I have an older, 1998 GE Microwave oven. By the LG parts inside,
probably just a rebadged LG. Lately, it's tended to 'thunk' when the
magnetron turns on. A few times, it started rapidly cycling. Once, I
didn't stop it in time and it blew it's 20A fuse. Another time, just
starting normally, it tripped a circuit breaker.

I opened it up and checked. So far, the only obvious thing I can see is
that the large diode between the transformer and ground may be open. I
get infinite resistance in both directions. I assume there is nothing
special about testing these types of diodes?

It's an HVR-1X diode. This appears, based on some Googling, to be very
common, and the HVR-1X3 seems to be a common replacement.

Do the symptoms I describe seem to go with the way the microwave is
acting? I'm not certain just what the true function of the diode is in
this case (I know how a diode itself works). There is also what looks
like a starter cap. It's rated at 1uF, and my meter says 1.15uF or so.

Do you think it's worth trying to replace it? I did buy a replacement
microwave, but regret it. It's basically the same, but has some issues I
don't like (display too far back from front, so you have to squat down
to see it while typing on the panel. Fan goes REAL fast when magnetron
off, then seems to nearly die when it's on.)

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