, this counter top microwave started heating intermittently for the customer and when it did heat, it seemed to take longer than before. Microwaves generally work or not work, so the intermittent part of the problem had me interested. But once I started a cycle myself to see what was going on, the telling sound of a high voltage arc led me right to the problem.
Microwaves use very high voltages to the magnetron for the purpose of creating an oscillating energy wave which is used to cook items within the oven cavity. This high voltage is produced by stepping up our 115vac house voltage using a transformer, then rectifying the AC voltage to DC voltage by using a diode, and finally doubling this voltage using a capacitor. All three components along with the magnetron, are considered part of the high voltage section of the microwave, and unless you are very comfortable working around electricity, none of these components should be touched without taking your personal safety into account.
Since I had heard an arcing sound, I removed the ovens case and while standing a safe distance from the unit, I started another cycle and found the source of the noise at the capacitor. After unplugging the unit, I used my screwdriver with an insulated handle to short across the capacitor terminals to verify that it had properly discharged (good habit to get into BTW). In doing so, I located the source of the noise, and the reason for the intermittent operation as the connection between the diode and capacitor.
The end of the diode has a terminal connector that provides a positive, low resistance connection to the spade terminal of the capacitor. This connector, however, was not properly connected allowing for voltage to build up and jump or arc from the terminal to the connector. Each time this happened, the magnetron was not getting it's necessary source voltage and as a result, would intermittently heat.
After removing the diode from the circuit, I was able to repair the connector with my pliers, and cleaned the carbon burn marks using my jewelers files. It was fortunate the customer called for service quickly because had they waited much longer, the diode most likely would have suffered permanent damage and would need to be replaced. After cleaning and reassembly, the microwave was once again working properly.
Standard Microwave Disclaimer - if you don't know what you are doing, DO NOT remove the cover or control panel of your microwave. A high voltage shock can not only hurt, it can kill you.