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Microwave advice

OK, I see there is a long history of microwave over repair questions. 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.

Reply to
Mr. Land
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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?

Reply to
Cydrome Leader

The large transformer gives the 2000 volt approx ac high voltage as it 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.

Reply to
Dani

Reply to
Jeroni Paul

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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Reply to
Samuel M. Goldwasser

I just fixed mine. 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...

Reply to
mike

I have new info.

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!

Reply to
Mr. Land

Yep, all resistances among transformer wires correspond to those shown in the block diagram enclosed in the unit.

I don't believe so.

uoted 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! %^)

Reply to
Mr. Land

Find the fuse,it's in line.

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Reply to
LSMFT

quoted text -

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.

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Reply to
Michael A. Terrell

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.

Reply to
Cydrome Leader

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