25 Oct 2003, 08:16, Jasen Betts (3:640/1042), wrote to Matt Mc_Carthy:

Hi Jasen.

 JB> whaty do you reckon would cause one to loose power ... it takes 
 JB> about three minutes to boil a glass of water that seems a little 
 JB> longer than it should be... do magnetrons fade? or could it be the 
 JB> capacitor?

Lots of variables, but for starters, three minutes is about correct for a
full 8oz glass of water.  That was the setting we used for years on my
daughter's large cavity, medium power microwave oven to warm her dog's
water for its food (dog has no teeth).  We needed the water right where it
was beginning to boil.

Her new microwave oven is still medium power, but has a much smaller
cavity.  The setting now to heat the water to the same point is 2.5
minutes.

One variable is the beginning water temperature.  During Winter here, our
tap water is about 12 degrees C, while in Summer it runs 25 degrees C or a
bit more.

Second variable is the cavity size of the microwave oven.  A large cavity
oven wastes a lot of the microwave energy by bouncing all over without
actually hitting anything that will absorb the energy.

Third variable is ambient temperature and airflow.  There are several
thermostats on the magnetron that prevent it from overheating.  If the
airflow is even slightly restricted, you may not be getting full-time power
cycles, but wasting time waiting for the magnetron to cool and cycle on
again.

Fourth variable is another thermostat that senses the temperature of the
cooking cavity.  If you were to heat six glasses of water consecutively,
the later ones would likely take longer than the first one, as the cavity
thermostat will sense the heat build-up, and may open.

The thermostats are easily checked by putting two to four liters of water
in the cavity, setting the timer for about six to ten minutes on max power
with an ammeter on the power line.  If no thermostats open, your Amps
should hold pretty steady for the duration, with minor 'flickers' when
other appliances turn off or on.

If you see any major Amp drops, one of the thermostats is likely cycling. 
Could be a dirt buildup in one of the fans or the cooling fins of the
magnetron itself, or on the exit grill(s), preventing the warm air from
getting out of the system.

If the oven works at all, the capacitor is good.  The capacitors are oil
filled and don't leak with age like the radio/TV/computer versions.  The
capacitor is the main part of the oscillator circuit, it works, or it
doesn't.

All that aside, the magnetron DOES age over time as the cathode looses some
of it's coating, but this is usually very slight and ends when the cathode
melts down and shorts.

     Good luck...  M.

--- Msged/386 TE 06 (pre)