There are a number of possible causes for this. Most of the troubleshooting requires a multimeter. However, first you should check whether the oven is actually not heating, or whether there is a problem with microwave distribution in the oven. In a large microwave safe cake pan or pie plate, put just enough water to cover the bottom. Put it in the oven and turn the oven on high. If you see one spot where the water boils away quickly, you have a non-functional microwave stirring fan. Food will not cook unless it is in this hot spot. To fix it, take off the top cover, and look for the big fan in the waveguide duct coming from the magnetron on the right side (not to be confused with the small magnetron cooling fan blowing air across the magnetron's fins). If memory serves me correctly, you have to remove a cover plate on top of the oven cavity, and the fan is belt driven by a small shaded pole motor near the back of the oven. Make sure the fan spins freely. Clean and lubricate the bearings with a light-weight oil if necessary. If the fan turns freely, make sure its motor has power when the oven is supposed to run. If all is fixed, the wet plate test should show more even heating, and food will cook properly.
The other possibilities, together with diagnostic procedures, are listed below.
- Backup door switch*: With the oven unplugged and the door closed, remove the top/side cover. There is a large transformer on the floor of the oven on the tight side. The primary terminals are towards the front. One terminal on the transformer is connected to neutral (white wire). The other goes to the backup door switch, and the other terminal on the door switch is connected to the controller relay (sometimes there is also a thermal cutout in this line, but that part is usually in the incoming power hot lead). Measure the resistance from the relay terminal connected to the door switch to the hot wire on the transformer. It should be much less than 0.8 Ohms with the door closed. If not, check directly at the door switch, and check any other devices wired between the relay and transformer.
- High voltage capacitor. This is an oval cylinder behind the transformer, on the high voltage side. It is about 3-4" long, 1" in the narrow dimension, and about 2-2 1/2" in the longer oval dimension. It has one lead to the transformer on one terminal, and a lead to a diode and another to the magnetron on the other terminal. Make sure it is safely discharged before testing. Measure its capacitance and compare it with the value printed on the side. If the capacitor is open, the magnetron won't work. If it is shorted, usually the fuse blows. Caution: when you reconnect the capacitor, do not interchange the two terminals. The capacitor is polarized, and applying a reverse voltage may cause it to explode.
- Relay. This requires a live test, with great care. Stay away from the backside of the transformer - 1200-2500 Volts is not a good thing to mess with. Measure the voltage at the transformer's hot terminal on the front (primary) side with the oven running (the other voltmeter lead can go to either neutral or ground; you are more interested in presence or absence than in accuracy). If you get nothing, check the relay terminal on the controller (make sure you are looking at the magnetron terminal, not the lamp/fan terminal). If you get nothing here, the relay is bad or has a bad connection to it.
- Magnetron or diode. If you have voltage on the transformer primary, but you get no heating power, the only items left are the magnetron and diode (assuming the capacitor passed its test). The only safe way to determine which, other than substituting parts, is to measure the DC voltage using a special high voltage probe on the capacitor's diode/magnetron terminal. If you have 0 Volts DC, or a DC voltage much less than 1000 volts, the diode is bad. If you have the full voltage, the magnetron is bad (only thing left). Warning: do not try to measure this voltage directly with an ordinary meter. It will destroy the meter and could be very dangerous for the operator. Use either a high voltage meter or a high voltage probe made for use with digital multimeters.
* The main door switch is not the problem. If it is open, the oven will not run. Its job is to turn off the fans and magnetron power relay if the door is opened while the oven is running. The backup switch is a second disconnecting means in case the relay is shorted.
