Lots of good information but NOT the jost important: SAFETY.

Don't even think about trying to probe the output of the HV transforme or
anything connected to it when the power is on. Experianced technicians have
been electrocuted doing that. Bleeder resistors can fail as well.

Microwave ovens are by far the most lethal of consumer appliances and
electronics.

Much more safety and troubleshooting info in the Microwave Oven Repair
Guide and other resouces at the Web sites below.

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default writes:

On 7 Feb 2006 13:17:38 -0800, "GT" wrote:

My capacitor dissipates w/i 30 seconds of being off, thank goodness.
I have checked the transformer for incoming power & it reads 115 vac,
cqn not get a reading for outgoing voltage.
I disconnected the leads going to the mag & it had ohm readings & not
infinity.
I checked the interlocks & they all seemed to reading correct ohms.
I had the guy at the parts store check the capacitor & he said it was
good.
I bought a new diode.
Bought new turntable motor. Still NOT turning after replacing with new
one.
It seems that I have checked everything I know.
I can't buy a new one because this microwave is in the same cabinet as
the oven as one unit. I did buy a cheap replacement but I was hoping
to repair this one so the entire unit would work & we could clear up
shelf space.
Thanks

The capacitors usually incorporate a bleeder resistor so they don't
hold a lethal charge indefinitely.

Regarding the transformer . . . you can't really have zero out for 115
in, so there's a couple of things to check the

The output should be on the order of 2,000 volts on the secondary side
- so your meter has to be able to read that high without damage.

The next thing to check would be the resistance of the primary winding
of the transformer - unplug the oven let the cap discharge. Disconnect
the primary wires from the transformer (you want to read just the
transformer and not the light, motor, fan, etc..) and check the
resistance, it should be close to zero ohms (so close as to be
indistinguishable from zero with most meters).

You could have a transformer with a shorted secondary or diode to give
you zero out with 115 in BUT I've yet to encounter a failed
transformer - I'm sure it happens, but it must be rare.

Your turntable motor still doesn't turn? That would lead me to
believe you're doing something wrong with your test procedure. It is
somewhat unlikely you have two coincident failures in the same
appliance.

Most of the motors I've seen are either "shaded pole" or a type of
synchronous motor (turns in either direction) - but they run from 115
volts just like the transformer. That's what suggests the problem is
upstream to the transformer and your reading of 115 on the primary is
incorrect.

Back to square one:

There are a series of safety interlocks that have to work for power to
be applied to the microwave - they all have to read zero ohms - that
is the "correct ohms." Zero on the lowest ohms range of your meter
(zero or as close as the meter reads when the meter leads are shorted
together) Zero for switches, zero for fuses, zero for over temp
cutouts . . .

A very few microwave ovens also sense airflow past the magnetron tube
before HV can be applied. Another switch - with a vane the size of
two postage stamps located in the air path or exhaust of the oven.
I've only seen them in commercial ovens but some home units may also
have them.

Read the switches individually if that can be done easily. Then, if
you can, check the whole string - they are usually wired in series and
all have to make (close) at the same time for power to get to the
oven.

Don't overlook things like a loose mounting or bent arm on a switch
that might prevent it from closing when the door (for instance) is
closed. Some microwaves incorporate two door switches just in case
one fails.

One last caveat - some interlock switches are "micro switches" (a
brand now associated with generic snap action switches). Some are
single pole two position switches - they only need one position
(normally open, as a rule) in a microwave but it is usually cheaper to
source two position switches - make sure you're checking the correct
switch position. For safety they are normally open and mechanically
held closed to operate.

When you think you've checked everything and it isn't working - you
must have overlooked something or the test you did was not definitive.
Try to approach a problem as if you're looking at it for the very
first time.

Microwave ovens are simple as schematics go - not that much to go
wrong. You have a lot of safety devices that all have to function, a
controller/timer, a nearly indestructible current limited transformer,
feeding a diode/capacitor voltage doubler feeding a magnetron tube.
The tube just wants to see high voltage and filament voltage to
produce microwaves.
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