, this over the range microwave would operate when started, but it was no longer heating any food. Several components can cause a no heat situation on a microwave, and it is just a matter of using your meter to figure out which one is causing the problem. Most microwaves function about the same using similar components, but this one is just a little different then most. Let me explain.
Microwave ovens in general do not heat food, but instead, use a magnetron tube to produce microwave energy that oscillates at 2450MHz, or 2.4 billion times a second. This energy is absorbed by the food items inside the oven cavity causing the molecules within the food to oscillate at the same rate. This vibrating movement creates friction which is what produces the heat used to cook our food.
To make this microwave energy, the magnetron tube requires a high voltage of approximately 4000 to 5000 volts negative DC. This high voltage is created from the 120vac house voltage through the use of a transformer and a voltage doubler circuit which consists of a diode and capacitor. Once the power enters the magnetron tube, the microwave energy is produced and the cooking begins. This particular microwave functions much the same as the rest, but it goes about a different path to create it's high voltage.
Inside the wrapper of this unit, you will not find a transformer, a diode, or a capacitor, but in their place you will find an inverter. An inverter is a generic term used for a component that turns or inverts DC voltage into AC voltage and vice verse. Most people have heard of or have used an inverter to turn 12vdc from their car or RV battery into 120vac to power normal house hold items. But the inverter inside this microwave is used to turn our house hold AC into a very high negative DC voltage.
The trouble shooting process is much the same as a typical microwave in that you first want to ensure 120vac is going to the inverter (or transformer) when a heating cycle is initiated. If voltage is not present, the problem is not in the high voltage section and you need to look at the interlock switches, thermal fuses, and control board for the failure. But if voltage is present, then we have a HV section component that has failed.
On this unit, I verified voltage to the inverter so I knew the source voltage was good, but then I needed to verify the control signal from the control board is being sent. This is how the control board turns on the inverter and sets the output power level. Because I cannot measure control signals with a basic multimeter, the easiest way to see if the inverter is working is to measure the current being used by the microwave using an amperage clamp. This can be done by clamping around the lead to the inverter which is conveniently behind the control panel. As a rule of thumb, if the current draw is high (1 amp or more) then the inverter is working and the magnetron has most likely failed. If the current draw is low (less than 1 amp) then the inverter is not working and needs to be replaced.
On this unit, I was reading less than 1 amp with 120vac present at the input so it was time to replace the inverter with a new assembly. If I had determined the inverter was functioning, the magnetron can be checked with a meter to verify nothing has opened or shorted. A resistance of less than 1 ohm should be read between the two terminals and infinity should be read between each terminal and the chassis. Anything else and the magnetron is bad. Even if the magnetron reads good, if your source components test OK, replace the magnetron.
After I installed the new inverter, the microwave was once again working properly for the customer.
Generic Microwave Disclaimer: Microwave ovens use very high voltage while operating and while the inverter driven models do not have a capacitor present, they still pose a shock risk 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. On models with a typical capacitor/diode set up, ensure the capacitor has been discharged with a screwdriver with an insulated handle prior to touching anything. There is no fun in getting shocked.