JJ wrote:

We have a GE built-in microwave above the range and all
of a sudden the cooking times needed to heat food have
drastically increased. Is it possible for a microwave
to gradually wear out? This unit is only 7 yrs old.

The maximum output of a magnetron tube drops about 1-3% a year in
average use, probably because the coating on its cathode wears out and
its permanent magnet weakens (sometimes even cracks). You can measure
the power of an oven by heating some water and measuring the amount of
temperature rise.

Fill a glass or plastic container with exactly 1 quart of cold tap
water, and measure its temperature in fahrenheit by stirring a
thermometer in it. Call this temperature T1. Set the microwave for
full power (100% power), and heat this water for exactly 60 seconds.
Immediately measure the temperature again by stirring, and call it T2.
The power in watts = 37 x (T2-T1).

For the metric version, measure temperatures in Celcius and use exactly
1 litre of water. Power = 70 x (T2-T1).

Most magnetrons are warranted for 5-10 years, probably parts only, but
someone _very_ familiar with electronics repair could change the
magnetron tube for as little as $35-50, provided it's bought from the
right source (prices vary greatly, MCM Electronics is good). For
another $25-30 the high voltage capacitor, high voltage diode, and
magnetron thermal cut-off can be replaced (worthwhile with an overhead
oven, which is hard to remove and reinstall). This work is dangerous
be cause magnetrons work at thousands of volts (capacitor can retain
voltage indefinitely after AC cord is unplugged), and an incorrectly
installed magnetron tube can let strong microwaves leak past its brass
braid gasket (can cause cataracts, even skin burns). Do not operate
the oven until its metal cover has been completely reinstalled, to
protect against an exploding capacitor or the worst of the microwave
leakage. See www.repairfaq.org for more information and precautions.