Hi again,The fact that a microwave oven uses a magnetron, and a magnetron is a type of valve - reducing the mains to the oven reduces the heater voltage, preventing the magnetron from oscillating. A common fault in microwaves is poor crimping on the push-on terminals for the magnetron heater, and the small loss of voltage caused by the 'high' resistance (a fraction of an ohm) prevents the magnetron oscillating reliably.
The first thing you do when repairing a 'not heating up' microwave (after discharging the HV capacitor!!!) is remove the terminals, give the tag crimps two or three squeezes with a pair of side cutters, and then solder them as well. Then you see if it now works, quite often it does, and if not you start fault finding properly.
That sounds very convincing. The only thing i can add is that maybe there is a critical point that we might reach where it no longer works, but up to that point the output power decreases from 100 percent down, even though not exactly in a linear fashion. As i said i noted that there was a certain voltage, and it was not too low really, where it would no longer cook at all.
It doesnt take much to try though. If we connect a variac to the oven and turn down the power little by little and check each time for cooking power, we see it go down more and more with input voltage and then stop altogether at some voltage like 90v or maybe it as 80v.
I tried cooking hamburger meat and boiling water for testing. Boiling water is a pretty good test because the less the heating power the longer it takes to boil the water and there is a formula we can use to calculate that too approximately.