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Protector01

GE JKP90 Microwave burning odor - transformer primary?

8 posts in this topic

Greetings!

 

I'm working on fixing the microwave on my GE JKP90 microwave/oven combo.  It develops a strong odor of burning insulation within seconds of operating - yet it is working.

 

When I opened it up, I noticed that the black lead that connects to the primary winding of the transformer was brittle and crisped - it seems that this is the source of the odor.  That made me think that the transformer was drawing too much current to make the black primary lead burn up close to the transformer.  I assumed that the transformer itself was bad - there is what looks like a burned area on the secondary coil of the transformer, so I thought that the secondary of the transformer had shorted out.

 

The measured primary resistance is 0.6 ohms (test table says it should be between 0.2-0.4 ohms), the secondary is 113.5 ohms (measured to ground - test table says it should be between 60-90 ohms), and the filament resistance is 0.4 ohms (test table says it should be less than 1 ohm).  The primary and filament windings show infinite resistance to ground.

 

So, with the resistance measures for the primary and secondary being outside of the ranges listed by the test table, I ordered a replacement transformer.  After installing the new transformer, the symptoms are still the same...the microwave operates correctly, heats water as expected, but develops the burning odor within seconds of operating.

 

Has anyone seen something like this before?

 

Any ideas on how to proceed are most appreciated!

 

Thanks - Joe

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I don't quite know how to describe it, but if it's kind of a sweet tangy burning smell, then it's the mag tube.

 

I don't work on microwaves much any more so I can't remember if the ones that had that smell still heated or not or just heated but not full power like they should.

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For a GE the output test is 1 liter of water (they specify a GE plastic beaker, I doubt it matters) heated for 2:03 should rise at least 40 degrees F. 

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Sensei - Thanks for the comment!  I don't know if the magnetron is at fault - I don't detect a sweet tangy odor.  It's more the odor of burning insulation - and when I look at the components, the black lead that connects to the primary of the transformer is really overheating - just like if the primary coil was drawing too much current.  The weird part is that the microwave is operating - it is heating water.

 

Sohei - I didn't run the specific GE test.  for one thing, I don't want to run the microwave longer than a few seconds, as the burning odor starts up within a second or two when the microwave is operating.  While I only used about a cup of water in a plastic container for the test, it is noticeably hotter after just a few seconds of operation.  While it's not the official GE test, it does show that microwaves are being generated - perhaps too many of them!

 

Again - thank you both for your comments!

 

-Joe

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For a GE the output test is 1 liter of water (they specify a GE plastic beaker, I doubt it matters) heated for 2:03 should rise at least 40 degrees F. 

 

And just in-case anyone is wonder why the odd extra 3 seconds - that is the time it takes the mag tube to power/oscillate up to full power, (if you listen closely after pressing start on a microwave you can hear the mag tube power up with a light buzz noise).

 

-----------------------------------------

 

Protector01, if you're smelling that smell after only a second of running I can pretty much guarantee you it is the mag tube.  Unplug the two wires going to the mag tube and make sure the are well away from everything and then power up the microwave. 

 

With a high 99% probability, I don't think you will smell that smell anymore - of course it won't heat either.

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I know that I'm resurrecting this prob the past, but as I was finally able to order and replace the magnetron, I wanted to report back.  Unfortunately, the problem remains.  Crap.

 

The microwave starts up, the fans run, then 3 seconds later the microwaves are emitted.  After 15-17 seconds, the smell of burning insulation starts up again.  I know that microwaves are being generated, as the test water heats up noticeably.

 

So I dug a little deeper into the microwave - and it looks like some other wire connections had also gotten too hot.  One of the plugs on the noise filter (Part number: WB02X11243

Part number: WB02X11243

) was burnt, as well as one of the leads on the thermostat (I think - it's mounted to the outside of the microwave cavity, and opposite from the magnetron).  I did not notice those before due to how they were hidden within the microwave.

 

I've also replaced the HV diode, and tested the capacitor with my multimeter (no shorts). The thermostat tests closed, and is not shorted to the case.

 

I'm hoping that someone might have any additional suggestions?

 

Thanks for your help!

 

-Joe 

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There are a lot of 18 gauge or smaller wires carrying the 15 or so Amps that it takes to run one of these microwaves. There's also 4 or more thermal cutouts, microswitches, relays, etc. that carry 15 Amps. That's a lot of power going through a lot of push on connectors, any one or more of which could be loose. I'd check them all for loose, burned, or arcing connections. I'd also wiggle the lugs on all these items. I had one come back a couple times and finally found a loose lug on a TCO.

 

EDIT: Although this addresses a loose connection in an oven the principle applies to your Microwave too: http://mastersamuraitech.com/loose-electrical-connections-and-heat/

Edited by olyteddy

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olyteddy,

 

I definitely have several burned connections in line on the transformer primary.  I'll take it apart again this weekend, and clean up these connections.

 

Hopefully this is all I'll need to worry about...

 

Thanks for your response!

 

-Joe 

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