A low hum usually means a short on the outboard side of the HV cap - HV diode
or magnetron. May not blow main fuse. A shorted HV cap will probably blow
the fuse.

The HV diode and capacitor don't generally fail slightly. They are
either good or dead.

Since you can run the HV transformer with no load without problems, probably
not that.

Could be the magnetron. Difficult to test without swapping in a known good one
though.

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writes:

Hello all,

YES, I'm -fully- aware of the dangers associated with HV and RF.

The symptom is: about a couple of seconds after heating starts, one can hear
a half second low hum then either heating stops or resumes normally, until
another random occurence.

In case where it abnormally stops, the control panel returns to the stanby
state, just as if the STOP touch had been hit twice.

Of course, I first thought of a bad switch in the door's interlocks, or some
intermitent somewhere. Nope, all looks fine and, by the way it wouldn't
cause the controller to return to stanby.

I have checked as much as possible:
-) the mains part is OK, no loose connection, no dying relay, no burnt
thermal protection.
-) the filament seems in good shape, no worn insulation, connectors fine.
-) the diode works as expected (under a simple 15 V test @ 50 mA).
-) the capacitor looks good, well: under low voltage test.
-) the magnetron has no evidence at all of any arcing, loose internal
filament connection or shortening, as far as I can see or test.
-) the mains and HV fuses are in brand new condition.

The symptom disappear completely when the HV circuit is open by
disconnecting the HV transformer output.

What seems more likely is that when symptom occurs (possibly due to
intermittent shortening internal to the capacitor, hence the little hum) the
HV is brought to its knees and the low voltage transformer being in
undervoltage, the controller somehow "jumps" to iddle. However, I note that
it doesn't go in the RESET state.

Which test do you suggest to pinpoint the defective part more positively?

I've thought of monitoring either the filament's current or the HV. But then
how could I determine whether it is the filament that is arcing or
shortening somehow, or the capacitor, or the magnetron that's the culprit?

I don't think I have a suitable HV load to put in place of the magnetron for
testing the thing in actual operation. Operating the beast with the
magnetron's HV disconnected might create a deadly overvoltage situation if
no load is connected.

The best I can do is "borrow" the capacitor of my other oven (a 0.84
uF/1900V) and try it for a SHORT period. I'm a little reluctant at this, due
to the sligtly inferior voltage rating (but I suspect it can't be hurt
really if test is over a short period).

For anyone asking gory details, my microwave oven is a Samsung type RE576TC
produced in 1989. It is a 1050/600 W (absorbed/RF), is micro-controlled,
uses magnetron Samsung 2M204-M3, a 0.91 uF/2100V capacitor and HVR-1X 3
diode.

Thank you in advance for any hint.
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