Frigidaire Microwave not Heating

[TechnicianBrian]

Model #PLMV169DCE, this over the range microwave was no longer heating when a cycle was selected and the customer complained that the unit now made a louder than normal humming sound when running. This sound would vibrate enough to echo within the cabinetry and the unit stopped working shortly after the noise started. Having a pretty good idea as to the cause, I got right to work.



Microwaves don't actually generate heat, but rather use a magnetron tube to generate microwave energy which is absorbed by the food. These microwaves cause the molecules within the food to vibrate and the food is cooked because of the heat generated from the resulting friction. Check out some of my other microwave posts for a more detailed description of the process, but that should be good enough for this post.

The magnetron uses very high direct current (DC) voltage to produce this microwave energy. Through the use of a transformer and what is commonly referred to as a voltage doubler circuit, the house hold 120vac voltage is stepped up to the approximate 4000vdc to 5000vdc needed by the magnetron. The doubler circuit consists of a capacitor used to store voltage, and a diode (often called a rectifier) which only allows current to flow in one direction. Through these two components, the high AC voltage from the transformer will charge the capacitor on the positive side of the AC sine wave through the diode. One the negative side of the AC sine wave, the voltage will combine with the voltage stored in the capacitor and will travel to the magnetron because the diode is now blocking the current path to ground(remembering diodes allow current flow in only one direction). This simple but effective method of creating the high voltage DC is common is most every microwave on the market.


Since the unit was not heating, I first checked for voltage to the transformer because it is a good place to separate the low voltage section from the high voltage section. With the control panel removed, I was able to easily access the transformer, which was convenient and rather unusual for most microwaves. When a cycle was started, I was reading 120vac into the transformer which tells me everything else from the door switches to the control panel was working properly. With the unit now unplugged and the capacitor discharged (use a screwdriver with an insulated handle) I used my meter to check the remaining HV components and found the diode was reading continuity in both directions. Remembering diodes only allow current to flow one direction, this would indicate the diode has failed. With the diode shorted, it was allowing current to flow in two directions (it wasn't a diode by definition anymore) thus allowing the high voltage charge to go right to ground instead of to the magnetron.

I replaced the diode (which comes as part of a wire harness) on this unit and was able to get it up and heating again in no time. Well not actually heating, but it does cook food now.  And it does it much quieter.

Just a note: when using a meter to do checks on these high voltage diodes, it may be necessary to use a 9-volt batter to put in series with the diode and your meter leads. Many, lets say lower quality meters, will not be able to break through the diodes resistance and may give a false reading.

Generic Microwave Disclaimer: Microwave ovens use very high voltage while operating they still pose a risk of shock if you are not careful. Do not operate a microwave oven with the covers removed, and never use your hands or tools inside the unit while it is plugged in. Always ensure the capacitor has been discharged using a screwdriver with an insulated handle prior to touching anything in the high voltage section. There is no fun in getting shocked.