Microwave will heat water but not food.

Please make sure your life insurance is paid up and check to see that working on this is not considered a suicide attempt. Personally I won’t touch one except to change a light bulb or fix a door as I have been knocked across the room just from a CRT discharge.

Solution 1:

Diode

The diode converts the A/C power output of the transformer to D/C, doubling the voltage to nearly 5,000 volts. This high voltage powers the magnetron to heat the food. If the diode burns out, the magnetron won’t receive enough voltage to operate, preventing the microwave from heating. When the diode fails, it is often visibly burned out. Inspect the diode and if the diode is burned out, replace it. If inspecting the diode proves inconclusive, test it with a multimeter that uses a 9-volt battery or put a 9-volt battery in series with the diode. When the leads are reversed, the diode should only have continuity in one direction. (Warning: The microwave oven can store a lethal amount of electricity in its high voltage capacitor, even after the microwave oven has been unplugged. Due to the high running voltage and the potential for electric shock, it is extremely dangerous to replace the electronic components in a microwave. Only a licensed technician should replace the diode.)

Solution 2:

Magnetron

The magnetron uses high voltage, high current DC power to generate the microwave frequency that cooks the food. If the magnetron burns out, the microwave won’t heat. The magnetron is not repairable—if the magnetron is burned out, you must replace it. (Warning: The microwave oven can store a lethal amount of electricity in its high voltage capacitor, even after the microwave oven has been unplugged. Due to the high running voltage and the potential for electric shock, it is extremely dangerous to replace the electronic components in a microwave. Only a licensed technician should replace the magnetron.)

Solution 3:

High Voltage Capacitor

The high voltage capacitor works with the high voltage diode to convert the output of the transformer to DC voltage and to double the output voltage. If the capacitor is burned out, the entire high voltage circuit will stop working properly, and the microwave won’t heat. To determine if the high voltage capacitor is defective, use a VOM meter with a capacitance testing capability. (“Warning: The microwave oven can store a lethal amount of electricity in its high voltage capacitor, even after the microwave oven has been unplugged. Due to the high running voltage and the potential for electric shock, it is extremely dangerous to replace the electronic components in a microwave. Only a licensed technician should replace the high voltage capacitor.)

Solution 4:

High Voltage Transformer

Microwave ovens produce a very high voltage in order to power the magnetron antenna, which emits the energy that cooks the food. When a high voltage transformer fails, it will usually arc and have a burning smell. (Warning: The microwave oven can store a lethal amount of electricity in its high voltage capacitor, even after the microwave oven has been unplugged. Due to the high running voltage and the potential for electric shock, it is extremely dangerous to replace the electronic components in a microwave. Only a licensed technician should replace the high voltage transformer.)

Solution 5:

Main Control Board

The main control board might be defective. However, this is rarely the case. Control boards are often misdiagnosed—before replacing the control board, check all of the more commonly defective parts. If you have determined that all of the other components are working properly, replace the main control board. (Caution: The microwave oven can store thousands of volts of electricity in its high voltage capacitor, even after the microwave oven has been unplugged. Due to the potential for electric shock, it is extremely dangerous to replace the electronic components in a microwave. Only a licensed technician should replace the main control board.)

I agree with Mayer that if you are going to open a MW with it plugged in, or going to test the HV with your finger instead of adequately discharging the capacitor, then ONLY a technician should work on anything but the light bulb. HOWEVER, if you can follow directions to be safe, replacing electrical components - switches, HV diode, HV capacitor, control panel (not cheap), turntable motor, fuse - is a piece of cake, once you get the cover off.

The magnetron and HV transformer are another story and also aren't cheap. In your case, it is either, the HV cap, the HV transformer, or the magnetron. The reasoning is....If it were the HV diode you would get nothing since there would not be any high voltage to run the mag. If it were the HV cap, you would get a reduced effective HV, which would reduce the output power of the mag. Likewise with the HV transformer. If it were the mag, you would get either reduced power or the wrong output frequency of the microwaves which translates into less heat power.

Economically, replacing the transformer or the mag is not worth it, especially since it would probably require a service company unless you have the technical skills.

Without special equipment, the only way to test the cap is to replace it. Since they are not very pricey, it is worth a shot.

One other item you might want to observe is if the power setting is less than 100%. If the sound changes periodically when it is running, it is going on/off and the power isn't 100%. In that case, you have a control board issue if you didn't set lower power.