Bob Simon wrote:

Can you give me some idea how much it might cost to replace a
magnetron? For the record, the microwave is a Panasonic NN-S760WA.
1300W. Counter top unit. Manufactured in May 2000.

Before I go shopping, I'm going to take it apart and look for a blown
fuse. Is there typically a big capacitor I should watch out for?

I wouldn't repair a countertop microwave-only oven because a
replacement is only $100-150, and keep in mind I'm so cheap that I own
a 28-year-old TV I've repaired a few times.

The magnetron may be fine but something in the power supply or control
circuitry may have failed. Panasonic has produced some ovens that
rapidly cycle the magnetron on and off for better control of the power
(most ovens simply use a relay and turn it on and off every few seconds
-- not good when the cooking time is just 20 seconds), and these ovens
have been much less reliable than average, maybe because a high-power
triac or SCR burns out in them. I would see if Panasonic has some type
of extended or secret warranty to cover these failures.

There is a large capacitor, approximately the size of a pack of
cigarettes, that can store over 2,000 volts indefinitely after the AC
cord has been unplugged, and it must be discharged to the chassis
ground before you can safely work on the oven. See www.repairfaq.org
for information. The oven's circuitry has a bleeder resistor to
discharge it, but you must always assume that this resistor has failed
and must manually discharge the capacitor to the chassis ground. This
can be done with a plastic-handled screwdriver by connecting the bare
end of a wire securely to some bare metal on the oven's chassis,
wrapping the other bare end several times around the screwdriver's
steel shaft, and only then touching the screwdriver blade to each of
the capacitor's electrical terminals. Of course, unplug the oven from
the AC before doing this, wear rubber-soled shoes, and keep one hand in
your pocket to minimize the risk of completing an electrical path that
goes through your heart and diaphram.

A new magnetron tube can cost as little as $35-50 (try MCM
Electronics), if the oven can use a common Goldstar-style tube (even
Sharps do), but may be $100 for other styles, and factory authorized
suppliers may want $200. I would also replace the high voltage
capacitor, high voltage diode, and thermal cut-off (mounted on
magenetron), about $25 total.

The magnetron tube is held in place with bolts or nuts and has a brass
braid sealing ring around its center to prevent microwaves from leaking
out. Be careful not to damage this ring on the new tube, and tighten
the nuts or bolts properly (not loose, not too tight, tighten all
evenly) so it forms a good seal. Be sure to reconnect all the wires
for the magnetron, diode, and capacitor so they don't arc and start a
fire.

The magnetron is cooled by a fan, and it should be checked by
disconnecting its terminals and applying the correct voltage to it.
Most work at 120VAC, but some may operate from a lower voltage or even
from DC. A bad fan will cause the new magnetron to burn out quickly.

Do not rely upon cheap microwave leakage detectors because they're
extremely unreliable and have been known to indicate great leakage even
when the magnetron was not operating or no leakage at all even when
sitting inside the oven. Detectors made with liquid crystals (LCD,
peel & stick against perimeter of door and against rear vent grill) or
fluorescent lights are more reliable, but I don't know how sensitive
the LCD type is, and the fluorescent type won't glow visibly except
when the leakage is very high and the room is dim.