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

Most digital meters will not bias a diode into conduction. That is why they have a didoe check on some of them. Even that may not check a microwave diode.

Reply to
Ralph Mowery
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The control board is more like a block diagram, but the other circuit shows all that should be there.

Reply to
Ralph Mowery

I don't see the thermal cutoffs anywhere on that diagram, and I'm the one who thinks that one of those is the most likely culprit.

Reply to
Smitty Two

What's the "magnetron thermostat" that is right after the fuse on the incoming AC? Sounds like a thermal protection device to me......

Reply to
trader4

Hehe. What good does "maybe" do??? ;-)

Reply to
croy

I was looking at what method Sam would use to test diode. I would simply hook 9 volt battery in series with diode and analog meter on high ohms.

On my mirror page.....

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Greg

Reply to
gregz

A microwave diode is a bunch of lower voltage diodes in series. I don't remember exactly how many, but a typical multimeter won't test it...unless it's shorted. Need more volts to forward bias it.

9V battery and a light bulb might do it. Two 9V in series probably will.
Reply to
mike

Perhaps it is, at that. But if that one were shot, there wouldn't be any power anywhere. My microwave has three thermal cutoffs.

Reply to
Smitty Two

Hi, Magnetron needs high voltage to function. That diode is HV diode some times in series to raise voltage rating. Usually Magntron buzzes when working. Is it silent when turned on? Also oven door has multi sequenced interlock switches which can go bad.

Reply to
Tony Hwang

The electronics great RepairFAQ.org has information about microwave oven repair:

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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.

Reply to
larrymoencurly

NO!!!! DO NOT UNDER ANY CIRCUMSTANCES ASSUME THAT THE CAP IS DISCHARGED...EVER...!!! Mine was dead for a week and still had enough charge go explode the wire I shorted across it.

NO!!!! DO NOT UNDER ANY CIRCUMSTANCES ASSUME THAT THE CAP IS DISCHARGED...EVER...!!!

If you wanna risk blowing up your Fluke, you can measure the voltage on the cap...and possibly blow up your meter.

Death is final...don't risk it...no matter how small you think the risk.

Reply to
mike

It gives you the option of not being killed?

Christopher A. Young Learn more about Jesus

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Hehe. What good does "maybe" do??? ;-)

Reply to
Stormin Mormon

I WONDER. IF YOU TOOK A COTTON SWAB!!!! AND TAPED THAT TO A ***WOODEN STICK**** AND THEN DIP!!!! THE SWAB INTO WATER, AND PUT THAT

NO!!!! DO NOT UNDER ANY CIRCUMSTANCES ASSUME THAT THE CAP IS DISCHARGED...EVER...!!! Mine was dead for a week and still had enough charge go explode the wire I shorted across it.

NO!!!! DO NOT UNDER ANY CIRCUMSTANCES ASSUME THAT THE CAP IS DISCHARGED...EVER...!!!

If you wanna risk blowing up your Fluke, you can measure the voltage on the cap...and possibly blow up your meter.

Death is final...don't risk it...no matter how small you think the risk.

Reply to
Stormin Mormon

I do believe I mentioned discharging the capacitor through a 110k ohm

1/2 watt resistor. O_o

TDD

Reply to
The Daring Dufas

What you mentioned is irrelevant. All that matters is what the OP read and is gonna do.

discharging the capacitor through a 110k ohm

Since you asked for a critique on your method, here it is. Check my math. Using round numbers...

4000V / 110k is ~36ma. 36ma * 4000v is 145454 mw = 145WATTS peak. Half watt resistor. Methinks a metal film might vaporize before that cap discharged. Carbon might take the energy, if you can find one.

I'd suggest, from the experience level suggested by the tone of the OP, that your advice is more harmful than helpful.

Reply to
mike

No need to worry about the power rating, a 1/2 watt resistor is only rated for less than 500 volts. When the resistor blows apart, the arc will finish discharging the capacitor.. Big grin..

The microwave is one place that if you do not have any idea what you are doing and how to do it safely , you beter keep your hands out of it or have a good insurance policy for your wife and kids to collect on. Once you pass about 1000 volts, many things that seem to be insulators are not.

Reply to
Ralph Mowery

I've probably dealt with a lot more high voltage sources than you have over the past four decades. I've never had a problem using a 100k resistor to discharge a cap in a microwave oven. The danger I would warn anyone about microwave ovens is to be careful not to expose yourself to the non-ionizing radiation from the output of an operating magnetron. Damage to the lens of the eye is quite possible. Of course, I've also been known to use The Jesus Method to discharge capacitors and find circuit breakers,........because it's fun. ^_^

TDD

Reply to
The Daring Dufas

Me, too. As to circuit breakers, I did make a plug in breaker popper. I don't use it on FPE Stabloc panels. Ah ain' 'tupid, y'know.

Christopher A. Young Learn more about Jesus

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I've also been known to use The Jesus Method to discharge capacitors and find circuit breakers,........because it's fun. ^_^

TDD

Reply to
Stormin Mormon

Your going to have to find the discharge time, capacitance, voltage, resistance. I don't see the cap value listed here. I'd be more inclined to use at least a 2 watt resistor, but I like sparks.

Greg

Reply to
gregz

no, 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.

The only person who can answer that question is the one with the tape measure and the candidate oven.

The standard answer is, microwaves are cheap, go buy a new one. 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.

Reply to
mike

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