Astriapo writes:

Sounds like one of the thermostats inside the case is bad. Could also
be a bad transformer.

Or an intermittent filament connection to the magnetron.

Put it on 50 percent power and listen carefully for the sound as the
magnetron kicks in. There should be a subtle change in the hum as
the filament heats up and the magnetron starts drawing power. I bet
that will be absent once it stops heating. Of course, could also
be a bad transformer, relay, other wiring, etc. This should be
repariable relatively easily though. Inexpensively unless it's
a bad transformer or an intermittent filament inside the magnetron.

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On 1 Aug 2003 19:32:50 -0700, (Beanie) wrote:

Okay, so I've tried everything possible to get a microwave that was
picked up off the street to work. It's a Kenmore 565.8925590, 1300W,
serial 1V7C01490, manufactured September 1991. The magnetron and
controller IC are made by Sanyo, so I imagine that the entire oven
was, too. So, here's the symptoms the microwave exhibits:
The microwave will happily begin to microwave a cup of water. After
about a minute or so, the fan and the turntable will stop. The light
stays on, and the counter keeps going. If I stop the microwave cycle
and let the oven cool down, I can start it up again, for about another
minute. So, I opened it up, and checked the usual suspects (fuse,
interlock switches, etc.) The primary interlock switch was always
open, so I replaced it. That didn't fix anything. All of the
switches appear to be making good contact with the actuators on the
door. There are two thermal protectors. Both offer a resistance of
zero, and according to the schematic (and the actual wire routing) if
either opened, it would kill power to the entire oven--so I know that
they are not at fault here. The magnetron does become hot to the
touch, but it's not _that_ (135 degrees centigrade) hot.
As far as I can tell, the only thing left to be at fault is the
relays, that are located on the controller board. I'm a little leery
of testing the relays on it with the oven live--especially because the
warning labels clearly state that the step-down transformer will give
me a jolt. :-) There is a burn mark on the PCB in the center, but the
resistors, diodes, and caps in that area all test fine.

Any suggestions?

Thanks!

--Chris