I am working on a microwave oven that starts to emit an odor after 2 minutes of full heating despite otherwise working correctly. The smell is the same as that "sweet", overheated coil varnish smell that anyone with much electronics experience knows well. It seems to be coming from the magnetron (inside the area where all the electronics are mounted, not the cooking cavity).
When I measured what seemed to be low voltage going to the magnetron, it dawned on me: Since magnetrons are somewhat mystical in their operation to me, is it possible that a lower voltage could cause them to overheat? Would it matter if the electrons took a different path inside the tube?
To assuage responders' questions about my methods, the following details are presented.
The magnetron's wave-guide looks clean via a bore-scope and the cooling fan spins.
With a multi-meter, I tested the magnetron, high voltage (HV) transformer, HV diode and HV capacitor while each was disconnected; all test fine that way.
I swapped, one at a time, the HV capacitor and the wave-guide cover, but the smell recurs after the 2 minutes. I replaced the magentron, too, and the smell changed slightly but was still present.
I can't see the HV diode causing such an issue, but I may try swapping it next.
I have an old, 30 kV, analog, DC voltmeter that I used to test the HV circuitry (I don't know of its accuracy). According to the oven's service manual for the oven, approximately 2400 volts should come from the HV transformer which is then "doubled" to about 4000 V and then fed to the magnetron. When disconnected, I measured 3000 V (maybe less) on the lead that is normally connected to the magnetron.
A new, replacement HV transformer is not available and a good used one may be hard to come by.
(If relevant, I remember replacing the HV capacitor years ago and noticed that that capacitor is rated for only 2100 V [what the appliance parts shop ordered for me back then]--I do not recall the original working voltage of that capacitor and it is not in the manual.)
For those who are curious about the end result of the repair, I fixed the magnetron overheating smell with an exact replacement, HV diode change. I didn't expect this. Again, the old diode behaves normally on a meter, forward-biased vs. reversed (though it measured 1/3 lower resistance in the conducting direction than did the new diode--I assume it has gone out of spec.). But I now measure 1 kV DC with the magnetron connected (versus 1.5 kV DC with the old diode), 2 kV DC when it's disconnected (versus 3 kV DC with the old diode) and there is no smell even after several minutes of full-power use. Thank you all for your helpful comments.