In sci.electronics.repair Danny D. wrote:
I just tested my microwave diode where, unfortunately, it seems to be good:
http://farm8.staticflickr.com/7358/9...dac4708b_o.fig

Forward biased with a 9 VDC battery, I measure 7 VDC across the diode;
reverse biased, I read 9 VDC across the diode (given a 200 ohm
current-limiting resistance in series with the battery).

I was hoping the diode was bad, because otherwise, I don't know what's
wrong with the microwave. It does everything but heat up the food.

It makes noise, the lights light up fine, the controls all work,
and it beeps when done, etc. - but it just won't heat up anything.

Did I test the diode properly?

Sort of. A 9 volt battery isn't really a good test for a high voltage
recifier. The forward drop will exceed that of that battery, at least a
rated current of 0.3 to 0.5A, depending on the diode and oven.

The results of your test though, show the diode isn't open and isn't
shorted, so that's good. I'd not bother to replace the diode at this point
of the oven was in front of me.

I'd test the heater coil of the magnetron. It should measure close to dead
short ohms if it's good. If it's open, well, that explains why there's no
heat. You'd also want to make sure the transformer is even being turned
on, you can usually tell this from just listening to the oven or watching
lights flicker when the oven turns on.

Be sure you discarge the cap before you mess around in a microwave oven.

As mentioned in this thread by others, a normal volt meter cannot be
safely used to test the HV section of a microwave. You really don't even
need to for the most part either.