on 2/16/2016, badgolferman supposed :
Our GE JEM25DM3BB 1.0 cubic foot microwave oven crapped out recently
and we couldn't find a suitable substitute. The oven functions would
work, but it wouldn't heat. The problem is the microwave fits inside
an enclosure of our kitchen cabinetry. When we moved into the house
eight years ago there was another microwave there which died a couple
years later. We had trouble finding a replacement then, but the GE
barely fit in there. Now that GE model is discontinued and the only
ones with dimensions small enough to fit are 0.7 cubic foot or less.
The wife was not pleased with the choices.

My first option was to cut out the enclosure a bit more to get a normal
sized microwave in there, but a woodworking friend pointed out it would
look funny because the hole would extend all the way up to the bottom
of the cabinet door above it. The wife was not pleased with that
choice either.

My next option was to repair the one we have. After researching
microwave ovens, the troubleshooting guides pointed to the overvoltage
diode and the magnetron as the likely suspects for the symptoms of this
oven. I disassembled the oven and took out the items. The diode was
open in both directions so I immediately suspected it. I checked the
magnetron for continuity and the leads had 0.000 ohms resistance
between them. Hmm, that looks bad too. The high voltage capacitor
seemed to be charging and discharging when I put the multimeter leads
on it. I ordered the parts with next day delivery and the total was
more than $200. Ugh...

When the new parts arrived I immediately ohms checked them and found
the very same results as the ones I took out -- the diode was open in
both directions and the magnetron was shorted. This time I was not
pleased!

Facing no other options I went ahead and installed the diode first and
heated up a cup of water for a minute. Still cold. Next I installed
the magnetron and held my breath while I heated up the cup of water.
After half a minute I noticed steam inside! When I took out the cup
after a minute it was hot. I showed the cup to my wife and this time
she was pleased. The microwave oven is back in the kitchen cabinet and
working fine. Warming up leftovers in a pan or heating up the leftover
morning coffee were good incentives to get this resolved.

I'm still baffled as to why the diode has no continuity. Is it because
my 9-volt multimeter doesn't have enough current to get through the
junction? I don't understand the magnetron properties so perhaps ohms
checking it was a useless test.

My built-in microwave stopped heating after 12 years use. I looked into
fixing it and found the magnatron to be a very expensive part, so I
bought a replacement microwave [Same size same brand same options] for
less than half the cost of a replacement magnatron, and installed the
oven myself. That was two years ago.
Sometimes fixing something is just to satisfy ones ego when a complete
replacement is cheeper.
In your case, a replacement oven wasn't an option. Glad you got it
fixed .