power but no heat

You couldn't pay me to repair one of these, just to dangerous. I'd buy a new one. But here's what needs to be done:

Cause 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.)

Cause 2

Door Switch

Most microwaves have three or four door switches. When the microwave door closes, the door switches actuate in sequence to ensure that the door is closed properly. If any of the door switches fails, the microwave will not start or heat. To determine if any of the door switches are defective, use a multimeter to test each of the switches for continuity. If any of the door switches does not have continuity, replace it. (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 a door switch.)

Cause 3

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.)