Microwave advice
Mr. Land
But I don't seem to see a thread that has my question:
So I have an older GE JVM1190 over-the-stove microwave unit.
Symptom is no heat.
When powered up and programmed to cook, the unit makes the expected
humming noise (the same it's always made) but a cup of water in the
chamber will not heat up.
I've studied the microwave FAQ repeatedly and I believe I understand
the dangers.
I constructed a long wooden-stick-based discharger for the HV cap,
discharged it (didn't get any spark at all), then ensured there was no
residual voltage on either of its terminals. Then I shorted the
terminals.
First I "tested" the magnetron: infinite resistance from either
cathode connection pin to ground, very low resistance between the two
pins themselves. Seems OK.
Then I tested the HV diode by placing it series with a 390 ohm
resistor and applying 15 VDC, and meauring the voltage drop across the
device. -15V negative biased, about 10 VDC forward biased...this
seemed to be within range.
Finally I tested the HV cap. It reads infinite resistance to the
chassis from either terminal. Between the terminals my capacitance
meters reads around 0.86 uF... which seems to be correct.
I've heard mention of a possibly bad HV fuse. AFAICT, this unit
doesn't have one of those.
So...the HV cap seems good, the mag seems good, the diode seems good,
the unit seems to draw appropriate current when in cook mode,yet the
over won't heat food.
What am I missing???
Could the magnetron still be "bad" despite the fact that it doesn't
read shorted/open?
Thanks.
Cydrome Leader
Is the transformer secondary good? check its resistance from the HV
terminal to the chassis. Open = bad.
Is that microwave old enough to use a reed relay for defrost mode?
Do you have a way to measure the HV from a microwave oven safely?
Dani
gets about 120 vac input. A relay controls this. You could have a bad
connection now burned at the relay output, or more likely a weak
magnetron. If you have the 120 vac to the input of the big high
voltage transformer, your magnetron is likely bad, considering your
measurements. Please be carefull. Hope this helps.
Jeroni Paul
connectors to the magnetron were somewhat oxidized and did not make
proper contact. I could not see oxide but I was measuring a few ohms
of resistance in the contact. I cleaned both surfaces and it worked
again.
Samuel M. Goldwasser
As someone else suggested, double check the continuity of the filament
circuit - transformer and magnetron. Since that's high current bad
connections are quite possible.
However, the unit would draw less AC current if the magnetron was not
getting filament power.
But yes, everything could seem to check out but the magnetron could still be
bad.
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mike
The push-on wire connection to the interlock switch had developed
some resistance, melted the wire and trashed the connector.
Had the same problem at the wire from the power cord to the main switch.
It hadn't failed...yet...
Mr. Land
Firstly, though THANK YOU ALL for taking the time to try to help. I
appreciate it.
So....I have replaced the magnetron in this before. AND, I read that
there seems to be a rash of
problems with these magnetrons.
But, the new info: my wife told me she'd heard the uWave emit this
"loud arcing noise" just before it
stopped heating food.
I couldn't find any evidence of arcing within the HV section itself,
however, when I removed the magnetron and
inspected the "antenna" (?) it looked as though it had been arcing:
there is a brass-colored conical tip that's flattened at the top
with a hole in it. That extends down towards the body about 1/2", then
there is a violet-colored band of some
material I can't identify...it appears to be some sort of insulating
material. Then under that is the RF braided gasket.
Well, where the purple insulating bushing and the copper tip
meet..there is a slight blackening and pitting...as
though arcing had occurred part of the way around that seam.
Does it make sense that the magnetron would arc there?
Ordering a new one tomorrow...
Thanks!
Mr. Land
Yep, all resistances among transformer wires correspond to those shown
in the
block diagram enclosed in the unit.
> Is that microwave old enough to use a reed relay for defrost mode?
>
I don't believe so.
> Do you have a way to measure the HV from a microwave oven safely?- Hide quoted text -
>
I have a TV HV probe, but that's not safe...to use it with neg voltage
I'd have to reverse it, but then
the working end wouldn't have anywhere near enough HV insulation/
standoff...it'd be just an alligator
clip...no thanks! %^)
> - Show quoted text -
LSMFT
Find the fuse,it's in line.
--
LSMFT
I haven't spoken to my wife in 18 months.
I don't like to interrupt her.
Michael A. Terrell
Some HV probes have a polarity switch, but you could add a small full
wave bridge for the meter movement so that it doesn't matter.
--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.
Cydrome Leader
yeah, that's pretty common. I don't recall why it happens though. The
microwave here at work died that way, I still have the magnetron laying
around.
It's possible it could have been cleaned up and used again, but a new $100
microwave oven showed up instead.