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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
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
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
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
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
opération lorsque le bouton de démarrage est enfoncé. Localiser et remplacer défectueux
commutateurs et / ou réaligner la porte.
Remplacez le couvercle de guide d'ondes carbonisé ou endommagé. Lisser les bords en métal rugueux.
Retoucher la peinture intérieure.
Dans certains cas, le fusible principal peut même être intermittent, ce qui entraîne des conséquences très étranges.
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.
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.
Installation et maintenance préventive
Installation et utilisation du four à micro-ondes
Pour assurer la sécurité et la commodité, suivez ces recommandations:
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.
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.
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).
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
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.
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.
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. 🙂
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.
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).
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
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
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.
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
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:
four – il est soit produit ou est inexistant.
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.
Dépannage du four à micro-ondes
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
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
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.
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!
peut être essentiel.
système connecté à la ligne ou haute tension.
circuits et conduisent le courant, ou se coincer dans les pièces mobiles.
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!
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.
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.
débranché. Utilisez des fils à pince ou des fils de soudure temporaires pour atteindre les endroits exigus
endroits difficiles d'accès.
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.
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.
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.
négligent, mais votre principal outil de diagnostic – le raisonnement déductif –
ne pas fonctionner à pleine capacité.
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
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
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":
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 ".
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.
tests de puissance.
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
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
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
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.
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.
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.
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
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:
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.
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.
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:
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
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.
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
From: Barry L. Ornitz (email@example.com).)
"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
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
(From: Filip (I'll buy a vowel) Gieszczykiewicz (firstname.lastname@example.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
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).
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.
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.
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.
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:
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
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.
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.
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
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
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
Magnetron construction – basic textbook
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.
supplied with pulsed or continuous power of many thousands of volts (negative
with respect to the anode.
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).
microwave energy to the waveguide.
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.
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
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
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
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:
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.
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
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
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.
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.
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
Magnetron construction – common features
The following items apply to all types of magnetrons.
in a vacuum.
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.
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
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.
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:
should the Primary Interlock not open due to incorrect sequencing of the
door switches or a failed switch.
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.
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.
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
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
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.
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
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
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
Of course, any number of other pre-existing or induced problems can result
in the oven playing dead after it has been "repaired". :
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.
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
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!
check the power supply for the controller next.
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
No response to any buttons on touchpad
There can be many causes for this behavior (or lack of behavior):
buttons – even setting the clock – unless the door is securely closed.
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
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.
the door is open and ignoring you.
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).
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
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
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:
(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).
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.
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.
There are three different situation:
cycle, suspect the controller power supply or bad connections.
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.
the section: Oven heats but power seems low or
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
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:
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.
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.
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 (email@example.com).)
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
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 (firstname.lastname@example.org).)
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:
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:
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.
continuity. It should read as a dead short – near zero ohms. Voir le
section: Testing thermal protectors and thermal
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.
high voltage capacitor. A shorted HV capacitor would likely immediately
blow the fuse.
or deteriorated press (Fast-on) lugs for the filament connections and not
an actual magnetron problem. See the section: Testing
Testing the high voltage transformer.
circuitry (rather than the primary) to regulate cooking power. This may
have dirty or burnt contacts, a defective coil, or bad connections
Testing the magnetron.
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),
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 (email@example.com).)
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
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
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
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
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:
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.
the high voltage transformer.
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 (firstname.lastname@example.org).)
charge but never hurts to be safe).
to the magnetron.
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.)
but these ovens are rare now because of their age.
Arcing in or above oven chamber
There is often a simple cause:
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.
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. 🙂
only way such damage could occur as part of the oven (not added knives or
forks!) would be through physical abuse.
be stuck. The result will be an uneven distribution of microwave energy and
localized heating, arcing, and possibly melting plastic or metal.
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
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
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 (email@example.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 (firstname.lastname@example.org).)
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 email@example.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.
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%.
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.
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.
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.
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!
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.
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.
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.
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.
poorly designed or oven where the components are marginal.
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:
to be removed. This is the easiest.
a screw or two on the back of the oven.
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:
When any of these do not operate properly, the most likely causes are:
affected part(s). Clean and lubrication as needed. Also confirm that
there are no other mechanical problems (e.g., turntable improperly
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.
If open, winding is bad but check for break at terminal which you can
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.
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
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 (firstname.lastname@example.org).)
Here are the door disassembly instructions from the Amana service manual.
Many others are similar:
panel (this may be all that's needed to replace the handle).
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
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:
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.
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.
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.
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.
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:
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.
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.
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.
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 (email@example.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.
inside the cable particularly near the ends where it gets flexed.
convection portion of a microwave/convection oven. Steam/humidity probes
may also behave similarly.
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.
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.
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
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.
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:
exactly 1 degree Centigrade (DegC) or 9/5 degree Fahrenheit (DegF).
(For Avoirdupois ounces, use 28.35 g.)
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 Â°C.
Or, if your prefer Fahrenheit: T(rise) = 109.8 Â°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
Or, from a Litton microwave handbook:
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
Testing the main fuse
Where the oven is dead or mostly dead, the main fuse is the place to start:
1" x 1-1/4" ABC ceramic type directly in-line with the Hot (black wire) of
the power cord.
capacitor, or high voltage wiring.
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!
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.
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:
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).
(From: Michael Caplan (firstname.lastname@example.org).)
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
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
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
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.
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.
switch on the HV transformer.
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.
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
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.
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.
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
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
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
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
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
(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
See the section Safe discharging of the high voltage
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
See the section Safe discharging of the high voltage
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
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.
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:
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.
burnt odor) usually indicates a bad magnetron.
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:
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.
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.
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.
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.
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.
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 (email@example.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.
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.
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
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!
Symptoms: No heat.
See note about HV capacitor in (2) above.
Symptoms: No heat, loud buzz due to arcing when entering cook cycle,
possible blown HV fuse.
See comments about fuses in (1) above.
possible blown HV fuse.
Symptoms: No heat, loud hum once it occurs.
See comments about fuses in (1) above.
Symptoms: Reduced cooking power.
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
oscillate at frequencies slightly higher or lower than 2.45 GHz.
Same as (7a) above.
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.
or antenna terminal.
Symptoms: Arcing, burning smell from magnetron, loud hum, no heat.
Symptoms: Reduced or no cooking power, magnetron overheating, occasional
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
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 (firstname.lastname@example.org).)
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
See the section Safe discharging of the high voltage
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
Testing the high voltage transformer
See the section Safe discharging of the high voltage
transformer may result in a blown fuse, loud hum, overheating, audible
arcing, a burnt aroma, or simply no heat.
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:
detectable with your multimeter. The only measurement easily made would
be that there is no short to the chassis.
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.
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.
all secondaries disconnected. See the section:
Testing the HV transformer using an AC current
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
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
See the section Safe discharging of the high voltage
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
Try not to drop either the old or new transformer on your foot!
Testing and repairing the wiring and connections
See the section Safe discharging of the high voltage
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:
at a preset temperature. They should reset when they cool off. cependant,
like a relay or switch, the contacts sometimes deteriorate.
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:
closed or the power being stuck on high no matter what the touchpad setting.
heat and possibly other things like the fan and turntable not working as well.
continuing to run even after the timer counted to zero.
acting as a rectifier pumping DC through the HV transformer.
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:
coming on as soon as the door is closed or the power being stuck on high
no matter what the touchpad setting.
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
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
You can find complete diagrams and parts lists at:
On-Line Parts Store. Just enter the the RC22LW model
number and then click the radio button for the P1198611M manufacturing
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
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 (email@example.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
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):
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:
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 (firstname.lastname@example.org).)
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
(From: Ren Tescher (email@example.com).)
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:
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
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 (firstname.lastname@example.org).)
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
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
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
The most important considerations are the door and door seal.
(From Barry Collins (email@example.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 (firstname.lastname@example.org).)
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
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
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
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
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 (email@example.com).)
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
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.
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
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
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 (firstname.lastname@example.org).)
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
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
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:
115 VAC but some may use low voltage DC. They can easily be adapted to
AC power. See the section: Using the control panel from
defunct microwave oven as an electronic timer.
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).
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 Â°C), tungsten oxide (yellowish/white coating)
can also form. Generally this is not harmful but smart to avoid anyway.
Hmmmm 100 kW. I guess I shouldn't run one of those exposed on a work
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
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:
screws or some metal tabs which are easily bent out of the way.
This usually requires peeling off the sheet metal around the edges.
(The thick copper coils are RFI chokes and prevent any microwave energy
from escaping via the filament circuit.)
fins. CAUTION: the sheet metal fins may be sharp!
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.
what might be in your back pocket, mechanical wrist watches, and color
computer monitors and TVs.
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.
together since the attractive force when nearly touching is substantial.
least 4 inches on all sides. Clearly mark: powerful magnets with appropriate
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
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 (email@example.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 (firstname.lastname@example.org).)
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 (email@example.com).)
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
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
What is wrong with radiant heat???
(From Barry Collins (firstname.lastname@example.org).)
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 (email@example.com).)
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. 🙂
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
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
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.
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:
to $50. It should be possible to replace these with the $2 variety
with wire leads);
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
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
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:
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.
must be able to mount it securely and flush against the same surface as
the old one.
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.
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
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.
specifications meet or exceed those of the original. Creative mounting
may be required.
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.
rating. Mounting should not be a problem but don't just leave it
loose – you could end up with a disaster.
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.
replacement has at least as high a current rating as the original.
Observe the color code!
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
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
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
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
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.
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!
International)U.S. Phone: 1-800-522-1264U.S. Fax: 1-800-442-3601Int. Phone: 1-515-448-5311Int. Fax: 1-515-448-3601Email: firstname.lastname@example.orgWeb: 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
Here is another one:
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.
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.)
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