Microwave stopped heating and I fixed it. Answered
Our microwave is about two years old and it stopped heating. The fan ran and the carousel went around, but food did not warm.
I took it out of the upper cabinet mount and opened the case. I know about the big capacitor and discharged it with a screwdriver after disconnecting the power. For extra safety I clipped a jumper wire on both terminals.
I watched numerous videos at YouTube on troubleshooting a microwave and on testing the various major components at home, but no component showed any signs of failure. Then it began to work again as suddenly as it had stopped working. About ten days later, it stopped heating again. After more testing, still no components showed signs of failure.
But, I did squeeze the female spade crimp connectors a little with a pair of pliers before connecting them again so they fit more tightly when reconnected. The microwave has continued to work as it should. My wife remembers a time a few months ago when she ran it and ran it to get something to cook as she wanted it and the microwave stopped because it was overheated. I suspect a connection was weak and heated the metal in the connector enough that it became weak. Later it failed intermittently.
We could have called warranty service, but we did not want to wait. Knowing what I know now, I doubt a serviceman would have patiently checked for weak connections, but would probably have simply replaced some parts. When those did not really solve the problem, the microwave would have been condemned and replaced. A replacement oven may not have matched our other appliances as well as our present microwave does.
Test procedures for components in a microwave are different than they are for similar components used in (for example) a sound amplifier. This applies especially to the high voltsge diode. Do not connect a voltmeter to the high voltage side of the transformer. When running the microwave, be certain to have a dummy load in the cavity, like a cup of water. A very useful test that checks for power to the transormer primary is to disconnect the primary side transformer connectors and attach aligator clips from a voltmeter to the transformer leads. Then run the microwave for a few seconds and check for a full line voltage reading each time.
A clamping ammeter to measure line current helps because the sound with the magnetron working is not too much differnt from the fan and carousel running without the magnetron working, but, it is easy to see the extra needle deflection when the magnetron is also drawing current.
What seemed to be an expensive problem had an easy inexpensive solution.