tag.

    The oven cavity is 15" deep x 13" wide x 9" tall. Most of the comments I
    hear about it are from people who are scared of the fact that the light,
    blower, etc. all turn on as soon as the door is opened/closed or the stop
    button is pressed and shuts off about 30 seconds after the door is left open
    or shut without pressing any buttons. They are used to consumer level
    models where the fan and light are only on while cooking.

    It operates on a standard 230 VAC 20 A circuit and everything inside
    including the cavity light bulb are 230 V as there is no neutral conductor
    in the cord. It has 2 magnetrons firing down from the top of the cavity
    and 1 firing up from under the ceramic floor. It is wired so that the top
    2 fire on the positive alternation of the AC cycle and the bottom 1 on
    negative. It has auto-sensing for incoming line voltage and frequency
    including 208 and 230 VAC terminals on the HV transformers and a small
    buck/boost transformer to boost the voltage to the antenna motors, cooling
    blower, and cavity light to 230 when connected to a 208 supply. The timer
    compensates for the filament preheat time using a current transformer on
    one of the main supply wires to sense when the magnetrons are actually
    producing output power (i.e., it waits ~2 seconds before starting to count
    down and 50% power is ~7 seconds on 5 seconds off). It has 2 thermal
    cutouts on each magnetron, 1 high voltage in the primary for that
    transformer, and 1 low voltage that causes the cook cycle to stop and the
    display to read "HOT" when unplugged/tripped. The 3 low voltage cutouts
    are simply wired in series. There is also a thermal fuse in the air
    exhaust duct and a 30 amp line fuse.

    How I acquired it is that my mom worked at a Hardees (Carl's Jr. in the
    western USA) restaurant and they gave me 2 of them because they had died. je
    discovered that one had the magnetron antennas burned from underloading and
    the other had the door interlock switch contacts welded from opening the
    door while operating, so I combined them into a working unit.

    I used the information here for testing output power via the temperature
    rise of water and came up with a value of 1.9 kW so I will be investigating
    that next but otherwise it works great and I love stainless steel appliances.
    I still love to impress people by popping a bag of popcorn in 72 to 75
    seconds.

    You can find complete diagrams and parts lists at:
    Maytag Services
    On-Line Parts Store. Just enter the the RC22LW model
    number and then click the radio button for the P1198611M manufacturing
    nombre.

    Microwave leakage meters

    A routine test for radiation leakage should be done before returning an oven
    you have worked on especially if the door or magnetron/waveguide were disturbed
    during the repair process. Use it around the door seem and ventilation holes
    in the cabinet. An inexpensive meter is better than nothing but will not be
    as sensitive and will not allow you to quantify the amount of any leakage.

    If you work on microwave ovens, such a meter is a *must* for personal safety
    reasons as well as minimizing the risk of liability after returning them to
    your customers.

    These should be available wherever you buy quality test instruments. Ils
    are usually made by the same companies that manufacture other service
    équipement. Prices and capabilities vary widely. MCM Electronics sells an
    inexpensive unit suitable for quick checks on a go/no-go basis for $6.99
    and an FDA approved unit (including calibration), for $388.

    Note: you should also perform an electrical leakage test to assure that all
    case parts are securely connected to the Ground of the AC plug.

    Comments on microwave leakage meters

    (From Barry Collins (bcollins@mindspring.com).)

    I found an old manual for a Narda 8100B Electromagnetic Leakage Monitor. (I
    used to work for a manufacturer of Microwave ovens.) While I don't personally
    recall ever having damaged a probe while checking for leakage, I do know that
    it is possible to do so and did happen on rare occasions.

    The Narda manual states that their probes use an antenna/thermocouples design.
    Holaday (sp?) makes another line of detectors and those may use a thermistor
    array.

    I have confirmed that by removing the styrofoam cone from the end of a Holaday
    uW leakage detector's probe and then bringing its tip near a heat source (40W
    bulb) caused the meter to have a significant deflection. Thus, the cones are
    not only used as spacers. They prevent radiant heat sources from affecting
    the meter reading, as well.

    The Holaday probes that I used had 8 diodes in the tip that formed an array.

    Newer designs (Holaday) claim to be more or less immune to damage resulting
    from placing them into high energy fields. I do know that the older Narda
    equipment was prone to such damage.

    There is a section in the Narda manual that details how to select the proper
    probe to measure "unknown" leakage levels. In a nutshell, one should start
    with the highest power rated probe and work toward the lowest power rated
    probe (three listed in all). The goal is to have a meter deflection of more
    than 10% of it's scale while not going off scale for sake of accuracy. While
    it didn't specifically mention damage to the probes, there were overtones
    throughout the text that implied such (watch needle, listen for alarm, stop
    and replace probe, etc…).

    The three probes were listed as (high/low range for each):

    Probe Range
       —————————————–
         8120A 0.2 mW to 2.0 mW/square cm
         8121A 2.0 mW to 20.0 mW/square cm
         8122A 20.0 mW to 200.0 mW/square cm

    This is from memory, but I believe that the maximum leakages we were allowed
    by the governmental agency were:

  • Less than 2.0 mw/square cm off of our assembly line
  • Less than 3.0 mw/square cm leaving the warehouse
  • Less than 5.0 mw/square cm in consumers home
  • As you no doubt know, with a hole cut in the oven (in reference to those who
    want to modify one – see the section: Microwave ovens for
    non-standard applications — sam), the density can easily reach several
    times these numbers, especially on the newer 1,000 watt plus models. Damage
    would occur where one intentionally held the lower power rated probe in the
    strong field until the thermocouple (or thermistor?) overheated.

    Simple microwave leak detectors

    Since these do not really provide an absolute measurement, their utility is
    somewhat limited. All microwave ovens leak to some extent. Determining by
    how much is why you pay the big bucks for a real leakage meter!

    WARNING: These are no substitute for a properly calibrated commercial unit!

    (From: Leon Heller (leon@lfheller.demon.co.uk).)

    A very simple design I saw somewhere (Electronics World, probably) consisted
    of a half-wave dipole with a Shottky diode detector between the two elements.
    I think one measured the voltage across the diode via a resistor and capacitor
    smoothing arrangement using a 50 uA meter. You can buy these detectors quite
    cheaply.

    (From: Ren Tescher (ren@rap.ucar.edu).)

    I saw an article about it in Modern Electronics in the early eighties. Il est
    simply a Schottky Barrier Diode (SBD) and an LED wired together. The leads of
    the SBD are left intact and straight and act as a 1/4 wavelength dipole.

    Here's the circuit:

    SBD
               
                                 | |
                                 +-|>|-+
                                   LED

    The LED is soldered close to SBD using as short of leads as possible (being
    careful not to ruin either part with too much heat). (Note that the diodes
    are connected anode to cathode, not cathode to cathode.)

    I then taped/glued it 1 1/2 and perpendicular from the end of a popsicle stick
    (this gives it a 'standoff' distance).

    Put a large container of water (>=2 cups) in the microwave and run it on HIGH
    for 2 minutes. While it is running, slowly sweep the tester around the door
    seal, hinges and door latch. You may have to dim the lights to see if the LED
    lights up.

    Any leaking uwaves will be picked up by the dipole 'antenna', the SBD will
    rectify the waves, and when sufficient rectified voltage has built up, the LED
    will light up.

    I built 10 of these at home and then compared them to the commercial tester we
    had at work. The commercial tester had three ranges and the most sensitive
    range was divided into 3 color bands, red, yellow, green. The home-built
    testers all 'fired' at some point in the 'yellow' range. I attribute the
    variances within the yellow (caution) range to individual characteristics of
    the diodes – they all came from the bargain bin at Radio shacks….

    A solid glow would indicate excessive leakage, especially if the tester still
    glows if it is pulled beyond the 1-1/2 inch standoff distance to 3 inches.
    Typically the LED just flickers, around the hinge/latch areas. (US law allows
    increased leakage as the oven ages).

    You may notice that no radiation leaks through the viewing window, contrary to
    the old wives tale of not looking through the window while it's cooking. (Le
    screen really is a very good microwave shield — sam).

    Small leaks may be remedied by adjusting or cleaning the door and hinges
    and/or by distance (square law= doubling the distance quarters the power).
    Large leaks – trash the oven.

    (From: James P. Meyer (jimbob@acpub.duke.edu).)

    Get a small neon bulb. The NE-2 size is a good one. Use some resistors to
    make a voltage divider for 115 VAC to feed the bulb. Adjust the voltage
    across the bulb so that it's just barely glowing. Make the divider network
    resistance large enough to limit the current through the bulb to just a couple
    of mA. Put the bulb on the end of a line cord and plug. INSULATE everything
    complètement.

    Adding this onto a neon circuit tester is one option and will provide an
    insulated housing as well.

    Plug the whole thing into an AC outlet. Wave the bulb around the door gaskets
    and if it gets brighter when the oven is turned on, then you have located a
    leak. The bulb detector can be very sensitive. You may even be able to use
    it to find wires behind drywall in your house.

    How safe is a repaired microwave oven?

    So you fixed up Aunt Minnie's Radarange or picked up a microwave at a yard
    sale or scavenged one off the curb. The only problem you could find was a
    blown fuse, truly horrible mess of decayed burnt-on food, or a thriving
    community of cockroaches inside. How safe is it to use (assuming you evicted
    the cockroaches)?

    As long as there is no serious damage to the door (a 6 inch hole would quality
    as serious damage) and the door fits square, it should be properly sealed. Comme
    long as the waveguide is tightly mounted and undamaged, there should be no
    leakage from there. Make sure the metal cover has all its fingers engaged
    around the front (though with a properly installed magnetron, there should be
    minimal microwave leakage into the electronics bay).

    An inexpensive leakage tester – around $8 – will not be as sensitive or
    accurate as the $500 variety by may provide some peace of mind. Cependant, comme
    noted below, they may indicate dangerous leakage even when your oven is within
    acceptable limits.

    The most important considerations are the door and door seal.

    (From Barry Collins (bcollins@mindspring.com).)

    Those inexpensive hand held meters (from Radio Shack, etc..) can give very
    inaccurate readings. While they definitely serve a purpose, they have caused a
    more than a few people to unnecessarily fear microwave ovens over the years.
    Also, I just changed jobs from working for a company that made gas ranges. CO
    detectors caused similar panic among users of the appliances. I'd highly
    recommend anyone with gas heat or appliances to purchase a quality CO
    detector, but not one of those inexpensive type that go off whenever there is
    a thermal inversion of smog a city.

    Efficiency of microwave ovens

    The efficiency of an electric heating element is 100% – period. However, using
    an electric stove to heat 1 cup of tea may result in much wasted energy as the
    element and pot must be heated as well and there are losses due to convection
    and conduction to the surrounding environment. Furthermore, you won't heat
    just *1 cup* of tea but more likely 2 or 3 just to be sure you have enough!

    A microwave oven is not likely to be more than 60% efficient – possibly as low
    as 50 percent or even less. While the magnetron tube itself may have an
    efficiency rating of 75%, there are losses in the high voltage transformer,
    cooling fans, and turntable motor (if used). The light bulb and controller
    also use small amounts of power. These all add up to a significant overhead.
    In addition, the waveform applied to the magnetron by the half wave doubler
    circuit is not ideal for maximum efficiency.

    However, you are not heating the surrounding countryside as the microwaves only
    affects what you are cooking and not the container or oven cavity itself and
    you are more likely to only load the amount of food you expect to be eating.
    For a single cup of tea, the microwave oven may use 1/10th the energy of a
    typical electric cooktop element to bring it to a boil!

    Therefore, it makes sense to use a microwave oven for small short tasks where
    the losses of an electric or gas oven or cooktop would dominate. cependant,
    gastronomic preferences aside, a conventional oven is better suited for that
    20 pound turkey – even if you could distort its anatomy enough to fit the
    typical mid-size microwave!

    Microwave oven design and cost reduction

    (From Barry Collins (bcollins@mindspring.com).)

    Microwave oven design is a black art. What one hopes for is to deliver all
    the power from the magnetron into the food and not have a high SWR reflect
    back into the magnetron and burn it out. Size, shape, placement of food items
    affect the SWR. The microwaves are designed for the most part to work
    optimally with an average load. Models equipped with turn-table models
    compensate for this by breaking up the SWR as the food revolves. My oven has
    a stirrer fan design and has been working for going on 18 years now without
    the first hint of a problem (maybe a little less power). I personally know
    that it had one of the lowest SWRs available at the time. Not to mention it
    has an older design, non-cost reduced, cooler running, more efficient
    magnetron (that cost $13.00 instead of $9.45). The thing that I found
    disturbing about microwave oven design was the trends to go with hotter an
    hotter insulation classes on the components used in them. L'original
    transformers were class H while the newer ones are now class N. This was all
    done in the name of cost reduction to remain competitive. The windings AWG
    got smaller and the temperature rise went up accordingly. The magnetrons were
    cost reduced in a similar fashion. Size was reduced and the number of fins
    were reduced. Their temperature went up while their efficiency went down.
    But then the cost went from $300 to $149 while life went from 10 years-plus to
    5 years or less and they became disposable items. That's one area, I'd
    almost hesitate to hope the Government would have mandated an efficiency.

    Problems with running a microwave oven with metal inside or totally empty

    Metal in microwave ovens may or may not be a problem depending on the specific
    situation. Sharp edges and points create strong field gradients which tend to
    spark, arc, or create other fireworks. With some food in the oven to absorb
    the power, this is probably not likely to damage the oven. You will note that
    some ovens come with metal fixtures in addition to the oven walls themselves
    (e.g., Sharp convection/microwave combo).

    Having absolutely nothing in the oven chamber or just metal is the potentially
    more likely damaging situation for the magnetron as you are dumping several
    hundred W to over a kW of power into a reflective cavity with no load. dans le
    worst case, you could end up with a meltdown inside the waveguide requiring
    replacement of various expensive components including the magnetron.

    Older microwave ovens with used glass magnetrons were perhaps more susceptible
    to these disasters (all modern overs use magnetrons with ceramic construction
    but I really don't know how much this matters) but it's still a good idea to
    avoid running a microwave empty. They don't need preheating! 🙂

    More on metal in the microwave

    (From: Don Klipstein (don@Misty.com).)

    Mainly, you need exposed water or food to absorb the microwaves. Otherwise,
    they just reflect around the oven and get back to the magnetron tube. Ce
    may be bad for the tube, and in an unpredictable manner.

    It is even not too good to run a microwave empty. The walls of the main
    cooking chamber are metal.

    In the event the microwave runs empty OK, adding metal objects change the
    microwave reflection pattern and might possibly unfavorably change things.

    If you have exposed food or water, the tube should not mind some stray metal
    too much. If the added metal does not interfere with microwaves mainly
    getting from the tube to the target food or water and being absorbed, the
    magnetron should be OK.

    Even if the tube does not mind, there is another concern. Metal objects close
    to other metal objects or to the walls of the cooking chamber may arc to these.
    Any arcing is generally not a good thing. If you add metal objects in a manner
    safe for the tube, try to keep these at lease a half inch (a bit over a cm.)
    from the walls to avoid arcing. Safe distances are uncertain and are usually
    less if the metal objects are small and a large amount of food or water is
    exposed.

    If any metal object has major contact with a microwave absorbing food target
    and such target is still heavily exposed, you should be OK. Examples would
    be wrapping foil around the wingtips of a whole chicken or whole turkey, or
    a bottle of liquid (on its side) with a metal lid with liquid contacting much
    of the lid. This is usually OK. Just avoid unrelated problems due to major
    temperature change of anything in contact with a non-heat-rated glass
    récipient.

    A plain glass bottle if ice-cold stuff might possibly break from thermal shock
    when heated, but any metal lid on a bottle largely full of microwave-absorbing
    stuff should not present a problem especially if the bottle is on its side so
    that stuff is contacting or very nearly contacting much of the lid.

    Burnt smell from oven – after incident

    "My daughter tried to heat up one of those 'soup in a box' containers and it
     burned – actually charred. I wasn't home at the time, so I don't know if it
     was neglect or inappropriate use, but the lasting effect is that there is a
     strong odor, similar to that which you smell after a fire that I cannot seem
     to get rid of. What do you recommend. I have a Sharp Convection/Microwave,
     that even after the incident described still performs well."

    Start by cleaning the interior of the oven thoroughly with mild detergent and
    water. You may have to do this several times to get all of the sticky film
    left behind. If this doesn't help enough, smoke may have gotten into the
    waveguide above the oven chamber. If possible, remove the waveguide cover and
    clean it and as best as possible the accessible part of the waveguide.

    However, the odor may persist since the smoke can penetrate to places you
    cannot access for cleaning. With a combination convection and microwave oven
    especially, there are many passages where the air would normally circulate in
    convection mode which will be coated even if the oven was used in microwave
    mode. However, I would expect that the smell will decrease and eventually go
    away. Most likely, nothing in the oven has actually sustained any damage.

    Some have suggested boiling a cup of lemon scented water or vinegar to help
    speed things along. It won't hurt – maybe even help. 🙂 Also, putting a
    container of used coffee grounds in the oven overnight for several nights
    should help clear the odor.

    Microwave ovens and grounded dedicated circuits

    A microwave oven should be used only on a properly wired 3 wire grounded
    circuit. Check with a circuit tester to make sure your 3 prong outlet is
    correctly wired. Many are not. Install one if it is not grounded. Là
    is a very important safety reason for this requirement: the return for the
    high voltage is through the chassis. While unlikely, it is theoretically
    possible for the entire high voltage to appear on the metal case should
    certain internal connections come loose. With a properly grounded outlet,
    this will at most blow a fuse. However, with the case floating, a shocking
    (or worse) situation could develop – especially considering that microwave
    ovens are usually situated near grounded appliances like ranges and normal
    ovens and wet areas like kitchen sinks.

    A dedicated circuit is desirable since microwave ovens are significant users
    of power. Only about 50 to 60% of the electricity used by a microwave oven
    actually gets turned into microwaves. The rest is wasted as heat. Donc,
    a 700 W oven will actually use up to 1400 W of power – nearly an entire 15 Amp
    circuit. Convection ovens have heating elements which are similar energy hogs.
    At least, do not put your refrigerator on the same circuit!

    Microwave ovens and GFCIs

    A Ground Fault Circuit Interrupter (GFCI) protects people from shocks should a
    situation develop where an accessible part of an appliance should short to a
    live wire. Touching this may result in a shock or worse. A GFCI detects any
    difference between the currents in the Hot and Neutral wires and shuts off the
    power should this difference exceed a few mA.

    A GFCI is not needed with a properly grounded microwave oven as any such fault
    will blow a fuse or trip a circuit breaker. In most cases, it will not hurt
    to have a GFCI as well. However, with some combinations of oven design and
    your particular wiring, due to the highly inductive nature of the high voltage
    transformer, nuisance tripping of the GFCI may occur when you attempt to cook
    anything – or at random times. However, this usually does not indicate any
    problème. Plug the oven into a properly grounded circuit not on a GFCI.

    Can a microwave oven be built into (or hung under) a cabinet?

    Assuming it is a regular microwave and not a convection/microwave combo, the
    major issues are:

    There are special (likely highly overpriced) models available for this type of
    mounting.

    To use a normal microwave, my recommendation would be to build a shelf rather
    than a totally sealed, enclosed, conformal cabinet. It can have sides and a
    top as long as you leave a couple of inches all around. This will result in
    a microwave oven that is much more easily serviced should the need arise and
    replaced in the future with a model that is not quite identical.

    Just make sure it is securely supported – the microwave weighs quite a bit and
    must endure a fair amount of abuse from heavy casseroles and the inevitable
    door yanking/slamming!

    Note that one of the advantages of buying a microwave oven designed for
    under cabinet or wall mounting is that it may provide convenient access for
    servicing from the front – not having to remove the entire unit to check
    or change a fuse! For example, some GE units have a hinged front panel –
    remove a couple of screws and most of the internal components can be accessed
    for service. This would not be possible where a countertop oven is used in
    a permanent installation.

    (From: Roy Smith (roy@popmail.med.nyu.edu).)

    I've installed a GE over-the-range microwave. It really was quite
    straight-forward. There is a backplate which you attach to the wall with
    whatever combination of lag bolts, screws, expansion bolts, etc you can
    get to work (i.e. wherever you can find studs, etc). It comes with a
    template to make this easy. The rear-bottom edge of the oven then clips
    onto the backplate to form a kind of hinge, and you pivot the oven up into
    endroit. There are two long bolts that run the depth of the oven near the
    top which you use to complete the attachment of the oven to the
    backplate. You then bolt it into the cabinet above it for additional
    security.

    Taking a microwave oven overseas (or vice versa)

    Microwave ovens are high power appliances. Low cost transformers or
    international voltage adapters will not work. You will need a heavy and
    expensive step down or step up transformer which will likely cost as much
    as a new microwave oven. Sell the oven before you leave and buy a new one
    at your destination.

    Furthermore, for microwave ovens in particular, line frequency may make a
    différence. Due to the way the high voltage power supply works in a microwave
    oven, the HV capacitor is in series with the magnetron and thus its impedance,
    which depends on line frequency, affects output power.

    High voltage transformer core saturation may also be a problem. Even with no
    load, these may run hot even at the correct line frequency of 60 Hz. So going
    to 50 Hz would make it worse – perhaps terminally – though this is not likely.

  • Going from 50 Hz to 60 Hz at the same line voltage may slightly increase
     output cooking power (and heating of the magnetron). The line voltage
     could be reduced by a small amount to compensate. This is best done with
     a buck/boost transformer rated for the maximum current input to the microwave
     oven (usually 15 A). Alternatively, it may be possible to replace the HV
     capacitor with one that has about 5/6 the uF value, at the same or higher
     operating voltage.
  • Going from 60 Hz to 50 Hz may slightly decrease output power and possibly
     increase heating of the HV transformer due to core losses. Using a slightly
     lower line voltage will reduce the heating but will further decrease the
     cooking power.
  • The digital clock and timer will likely run slow or fast if the line frequency
    changes as they usually use the power line for reference. Of course, this may
    partially make up for your change in output power! 🙂

    Microwave oven test-mode

    (From Mark Paladino (paladino@frontiernet.net).)

    Some microwave ovens have a self-test feature. This self-test is usually
    accessed by pressing a couple of keys on the touch pad. You can usually test
    things like keys, switches controller etc. Check the manual for any
    self-test info. Some microwaves have this information tucked in a pocket
    or hidden somewhere behind panels.

    High frequency inverter type HV power supplies

    While the vast majority of microwave ovens – perhaps every single one you will
    ever see – use minor variations on the tried and trusted half wave doubler
    circuit, a few models have been designed using solid state high frequency
    inverters – in many ways similar to the deflection/HV flyback power supply of
    a TV or monitor. This number is will likely increase as it becomes cheaper to
    use semiconductors than iron. It's not clear if inverter microwaves provide
    any real advantage in terms of performance. But there is definitely a
    marketing benefit and they do weigh less. 🙂

    A typical circuit (from a Sharp microwave oven) uses full wave rectified
    but mostly unfiltered pulsating DC as the power to a large ferrite inverter
    transformer which sort of looks like a flyback on steroids. Voir
    High Voltage Inverter Power Supply from Sharp
    Microwave Oven. This means that the microwave output is pulsing
    at both 60 Hz and the frequency of the inverter!

    Bridge Rectifier Inverter Transformer Magnetron
                                           o
      H o—-+—|>|——+——–+——-+ +————————–+
            ~| |+ _|_ Drive )::( Filament 1T #18 |
             +—|<|---+ | --- 25T ):: +--------------+------+ | 115 VAC | | | #12 ):: HV Cap | +-|----|-+ +---|>|—|–+ +——-+ :: +——-||—–+ | |_ _| |
             | | | ::( .018 uF | | / |
      N o—-+—|<|---+ Drive |/ C ::( 2,400 V __|__ | ___ | ~ |- o---| Chopper ::( HV __/_ +----|:--+ (Interlocks and | | E ::( 250T | HV |'-->
    fuses/protectors | | ::( #26 Sense | diode | uWaves
      not shown) +———–+ +–+—//—-+———+
                                                 o | 1.2 _|_
     (Except for filament, # turns estimated) o H1 – Chassis Ground

    The chopper transistor is marked: Mitsubishi, QM50HJ-H, 01AA2. It is a LARGE
    NPN Darlington transistor on a LARGE heatsink. 🙂 Others may use LARGE
    IGBTs or MOSFETs.

    Note the similarity between the normal half wave doubler circuit and this
    output configuration! Base drive to the chopper transistor is provided
    by some relatively complex control circuitry using two additional sets of
    windings on the inverter transformer (not shown) for feedback and other
    functions in addition to current monitoring via the 'Sense' resistor in the
    transformer return.

    It is not known whether power levels in the oven from which this
    particular inverter unit came were set by the normal long cycle pulse
    width modulation or by control over a much shorter time scale, or by
    pulse width modulation of the high frequency power. Cependant, le
    blurb for the current line of Panasonic Genius(tm) inverter microwave
    ovens does boast about providing actual power continuously at each
    setting though I've heard it may only be down to a 1/10th, but that's
    close enough. Panasonic has a several models like this. I don't know
    what other manufacturers (including Sharp) still do. I acquired the
    Sharp unit in the late 1990s.

    Compared to the simplicity of the common half wave doubler, it isn't at all
    surprising why these never caught on (what is diagramed above includes perhaps
    1/10th the actual number of components in a typical inverter module, which
    can be seen in the photo). Except for obvious problems like a tired fuse,
    component level troubleshooting and repair would be too time consuming.
    Furthermore, as with a switchmode power supply (which is what these really
    are) there could be multiple faults which would result in immediate failure
    or long term reliability problems if all bad parts were not located.
    Schematics are not likely available either. And, a replacement module
    would likely cost as much as a new oven!

    This may simply be a situation where a high tech solution might not have been
    the best approach. The high frequency inverter approach would not seem
    to provide any important benefits in terms of functionality or efficiency
    yet created many more possible opportunities for failure. The principle
    advantages claimed by the manufacturer are more even cooking and less
    overcooking of edges. The microwave distribution mechanism is at least
    as important in this regard. Another major advantage – reduced weight –
    is somewhat irrelevant in a microwave oven. Perhaps, this was yet another
    situation where the Marketing department needed something new and improved!
    But if it was a "must have", other companies certainly aren't jumping on
    the bandwagon. Possibly more have jumped off. 🙂

    (From: John De Armond.)

    Don't try to operate an inverter-based oven from a cheap generator with a
    less than perfect sine output. That's another excuse for the blue smoke
    to leak out.

    In my case I wasn't about to spend that kind of money to repair an oven that
    barely cost that much, especially since I used it in my restaurant always on
    high. Therefore I yanked out all those fancy electronics and installed the
    transformer/diode/cap assembly from another old oven. I drilled a hole
    through that nice touch pad and installed an Intermatic spring-wound timer
    from Home Depot.

    Viola, good as new and bullet-proof against nasty power.

    Dangerous (or useful) parts in a dead microwave oven?

    A microwave oven with its power cord cut or removed AND its high voltage
    capacitor safely discharged is an inanimate object. There are no particularly
    hazardous parts inside. Of course, heavy transformers can smash your feet
    and sharp sheet metal can cut flesh. And, the magnets in the magnetron may
    erase your diskettes or mess up the colors on your TV.

    Some may feel there is nothing of interest inside a microwave oven. je voudrais
    counter that anything unfamiliar can be of immense educational value to
    children of all ages. With appropriate supervision, an investigation of
    the inside of a deceased microwave oven can be very interesting.

    However, before you cannibalize your old oven, consider that many of the parts
    are interchangeable and may be useful should your *new* oven ever need repair!

    For the hobbiest, there are, in fact, some useful devices inside:

  • Motors – cooling fan and turntable (if used). These usually operate on
     115 VAC but some may use low voltage DC. They can easily be adapted to
     other uses.
  • Controller and touchpad – digital timer, relay and/or triac control of the
     AC power. See the section: Using the control panel from
     defunct microwave oven as an electronic timer.
  • Interlock switches – 3 or more high current microswitches.
  • Heavy duty power cord, fuse holder, thermal protector, other miscellaneous
     parts.
  • High voltage components (VERY DANGEROUS if powered) – Typical HV
     transformer (1,500 to 2,500 VRMS, 0.5 A), HV rectifier (12 to 15 kV
     PRV, 0.5 A), and HV capacitor (approximately 1 uF, up to 1,500 to 2,500 VAC
     (4,200 to 7,000 VDC).
  • Magnetron – there are some nifty powerful magnets as part of the assembly.
     Take appropriate precautions to protect your credit cards, diskettes, and
     mechanical wristwatches. See the section: The magnets
     in dead magnetrons.
  • DOUBLE WARNING: Do not even think about powering the magnetron once you have
    removed any parts or altered anything mechanical in the oven. Dangerous
    microwave leakage is possible.

    If disassembling the magnetron (or if it does this on its own for some
    reason), see the comments below.

    (From: Wayne Love.)

    I am a microwave engineer
    and manufacture high power magnetron (up to 10 kilowatts at 2.450 GHz and
    up to 100 kilowatts at 915 MHz.) Just some info. The filament in a 2.450
    GHz magnetron is generally made of thoriated (about 2% thorium) tungsten.
    The thorium is slightly radioactive but the tungsten is generally not
    poisonous. The lead-in to the vacuum envelope are generally molybdenum and
    also relatively inert. If the vacuum tube is compromised with the filament
    at temperature (around 950 °C), tungsten oxide (yellowish/white coating)
    can also form. Generally this is not harmful but smart to avoid anyway.

    (From: Sam.)

    Hmmmm 100 kW. I guess I shouldn't run one of those exposed on a work
    bench. 😉

    (From: Wayne.)

    The 915 MHz (actually 898 MHz in the UK and parts of the old eastern block
    countries) 100 kilowatt magnetrons are about 4 feet tall and weight a couple
    of hundred pounds and that is just the vacuum diode. Add a couple of hundred
    more pounds for electromagnet and electronic lead terminals and I am pretty
    sure it might crush your work bench. 🙂 They are used primarily for large
    industrial processing.

    The magnets in dead magnetrons

    The dead magnetron you just replaced is fairly harmless. There is no residual
    radiation but it does contains a pair of powerful ferrite ring magnets. Celles-ci
    can be removed without extensive disassembly and make really nice toys but
    should be handled with care. Not only can they pinch flesh (yes, they are that
    powerful) but they will suck all the bits right off your tapes, diskettes, and
    credit cards. If you do want to save the magnets:

  • Disassemble the magnetron assembly as follows:
  • Remove the top portion of the magnetron – it is either fastened with
      screws or some metal tabs which are easily bent out of the way.
  • Remove the cover over the box where the filament connections are located.
      This usually requires peeling off the sheet metal around the edges.
  • Cut the thick copper connections to the filament near the tube itself.
      (The thick copper coils are RFI chokes and prevent any microwave energy
      from escaping via the filament circuit.)
  • Spread the frame apart just a bit and lift out the tube with heat sink
      fins. CAUTION: the sheet metal fins may be sharp!
  • The magnets can now be pulled off. They may need cleaning. 🙁
  • The magnetron tube itself can be disassembled by grinding off the welds
      around the edges of the large cylinder or cutting around it outer edge
      near one end with a hack saw but it takes quite a bit of curiosity to make
      this a worthwhile exercise. There is a slight chance that the coating on
      the filament is poisonous so don't take chances. You don't need to get
      inside to remove the magnets.
  • Keep the magnets a safe distance away from any magnetic media including
     what might be in your back pocket, mechanical wrist watches, and color
     computer monitors and TVs.
  • Paint the magnets with plastic enamel or coat them with the stuff used on
     tool handles to reduce their tendency to chip. The chips are as magnetic as
     the overall magnet. The ferrite is basically a ceramic and fragile.
     Smack them too hard and they will shatter.
  • Take care not to get your skin between the magnets when you bring them
     together since the attractive force when nearly touching is substantial.
  • Store the magnets in a box packed in the center of another box with at
     least 4 inches on all sides. Clearly mark: powerful magnets with appropriate
     warnings.
  • Having said that, these magnets can be used to demonstrate many fascinating
    principles of magnetism. Have fun but be careful.

    Also see the section: Magnetron construction – modern
    microwave oven.

    Using the control panel from defunct microwave oven as an electronic timer

    It is usually possible to remove just the touchpad and controller board
    to use as a stand-alone timer with a switched output. Be careful when
    disconnecting the touchpanel as the printed flex cable is fragile. Avec
    many models, the touchpanel (membrane touchpad) needs to be peeled off of
    the front plastic panel or the entire assembly can be removed intact.

    The output will control a 10-15 A AC load using its built in relay or triac
    (though these may be mounted separately in the oven). Note that power on a
    microwave oven is regulated by slow pulse width modulation – order of a 30
    second cycle if this matters. If it uses a triac, the triac is NOT phase
    angle controlled – just switched on or off.

    Precise control of microwave oven power

    For heating a casserole, the 10 to 30 second cycle time typically used for
    microwave oven pulse width heat control is fine. However, for other purposes,
    this results in unsatisfactory results. This question was posed by someone
    who wanted to modify the circuitry to their microwave oven to provide
    continuous control and a constant heating rate.

    Just cycling faster (without any other modifications is not the answer). Un
    problem is that the filament of the magnetron is turned on and off as well.
    This would result in a very non-linear relationship between on-time and power
    as the cycle became shorter and shorter.

    It should be possible to put a Variac (variable autotransformer) on the input
    to the high voltage transformer – between the controller and HV primary. (For
    safety, DON'T attach it externally, DON'T bypass or disable any door
    interlocks, and make sure the cooling fan is always powered from the full line
    voltage.) The power to the filament will still be affected but there will be
    a range over which continuous control will be possible. My guess is that this
    would be between 60 and 80 percent and full voltage from the Variac will
    result in 0 to 100 percent of cooking power (the magnetron is a non-linear
    device – there is a threshold voltage below which no output is generated).
    However, there will be a lag as the filament heats and cools.

    Where manual control is all that is needed, this approach may be the adequate.

    If the filament were put on its own transformer (with appropriate insulation
    ratings), then instantaneous control of power should be possible using a Variac
    on the HV transformer primary or a phase control scheme using a triac – a high
    power light dimmer or motor speed control might even work. Alternatively, a
    triac or solid state relay can be turned on and off at the peaks of the AC
    (to minimize inrush) similar to the pulse width modulation that is normally
    used for the oven – but at a much higher frequency. This could easily be
    computer controlled with feedback from a temperature sensor.

    In any case, you want everything else – including cooling fans – to be on the
    full line voltage not affected by any power control scheme or timer.

    Has technology gone too far?

    Don't you just hate it when your kitchen appliances have the highest IQ in
    the household? What more could you want? Maybe, a microwave with a robot
    arm to retrieve the food from your fridge or freezer! But wait, you haven't
    seen it all. Just what the World needs is a smart microwave. You WILL see
    ovens (if they don't exist already) that with the help of a barcode or Dallas
    ID chip on the frozen package or food container, will contact a recipe
    database at the Web site for the product to determine exactly how to optimally
    overcook it and turn it into rubber. 🙂

    (From: Dave Marulli (marulli@rdcs.kodak.com).)

    We bought a Sharp unit with the Interactive Display feature.

    There is a list of common items that you might Defrost, Cook, or Reheat.
    You pick one of those tasks, choose a number from the list, enter the
    'quantity', hit start and it picks the time and power level. Il y a
    even an 'on-line' help feature. A typical session goes like this:

    Button Pressed Screen Output
       —————- ——————————
          CompuCook Enter Food Category
              1 Baked Potato, Enter Quantity
              4 Press Start

    Unit turns on and starts cooking. If the little word HELP lights
    up, you press the HELP button and it gives you little hints like,
    DO NOT COVER, or CUT IN HALF, etc.

    For things like CompuDefrost, you tell it what you are defrosting,
    how many pounds, and hit start. It will turn on for a while, then
    beep at you and tell you to break the pieces apart, cover the edges,
    etc. You do as you are told, close the door hit start and it continues
    until it's time for you to do some thing else.

    Same idea for CompuReHeat: Tell it how many slices of pizza or bowls
    of pasta you want to reheat, and it sets itself up and takes off.

    It even has the obligatory POPCORN button!

    Another neat feature is that you can hold the start button on without
    setting any time and it will stay on for as long as you hold the button.
    This is great for melting cheese, softening butter or chocolate, etc.

    But, does it run Lotus??? 🙂 — sam.

    (From: Steve Dropkin (sdropkin@isd.net).)

    The one we bought has an LCD screen that's maybe three inches square, takes
    you step-by-step through anything the oven can do, and includes 600 recipes
    (!). While that sounds like overkill, the attraction for me was that the
    menu-driven interface actually seemed simpler and more inviting than the
    ovens with timing buttons and 24 others marked "popcorn," "baked potato,"
    "hot dog," "frozen dinner," "beverage," "sandwich," "waffles," etc. They
    looked just way too busy. (Same argument I have against a lot of mainstream
    HiFi equipment these days. I just want to listen to the music, not
    reengineer the sound source …)

    (From: Andrew Webber (webbers@magma.ca).)

    Our microwave has a button for popcorn. As far as I can tell, all it does is
    automatically set 5 minutes. The manual says to monitor the popcorn anyway
    since it varies based on bag size, etc. So on principal I choose 5 minutes on
    high and stop it at 1:45 (why not set for 3:15? because the one time I tried it
    the popcorn was burnt!). I can choose 5 minutes with two presses (QUICK, 5)
    and popcorn with two presses (POPCORN, START).

    But that popcorn button sure is a good selling point! 🙂

    Microwave ovens for non-standard applications

    Occasionally, people ask questions about the use of a microwave oven to
    do things other than heating food. In general, these have to be taken on
    a case-by-case basis. Obviously, softening sticks of Dynamite is probably
    not to be recommended! (There actually is a reason for this – a microwave
    can develop hot spots – heating is not as uniform as with normal ovens. Faire
    your dynamite softening in a normal oven).

    Special kilns that will fit inside a microwave oven are apparently available
    to achieve really high temperatures. They consist of a ceramic (expanded
    alumina or something similar) insulating cylinder lined with a microwave
    susceptor – possibly a ferrite material. Temperatures exceeding 1000 degrees
    C (yellow-white heat) are possible after a few minutes on high.
    See for example Microwave
    Melting of Metals.

    If any modifications are made to the oven that would compromise the integrity
    of the door seals or provide other places where microwave radiation could
    escape, then special tests MUST be done to assure the safety of the users
    of the equipment. The following is one such case in point:

    "My Dad and I are using a microwave oven to heat oak strips by passing them
     through the microwave field of a 1000W oven. We cut out squares (4"x 4") in
     the glass front and metal back of the oven to allow these strips to pass
     through the field. I am concerned about potential microwave leakage of a
     harmful nature."

    Geez!!! You guys are out of your collective mind. Sorry, having said that
    I feel much better. 🙁

    My first recommendation (though this is too weak a term) would to not do this.

    My second (and up to N where N is a very large number) recommendation would
    be not to do this.

    However, if you insist, use a good conductive sheet metal such as copper or
    aluminum to reduce the size of the opening as close to the material as
    possible. The wood stock will tend to reduce leakage while it is in place
    but the opening will leak like crazy when there is nothing in the hole. le
    sheet metal must be in electrical contact with the mesh in the door and the
    metal back. The smaller the opening, the less will be the leakage. Également,
    make sure there is always a load in the oven (a cup of water, for example) to
    keep the magnetron happy.

    Next, borrow an accurate microwave leakage detector. A large appliance repair
    shop or electronics store may rent you one if you are persistent enough. Utilisation
    this to identify the safe limits front and back. Label these and don't go
    closer while the oven is in operation. The operators may have to remain
    further away or some additional shields may needed if these distances are not
    satisfactory. The leakage detector or microwave field strength meter should
    come with information on acceptable power limits. It is something like 2 mW
    per square cm a foot or so from the oven – check it out. However, there is
    no assurance that even this limit is safe.

    CAUTION (In addition to the loony nature of this entire project!): Since the
    leakage you encounter may be orders of magnitude greater than what is typical
    of even a misaligned microwave oven, start with the probe at a distance of a
    few feet and slowly move it closer while watching the meter or readout. Ne pas
    set it next the opening as you hit START! This will prevent the possibility
    of damage to the expensive leakage tester (which could be costly) and exposure
    risk to you as well.

    The only known confirmed danger from microwave radiation is from internal
    heating effects. The eye is particularly sensitive to this and it doesn't
    take much of an increase in temperature to denature the tissue of the central
    nervous system (i.e., scramble your brain). The human body does not have an
    adequate warning system since nerve endings sensitive to heat are somewhat
    sparse. Thus, while the dangers may be overstated, it doesn't make sense to
    take chances.

    What is wrong with radiant heat???

    (From Barry Collins (bcollins@mindspring.com).)

    You did the right thing to discourage people from breaching the integrity of a
    microwave oven, because there are so many factors involved that one has to
    assume personal (or property) injury (or damage) may result from such actions.

    I personally don't feel uncomfortable with what the person was doing, provided
    they had taken reasonable precautions (too numerous to list). Power does fall
    off with the square of the distance and microwaves, barring any reflective
    surface, are very directional by nature. Just don't stand in front of the
    source. (I met one of the Japanese engineers who had unintentionally placed
    his head in a test oven that was working. He reported warmth, but no lasting
    damage, aside from the resulting joke.) Field density and exposure time is a
    large factor. One tends to remove one's hand when one senses heat. je pense
    the story goes that this was how the heating affect was originally discovered.

    The number one precaution I've always held near and dear to me is to protect
    one's eyes. The Narda manual has multiple warning in it about this. le
    aqueous membranes of the eyes are perfect absorption material for stray
    microwaves. This can happen much faster than with fleshy parts of the body
    and don't heal anywhere near the way a flesh injury does. It is this that you
    might want to point out in your FAQ's.

    Short course on Amana

    (From: Charles Godard (cgodard@iamerica.net).)

    Everything depends on "Air Flow". If the stirrer does not turn, you will
    always get a "Hot! spot" on the left bottom of the door. In addition the
    stirrer bearing will sometimes arc and may melt at the spots where it arcs.

    If your blower is running up to speed, remove the cover and replace the foam
    gasket material. This forces air over the stirrer when the cover is replaced.
    If stirrer still does not turn, remove the grease shield and check the stirrer
    for burns that are causing it to stick. If this is ok or you correct it and
    stirrer still does not turn, then replace the grease shield with a later model
    that looks almost the same as the original, but has one small modification
    which you will see when you compare the two.

    Never let one go out of the shop unless the stirrer is turning. It will soon
    be back unless all they do is heat coffee. Next time it may be a cavity or
    magnetron overload that has opened due to the stirrer not turning.

    It's good work on a quality product. I wish I had a hundred restaurant
    customers using them. The older Amana's power stays near 1,500 watts forever.
    Retail customers are junking them because of $100 – to $125 repair bills.
    What a waste!

    Computer system near microwave oven?

    "Can placing my microwave oven in close proximity to my computer and printer
     do any damage to either of them? The back of the oven would be right next to
     the printer and about 16 inches from the computer. I have gotten conflicting
     answers from the guy who rebuilt my computer and the guys at Radio Shack."

    Did the kids at Radio Shack even understand the question??? 🙂

    Your request is certainly a bit unusual. My feeling is that it should be
    fine. The problem would more likely be the magnetic field from the large
    transformer in the microwave oven causing interference on your monitor
    (wiggling, jiggling, shimmering, etc. due to its effect on the electron beams
    in the CRT). There should be no significant microwave leakage from the oven,
    especially the rear. Keep in mind that there is a computer of sorts inside
    the microwave controlling it!

    However, you will need separate grounded electrical circuits for the microwave
    and computer equipment if you intend to ever use them at the same time.

    Why Microwave-Safe Containers Get Destroyed

    You probably have a cabinet full of so-called microwave-safe containers that
    look like they have been exposed to damage from a nuclear explosion. Pourquoi?
    It probably comes down to unequal heating of the contents or heating
    continuing long past the point where boiling takes place. I would assume
    that putting a microwave-safe container in an oven with a cup of water in
    a separate container wouldn't result in any damage to the microwave-safe
    récipient. But if the contents of the microwave-safe container are being
    heated, then some parts will get much hotter than others resulting in local
    melting and other damage. I doubt it is the microwave radiation itself doing
    anything to the material of the container directly and complaining to the
    oven manufacturer isn't likely to be very satisfying. 🙂

  • Back to Microwave Oven Repair FAQ Table of Contents.

    Service Information

    Advanced troubleshooting

    If the solutions to your problems have not been covered in this document,
    you still have some options other than surrendering your microwave to the
    local service center or the dumpster.

    Unlike most other types of consumer electronic equipment, a service manual
    is rarely required. A sufficiently detailed schematic is nearly always
    pasted to the inside of the cover and includes all power components,
    interlocks, fuses, protectors, and wiring. This is entirely sufficient
    to deal with any problems in the microwave generator. No adjustments or
    alignment should even be required so detailed procedures for these are not
    nécessaire.

    However, when tackling electronic faults in the controller, a service manual
    with schematics will prove essential. Whether these are available depends
    on the manufacturer. For legal reasons, some manufacturers are reluctant
    to sell service information or replacement parts for microwave ovens. Ils
    are concerned with litigation should an unqualified person be injured or
    killed.

    Suggested Reference

    I know of at least one book dealing specifically with microwave oven repair.
    It is very complete and includes many actual repair case histories. Là
    is a good chance that your specific problem is covered.

  • Microwave Oven Repair, 2nd EditionHomer L. DavidsonTAB Books, a division of McGraw Hill, Inc., 1991Blue Ridge Summit, PA 17294-0850ISBN 0-8306-6457-2 (hard), ISBN 0-8306-3457-6 (pbk.)
  • This may be available at your public library (621.83 or 683.83 if your
    library is numbered that way) or from a technical bookstore.

    Cost of repair parts

    Assuming you have located one or more bad components, the question is
    whether an oven that is a few years old is worth fixing. Typical parts cost
    for generic replacements:

  • HV diode: $2-5 (except for the bolt-on variety which can range up
     to $50. It should be possible to replace these with the $2 variety
     with wire leads);
  • Power fuse: $.40.
  • HV Capacitor: $10-20.
  • Magnetron: $30-100. Common generic replacements are $30-40.
  • Overtemperature thermostat (thermal protector): $4.50.
  • Interlock Switch: $2.50.
  • Triac: $12.00 (unless original replacement in which case you will
     need to take out a mortgage – try the generic variety).
  • Parts suppliers like MCM Electronics can provide these components to fit
    the vast majority of microwave ovens.

    Touchpads and controller parts like the microprocessor chip are usually only
    available from the manufacturer of the oven. Prices are high – a touchpad
    may cost $30 or more.

    Sensors and other manufacturer specific parts will be expensive.

    While the HV transformers are fairly standard, they are not readily available
    from the common replacement parts sources. However, they do not fail that
    often, either.

    Here is one place that seems to stock some: AMI Parts, Eagle Grove, IA. Voix
    phone: 1-800-522-1264. However, they won't be cheap – expect to pay $50 or
    more!!! In addition, MCM Electronics now lists at least one Goldstar model
    replacement.

    With the prices of microwave ovens dropping almost as fast as PCs, a few year
    old oven may not be worth fixing if the problem is a bad magnetron or touchpad.
    However, except for a slight decrease in power output as the oven is used over
    the years and the magnetron ages, there is little to go bad or deteriorate.
    Therefore, you can expect a repaired oven to behave just about like new.

    Interchangeability of components

    The question may arise: If I cannot obtain an exact replacement or if I
    have another microwave oven carcass gathering dust, can I substitute a
    part that is not a precise match? Sometimes, this is simply desired to
    confirm a diagnosis and avoid the risk of ordering an expensive replacement
    and/or having to wait until it arrives.

    For safety related items, the answer is generally NO – an exact replacement
    part is needed to maintain the specifications within acceptable limits with
    respect to line isolation, radiation emission, and to minimize fire hazards.
    For microwave ovens such parts include the power fuses, interlock switches,
    and anything else that could potentially lead to microwave radiation leakage –
    like a magnetron which did not fit the waveguide properly.

    Fortunately, while an exact match may be required, it doesn't have to
    be from the original manufacturer – most parts are interchangeable.
    Thus the organs from that carcass may be able to provide renewed vitality
    to your ailing microwave.

    Here are some guidelines:

  • Fuses – exact same current rating and at least equal voltage rating.
     This will probably be a ceramic 1-1/4" x 1/4" 15 or 20 A 250 V fast
     blow type. For the repair, use an exact replacement. For testing
     only, a similar type may be used.
  • Thermal protectors – same temperature and maximum current rating. Vous
     must be able to mount it securely and flush against the same surface as
     the old one.
  • Interlock switches – must have the same terminal configuration and
     at least equal current rating. Of course, a secure fit is very
     important as well for it to perform its safety function. Many of
     these are interchangeable.
  • HV capacitor – similar (within 5%) and at least equal working voltage.
     Note that the working voltage rating of these capacitors is not consistent
     with the way capacitors in other electronic equipment are specified and
     is usually the RMS voltage of the AC input from the HV transformer.
     Therefore, it is not possible to substitute something from your junkbox
     unless it is from a microwave oven. In addition, this is one situation where
     higher capacity (uF) is not better. The power output is related to
     capacitance. Thus, the value should be matched fairly closely or else
     other parts may be overloaded. However, a smaller one can be used for
     testing.
  • HV diode – most of these have similar electrical ratings so a substitution
     is possible if you can make it fit physically. This would be particularly
     desirable where your oven has one of those chassis mount $50 dollar
     varieties – it may be acceptable to use a $2.75 generic replacement.
  • Relays and triacs – substitutes will generally work as long as their
     specifications meet or exceed those of the original. Creative mounting
     may be required.
  • Magnetrons – a large number of microwave ovens use the same basic
     type but the mounting arrangement – holes vs. studs, orientation of
     the cooling fins, etc., differ. You can safely substitute a not
     exact match for testing purposes IF you can make it fit the waveguide
     securely without gaps. However, if the cooling fins end up being on
     the wrong side, it will heat up very quickly – 50% of the input power
     goes to heat – and will not be suitable as a permanent replacement.
  • HV transformer – same (within 5%) voltage and at least equal current
     rating. Mounting should not be a problem but don't just leave it
     loose – you could end up with a disaster.
  • Fans and motors – speed/power and direction must match and mounting must be
     possible. Speed isn't so critical for a turntable but for a magnetron
     cooling fan, inadequate air flow will result in overheating and shutdown
     or failure. Common shaded pole type motors may be interchangeable with
     other appliances or if a mounting arrangement can be cobbled together.
  • Mica waveguide cover – cut to match.
  • Turntable and mode mixer components – if they fit, use them.
  • Light bulb – similar ratings and base.
  • Temperature sensors, thermistors, etc. – depends on the particular
     model.
  • Mechanical timers – compatible switching and mounting arrangement.
  • Cordsets – must be 3 wire heavy duty grounded type. Make sure the
     replacement has at least as high a current rating as the original.
     Observe the color code!
  • Controller and touchpad – small parts like resistors, diodes, capacitors,
     and so forth can often be substituted. Forget about the controller
     ICs or display. The touchpad is likely to be custom both electrically
     and physically as well unless you have a similar model microwave to
     cannibalize.
  • Can I substitute a slightly different HV capacitor for a blown one?

    It is not always possible or convenient to obtain an exact replacement
    high voltage capacitor. What will the effects be of using one that is
    a slightly different value?

    First, the voltage rating must be at least equat to that of the original.
    It can be higher but never never lower or you will probably be replacing
    it again in the very near future.

    Now for the uF rating:

    Unlike a conventional power supply filter capacitor, the capacitor in a
    microwave is in a voltage doubler and effectively in series with the
    load (magnetron). Therefore, its value **does** have an impact on output
    power. A larger capacitor will slightly increase the output power – as
    well as heat dissipation in the magnetron. Too small a capacitor and
    the doubler will not produce full output.

    As an example, the impedance of a 1 uF capacitor at 60 Hz is about 2.5 K ohms.
    The cap is in effect in series with the magnetron. A 1 kW magnetron running
    on just over 3 kV RMS is about 10 K ohms. These are really really rough
    calculations.

    Thus the power difference is not a straight percent for percent change – I
    estimate that it is about a 1:4 change – increase the capacitor's uF rating
    by 10 percent and the power and magnetron heat dissipation will go up by 2.5%
    (assuming the relationship is linear right around the nominal value). j'ai
    not confirmed this, however.

    Therefore, I would say that using a capacitor with up to a 10-15% difference
    (either way) in uF rating is probably acceptable but a closer match is better.

    Obtaining replacement parts for microwave ovens

    For general electronic components like resistors and capacitors, most
    electronics distributors will have a sufficient variety at reasonable
    cost. Even Radio Shack can be considered in a pinch.

    However, places like Digikey, Allied, and Newark do not have the specialized
    parts like magnetrons, HV capacitors and diodes, interlock switches, thermal
    protectors, etc., needed for microwave oven repair.

    Your local appliance distributor or repair parts outlet may be able to obtain
    an exact replacement or something that is an ecceptable substitute. cependant,
    the cost will be higher than for generic parts from the places listed below
    if they carry what you need.

    Going direct to the manufacturer is a possibility but expect to pay more than
    might be charged for generic replacement parts by an independent company.
    Also, some places like Sears, may refuse to sell you anything microwave oven
    related due to safety concerns – unless they are convinced you are a certified
    repair technician, whatever that might mean. Their prices are inflated as
    bien.

    Another alternative is to determine who actually made your oven. This is
    obvious with name brands like Panasonic and Sharp. However, Sears doesn't
    manufacture their own appliances, but an inspection inside may reveal the
    actual manufacturer. Then, go direct to the horse's mouth. Many companies
    will be happy to sell service parts but availability may be a problem on
    older ovens. I had to give up on a Sharp microwave/convection oven that
    was 15 years old because specialized replacement parts were no longer
    available from Sharp.

    Note: I have heard that in other parts of the world, there may be restrictions
    on who can actually purchase microwave oven parts other than things like light
    bulbs, turntables, and standard door switches. In the U.S., certain companies
    (like Sears) may set their own rules – you have to convince them that you have
    at least the intelligence of an average carrot and possibly sign a 100+ page
    document written by too many lawyers. 🙂

    Sources for replacement microwave oven parts

    See the document: Major Service Parts Suppliers
    for some companies that I have used in the past and others that have been
    conseillé. They may include microwave oven parts in their catalog but
    don't specialize in them. Also see the "Microwave Oven" sections of
    Sam's Neat, Nifty, and
    Handy Bookmarks.

    The following suppliers have web sites with on-line catalogs and list a very
    extensive selection of microwave oven parts. There is a chance that they may
    not want to sell to the general public. I suppose this may be due to several
    factors including the potential liability issues, complaints/attempts to return
    parts when a repair doesn't work, and the small quantities involved. cependant,
    it is definitely worth checking as the public web sites implie a desire to deal
    with the entire Internet community.

  • Global/MPI/All Appliance
     PartsPhone: 1-800-325-8488Web: http://www.allapplianceparts.com/

    Their web site includes a very extensive selection of microwave oven parts.
     For example, nearly 50 different magnetrons are listed along with little
     photos of each!

  • AMI (Appliance Maintenance
     International)U.S. Phone: 1-800-522-1264U.S. Fax: 1-800-442-3601Int. Phone: 1-515-448-5311Int. Fax: 1-515-448-3601Email: ami@amiparts.comWeb: http://www.amiparts.com/

    Distributor of consumer and commercial microwave oven parts. Extensive
     on-line catalog of microwave oven parts with on-line parts lookup and
     ordering.

  • Here is another one:

  • Electronix, CorporationWeb: http://www.electronix.com/

    Magnetrons, interlock switches, lamps, glass trays, diodes, thermal fuses,
     couplers, latches, rivets, stirrers, fans, waveguides, more…
     Also: Techweb, $6/month.

  • The following company will definitely not sell you anything but should be able
    to provide the name of a local appliance parts distributor.

  • QB ProductsPhone: 1-800-323-6856

    Master distributor, they sell only to appliance and electronics parts
     distributors like Marcone, Tritronics, Johnstone, etc. You can call them to
     find the nearest distributor.)

  • Back to Microwave Oven Repair FAQ Table of Contents.

    — end V3.65 —

  • Remarques sur le dépannage et la réparation des fours à micro-ondes
    4.9 (98%) 32 votes
     

    Laisser un commentaire