How Everything Works
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QUESTIONS AND ANSWERS
 
Question 653

I recently acquired a microwave that "doesn't cook as fast as it used to." Does this sound right? What type of service might need to be performed? - W
It is possible for a microwave to lose cooking speed. If the microwave source isn't able to produce as intense microwaves as before or if it doesn't turn on reliably and steadily, it won't cook as fast. For the source to produce less intense microwaves, the high voltage power supply would probably have to be weak. Its storage capacitor could have failed or one or more of its high voltage diodes could have burned out. According to a reader, the most likely cause of weak cooking in a microwave oven is a failed capacitor—with no ability to store separated charge in its capacitor, the oven produces pulsing rather than steady microwaves and delivers less average power. I suppose that the magnetron itself could be dying, with the most common failure (according to that same reader) being shorting out, the result of electromigration of the filament material. For the source to not turn on reliably, it would probably have to have a bad connection to the power line. One good possibility is that the relay that turns on power to the high voltage power supply is not making good contact.

Listen to the microwave as it operates on a medium setting. It should cycle on and off every five or ten seconds. You should hear it hum softly during the on half of the cycle and then stop humming during the off half of the cycle. Different power levels simply vary the fractions of on time and off time. If you don't hear the hum or the hum is intermittent, then something is probably wrong with the power relay or with something else in the high voltage power supply. If the relay is flaky, a little cleaning of its contacts may cure the problem. Be careful of the high voltage capacitor, which can store a lethal charge even when the unit is unplugged.

         

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