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Sears Microwave stopped Cooking
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4 Feb 2006 10:23:41 -0800
sci.electronics.repair
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GT...
James Sweet...
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Magnetron failures are quite rare, I've never actually encountered one.
Much more likely there's a bad connection in the controller that
switches power to the transformer, or a bad capacitor or diode.
Sam Goldwasser...
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Or even a bad connection at the magnetron filament terminals.
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Sam Goldwasser...
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You could check the INPUT to the high voltage transformer to see if the
microwave generator is getting power when it should be on.
WARNING: Microwave ovens are by far the most dangerous home appliance or
electyronic equipment to be troubleshooting!!! Make sure you understand
ALL the safety issues!!!
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Very unlikely.
A magnetron will usually fail it two modes: filament opens or emission
drops too low. In the former case you can check it easily with an ohm
meter, a tube with low emission will work but won't heat as much and
it fails over a long period of time.
Bad filament (or filament winding on the transformer, or bad filament
connection) and you'd have high voltage DC at the filament, but no
microwaves would be produced - and your turntable would probably turn.
The magnetron only has a few internal parts - a filament to emit
electrons and a plate with slots/cavities cut in it. Electrons leave
the filament and are attracted to the plate, before they get there,
the magnets bend their path so they cruise by the cavities. The
cavities resonate and produce high frequency radio waves. There's an
antenna probe to couple energy out of the tube and guides it into the
oven chamber. Very simple; not much to go wrong.
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Bought in 1999 it is a microwave/oven unit.
I have checked the following parts & they have readings on my ohm
meter.
Door switch sensors, upper & lower. There are two separate sensors for
the bottom switch. They read totally different from each other
depending if door is open or shut.
HV Capacitor
Replaced Diode (connecting to capacitor) could not get a reading on ohm
meter & guy at parts store said this is normally why they stop heating.
Magnatron
Fan runs
Heating sensors (one on top of magnetron & other just inside unit)
Bad: Turntable motor, I took it out & it had an infinity reading.
Question what else is there to check to see why the microwave is not
cooking?
Thanks for any insight
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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 there:
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.
James Sweet...
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I've seen a transformer failure in a microwave exactly once, it produced
plumes of nasty smelling smoke from the winding insulation burning, it
was pretty obvious.
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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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Garry
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Like Sam says, check the input to the primary of the transformer. If
you don't have line voltage there (when it should be running) nothing
will heat. If you don't have line voltage it means one of the safety
devices (over heat, fuse, interlock switches etc is open) or the
controller isn't calling for heating.
1999 means it probably has a micro processor controlled clock, timer,
heat modulating controller. Generally cheaply made, but reasonably
reliable.
How did it fail? Just quit or died over a period of time? Quit
suddenly - check the magnetron filament for an open (check the primary
to the HV transformer FIRST for line voltage). Keep your eyes open...
any evidence of lightning strikes? (clock and microwaves seem to be
the first to go)
If you can find that the controller isn't calling for heat when it
should be, then you might (if you have the skills to stay safe) bypass
the controller and see if it heats a cup of water.
Dying over a period of a year or two - probably a bad magnetron
(rarely a problem - but possible - with the cap or diode) That's
where current and voltage measuring come in handy.
I've fixed a few microwave ovens - it is frustrating having to deal
with symptoms when you can't actually check things (like current to
the magnetron or high voltage).
Most of the problems I've had are safety interlock problems (door
switches, or over temp fuses that opened because some food occluded
the mica window that lets the waves in), two controller problems (one
that I could fix - the other in a proprietary chip that was too
expensive)
Like Sam says - worry about your safety. My own practice is to keep
one hand in my pocket, my feet on an insulator, and one hand to work
the test probe. The very idea of discharging a cap in a microwave
give me the willies, and I worked on megawatt CW transmitters in the
Navy, and play with Tesla coils at home.
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