Microwave Oven Magnetron Test

I have a non-heating microwave oven that I just opened up. I tested the HV diode and it seems to be acting as a good diode. The capacitor measures 1.0 4 uF and >30 MEG-ohm, so it should also be good. That leaves the Magnetron. Is there a way to check the tube without powering it up with high voltage? Any visual signs might indicate it's bad?
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On Monday, December 30, 2013 7:01:46 AM UTC-6, panabiker wrote:

.04 uF and >30 MEG-ohm, so it should also be good. That leaves the Magnetro n. Is there a way to check the tube without powering it up with high voltag e? Any visual signs might indicate it's bad?
check the fuse inline to the heating element.
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On 12/30/2013 8:01 AM, panabiker wrote:

Doesn't that also leave the transformer and wiring and connections and whatever component(s) switch the HV/RF circuitry on? The diode and capacitor may well be perfect but there are other things to consider. I have experienced more seemingly-dead microwave ovens caused by defective interlock switches than any other single cause.
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On Monday, December 30, 2013 9:49:58 AM UTC-5, BenignBodger wrote:

1.04 uF and >30 MEG-ohm, so it should also be good. That leaves the Magnet ron. Is there a way to check the tube without powering it up with high volt age? Any visual signs might indicate it's bad?

Well, this MW oven had a slow dying over a couple of weeks. It started taki ng longer to heat up things and getting louder when starting up to finally only being able to heat a cup of water to lukewarm after 5 min. So I don't think it's a switch/interlock type of problem. I also checked all microswit ches in the interlock mechanism to make sure.
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On 12/30/2013 7:06 AM, panabiker wrote:

A shorted diode will cause a loud hum. A microwave diode has a bunch of diodes in series. If some short, you may not be able to tell with an ohm-meter. You'd think the problem would cascade rapidly, but stuff happens. You could check the filament for continuity. But there's not much you can do about it. If it's not bad connections, it's scrap. I don't think I'd risk getting microwaved or electrocuted for the low probability of being able to fix it.
That cap is a killer. Mine had been off for days. I was smart enough to discharge the cap. I wasn't smart enough to use a big resistor. Ripped my hand on the cabinet when the resistor exploded. Good times...
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Check that the filament is not open and that the filament is not shorted to the case. Matt electronics is a good source of parts. Mark
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On Monday, December 30, 2013 8:42:01 PM UTC-5, mike wrote:

I mean the humming noise is getting louder, especially during the first few seconds of power-on, over the last few weeks.

I tested the diode using a variable current-limiting power supply. Its forw ard voltage is around 6 volts when the current starts flowing. When reverse biased, no current is detected up to -25 volt. I can't test high voltage b ehavior at its operating condition though.

Yes, it's time to buy a new one. This is a cheap, 14-year-old $120 microwav e so it's not worth any money or effort to replace the magnetron. I was hop ing for either a failed diode or the cap as the cause.
Thanks to all who chimed in, and have a happy New Year.
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Yes, the OPost reminded me of those who read the computer codes on their cars and announce it's the MAP sensor or the CP sensor, when the books all say "MAP sensor circuit" and "CP sensor circuit" Then they replace the sensor, etc even though it might be working fine. When the codes said my MAP sensor circuit was bad, I pushed the MAP sensor onto the connector harder and it worked fine for 3 days. After that I strapped the MAP sensor to the connector as tightly as I could with one of those plastic ties (not twist ties of course) and it worked fine until I got rid of the car after 9 years.
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On 1/2/2014 7:20 PM, micky wrote:

Most of the problems I repair in any kind of equipment turn out to be a bad connection or cold solder joint. ^_^
TDD
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On 12/30/2013 9:49 AM, BenignBodger wrote:

I am on a roll here. My big old 1999 Sharp microwave/convection oven stopped a couple of days ago as I was doing some egg rolls for dinner. This thing is huge and heavy and is mounted inside an eye-level cabinet but I wrestled it out and put it on the table to check for troubles. Guess what? It was a failure of the secondary interlock switch. I think that when this beast fails permanently I'll replace it with something similar and I'll order a complete set of interlock switches at the same time and tape them inside the case. BTW: RepairClinic had the OEM switch for just a few dollars.
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Pan,
YouTube has lots of appliance repair advice.
Dave M.
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On 12/30/2013 9:51 AM, David L. Martel wrote:


http://www.youtube.com/watch?v=VqN1sDgY6I8

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On 12/30/2013 05:01 AM, panabiker wrote:

The only way to test a magnetron (other than testing the heater) is to sub in a known good one.
Jon
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On 12/30/2013 10:21 AM, Jon Danniken wrote:

From the do not try at home labe:

http://www.youtube.com/watch?v=0Z_NxYv2bPw

Very basic:
http://www.microtechfactoryservice.com/mag_test.html
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On 12/30/2013 7:01 AM, panabiker wrote:

It would need to be a pretty fancy MW to bother replacing the magnetron. The last time I looked up a replacement it was over $70 which gets you well into new MW territory. I assume you have checked the inline fuse where the power cord comes in the unit. This means that the lights come on and it all pretends to work, just not making things hot. Test the door lock microswitces and the flame/heat sensor(s). If they all check good, junk it. I don't know any test for the magnetron, but when you junk it, there are a couple of really nice strong magnets you can dig out of it.. There are lots of DIY projects you can mess with using the transformer. Exercise due care around the capacitor. I always thought the door was some type of glass, I was really surprised to find the door is a finely perforated steel sheet (I'd love to know how they punch the holes) with a thin piece of plastic on each face. Not much else worth keeping. The glass dish is great for a serving tray/dish and the turntable motor is about $20 if you have to buy one and they seem to be about the same, I've been able to fix a few for others. Yes, I've junked several.
My 2 cents.
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You left out the transformer.
Greg
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As someone said, scrap the one or two extremely strong magnets that are around the magnetron tube, they are VERY useful.
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Right, the problem seems to be the transformer. I measured transformer with an ohmmeter. The secondary (high voltage) coil is open. Since the oven had a slow death, I didn't suspect the transformer.
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