(Bob Ladbury) writes:

Thanks Sam for the quick response. I believe the charring was indeed
over the mica/waveguide cover (there's a large high voltage
transformer right behind this part of the sidewall). The cavity damage
is cosmetic. I was mistaken that it created a hole in the cover, it
just melted a bit, but didn't go through. It's a 1000w tabletop unit,
and I have melted cheese and crackers dozens of times before over the
years in it, nothing like this happened, so I'm still not sure why it
did this time.

But let's say the unit works and heats up water, is it still necessary
to have it serviced? I don't want any risk of stray microwave energy
hitting me when I'm near the device, due to a damaged oven. But with
the cost of a service technician, I'd just as soon buy a new oven as
to have it serviced.

If only the mica thing cavity paint job is damaged, then no danger.
Sand down the cavity smooth and repaint with some non-toxic enamel
or microwave cavity paint if you're a purist. The cavity cover
can be replaced for about $6 for a sheet you cut to size.

First, test to make sure it actually still works.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
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