Hi!

When does a microwave oven require a service - and what can be
serviced anyway?

When the bulb burns out. :-)

In all sincerity, microwave ovens are very reliable pieces of equipment. It
is not uncommon for one to go 20 or more years with no reduction in
performance. Things that might require service include the moving parts,
such as the fan or turntable (if equipped) motor/drive system. Since there
is a fan inside most ovens, it stands to reason that dust will be pulled
inside the oven. This should be periodically cleaned out. Grease from things
such as bacon will eventually plug air passageways in the oven, but this too
can be cleaned up.

Other things can go wrong--the relay or triac on the controller board could
fail, units with mechanical timers could have the motor or gear train jam or
fail, and the magnetron can go weak or bad. These are all relatively
unlikely to happen.

Replacing the bulb is unlikely to be terribly difficult. Some ovens have an
access door for the bulb. (I have a small GE microwave oven from 1986 that
has a door on the side that can be unscrewed.) Others may make you take the
top cover off to get at the bulb. (I have a 1985 model large Sears Kenmore
(Sanyo) oven that is like this. It uses a strange type of bulb with a
"permanently" fastened socket. I was able to get a new bulb, but haven't put
it in yet.)

The Kenmore microwave recently had its internal fuse die of old age.
Replacing it with a new fuse fixed it, so maybe I'll replace the bulb while
I'm in there.

William