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Microwave magnetrons keep failing

A year ago my GE microwave’s magnetron failed. I replaced it and it worked fine for a year and now I again have a short somewhere (loud hum then blows fuse). The magnetron resistances check out OK (<1ohm filament, no conduction to body), but then again, the old one that I had replaced also checked out OK this way, yet replacing it did fix the problem. Not seeing any arcing near the connector or at the antenna. The diode is fine (VF=9V, no cond. in reverse). The capacitor is fine (0.91uF, no shorts). Shorts in the transformer windings seem unlikely; I see no evidence of arcing or burning. Can’t be the door switch (the NC one), since if that was bad it would have blown the fuse immediately, not after a few seconds.

So it must be the magnetron again, but before I buy another replacement, any ideas why they keep failing?

Yanko

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Hi,

Is there any obvious visible damage to the magnetron itself e.g terminals blackened, cracked magnets, burnt antenna dome or any black heat marks on other parts close to it?

Blackened connections could mean loose or badly crimped connections causing heat. The other problems may be due to overheating caused by microwaves reflected back to the magnetron . You don’t state the model but has it got a stirrer blade and is it working OK?

Check the resistance of the magnetron with an Ohmmeter. Normal test is ∞ (infinity) i.e. infinite resistance or open circuit (OL) if you like. Any other reading means that it is faulty.

If the magnetron tests OK then you may have to look for another reason why the fuse blows.

Have you measured the resistance of the HV transformer?

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See my answer below to your questions.

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Hi, thanks for the response!

There is no visual indication of damage, no arcing at terminals or the antenna or anywhere else. Can’t see cracks in the magnets. The resistance between cathode and enclosure is infinite. Filament resistance less than 1 ohm.

The microwave model is GE JVM1660; it does have a stirrer, and the motor works OK.

It took several seconds before the fuse blew, with loud humming, so I’m pretty sure the issue is in the HV circuit.

I have not checked the resistance of the transformer windings, only continuity. Do you know what sort of values I should expect?

The only explanation I can think of is that the magnetron lost its vacuum, resulting in internal arcing. Probably the same with the last one since that one checks out OK too, but was definitely faulty a year ago.

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@Yanko

Here's a link that describes how to test a HV transformer.

It is quite good as it describes all the various scenarios.

Click on 7.16 in the Contents at the top of the page.

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Yanko crwdns2893898:0crwdne2893898:0
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