Recommended repair or magnetron replacement of broken microwave (Jenn-Air M170B)

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On Mon, 17 Dec 2012 19:45:33 -0800, hr(bob) snipped-for-privacy@att.net wrote:

I was wondering WHERE the magnetron lay.
If these are the magnetron leads, they read 0.4 ohms with the Fluke in ohms mode:

And, the diode reads OL in both directions, both forward biased and reverse biased with the Fluke 75 in the diode mode:

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wrote:

If you are only on the filiment of the megatron and not in the circuit it is probably ok. If the megatron is still connected, you may be meauring the filiment windings of the transformer.
Some high voltage diodes are made of several lower voltage units in series in the same case. The diode function of the Fluke may not have enough voltage to check this out and will show open. Here is one way to check to see if it is actually open. http://www.flickr.com/photos/zenzoidman/4724416181 /
They use a 9 volt battery and the voltage scale of the meter, One way will show open and the other will show a few volts less than 9 if the diode is good.
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On 12/17/2012 11:35 PM, Ralph Mowery wrote:

Um, isn't "Megatron" one of the living robot Transformer creatures? ^_^
TDD
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On Tue, 18 Dec 2012 00:35:25 -0500, Ralph Mowery wrote:

That's a nice procedure which I will try in the morning. Thanks for pointing it out - as I would have thought the basic diode function of the Fluke 75 would have sufficed.
At the moment, it shows the diode as bad - but - as you noted - that may be an anomaly due to the voltage of the Fluke in diode mode.
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On Monday, December 17, 2012 10:02:16 PM UTC-7, Danny D. wrote:

The electronics great RepairFAQ.org has information about microwave oven repair: http://www.repairfaq.org/sam/micfaq.htm
Those high voltage diodes will read open with a digital meter because they're made of several diodes in series to handle the thousands of volts, and usually digital meter won't apply enough voltage to make all those diodes conduct. So either try an old-style analog meter set to a higher ohms scale (x100, x1000, x10,000), or apply at least 6V - 9V DC through a 1,000 ohm resistor and measure directly across the diode. If the diode is open, you'll see the source voltage, but if it conducts, the voltage will be lower.
The big high voltage capacitor has to be discharged before handling it. It almost always contains a bleeder resistor to do that, but bleeders fail in use, so assume it's not there. Get a screwdriver with a plastic handle on it (it's not enough to wrap electrical tape around a metal handle; the voltage is way too high), and wrap several turns of BARE solid copper wire around it. Connect the other end of that wire to a 10,000 ohm, 10 watt resistor, and connected the other resistor lead securely to bare metal of the oven's chassis. Touch the tip of the screwdriver to each lead of the capacitor for 30 seconds. Finally, touch the screwdriver shaft to both leads of the capacitor simultaneously.
Prices for microwaves oven parts vary greatly, and sometimes electronics parts dealers are cheaper than appliance dealers. Some of the former include MCM Electronics, Dalbani, Premium Parts, and Parts Express.
You don't need an exact replacement magnatron, just somethign compatible mechanically and electrically, and for our Sharp onvection/microwave ovens I've used L-G magnatrons. Also any replacement high voltage capacitor should have the same microfarad (uF) rating as the original because using one with a different capacitance rating changing it changes the power output roughly proportionally, meaing the programmed cooking modes may not work right because they're not calibrated for a higher or lower power.
The magnatron is usually held in place with 3-4 bolts or nuts, and you absolutely want them to be secured so the magnatron is completely flush with the mounting surface. Otherwise microwavess will leak past the brass braided ring seal.
The magnatron has to be cooled with a fan, so be sure that fan works by feeling for air through the rear vent. There's probably a bimetal thermostat mounted on the magnatron to shut it off if it gets too hot, and you may want to replace it because I had one that got stuck in the on position. They're made with several different temperature ratings.
I wouldn't turn on a microwave without the cover completely reattached, including with all its screws, to prevent electrical shock and also injury in case the capacitor explodes.
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On Wed, 19 Dec 2012 01:02:09 -0800, larrymoencurly wrote:

Wow. Lots of good reading there! Thanks.

I am getting the test jig set up at Radio Shack & will report back.

OK. Now I see why a bare screwdriver isn't used. I'll get a 10K ohm 10 Watt resistor at Radio Shack.

I have that bimetallic switch mounted on my magnatron. Is it supposed to be on or off when the microwave is unplugged? Mine reads at 0.4 ohms resistance with the microwave unplugged.
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On Mon, 17 Dec 2012 17:04:25 +0000 (UTC), "Danny D."

When my JC Penny unit quit working, it turned out to be the fuse & fuse block--badly corroded. Replaced both 5 years ago, and they're still going strong.
--
croy

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On 12/17/2012 9:04 AM, Danny D. wrote:

First you have to determine that it's bad. Then why it failed to make sure the new one doesn't fail too. Then you have to find the exact magnetron...at an affordable price. If you know what you're doing, you might be able to find an equivalent magnetron...whatever that means...or cobble in a different magnetron. That's just not practical for most of us.

measure and the candidate oven.

If you must fit the same hole, your quest is complicated to the extent that most people won't be able to offer any useful advice. Call up someone who refurbishes kitchens and ask them.
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