Basic Microwave Repair?


Hi everyone. I read many web sites about testing and repairing microwave ovens and have concluded that my Sharp microwave has a bad diode and capacitor. I checked the magnetron and, according to what I read, it's probably OK as I get a reading of about .4 ohms across the terminals and it is not shorted to the case. However, I'm wondering if it is possible to test the magnetron with low voltage DC. I assume that what is usually fed into it is 120 VAC rectified which I think will result in about 170 VDC. If that's the case, then could 12 VDC be used just to verify the magnetron still works before spending money on other parts? I ain't gonna do nothin' 'les someone says it's OK and explains how ;-) The sympton right now is that it buzzes and blows the fuse so I'm thinking an open diode and shorted capacitor results in feeding 60 Hz AC to the magnetron thus causing the buzz. Does that sound right?
Thanks.
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On Sat, 03 Jul 2010 15:20:18 -0700, Ulysses wrote:

Sounds right to me. If you replace the cap and diode why not do the mag too? They sag in output as they age so if you like the oven you have do it justice and replace all three components. Just make sure you don't handle the leads on the capacitor. It can store a nasty jolt. And you probably have a shorted diode if it blows the fuse.
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A new magnetron at one site cost about twice as much as a new microwave. I don't like it THAT much ;-)
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much consumer stuff has parts costs the exceed the price of a brand new whatever with warranty. often its just not worth the money or time
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Nor should you. Microwave ovens have joined that great wave of cheap throwaway appliances that cost more to repair than replace. Unless you plan on doing a whole roast pig or an angel food cake, a crappy $50 microwave will do almost everything a high buck built-in, control-from-yer-Ipod, warn-you-when-yer-too-dumb-to-breath, model. It'll certainly warm coffee and cook tv dinners just as well. ;)
nb
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On 07/03/2010 05:29 PM, Chief Two Eagles wrote:

Speaking of capacitors holding a nasty jolt, when I was around 12 years old my dear old drunken, worthless father, that liked to beat and starve me, told me he was going to work on the TV and asked if I thought it would be alright with the plug unplugged. I had heard that capacitors help a charge even when something was unplugged, but I told him, it should be alright. He jabbed a screwdriver in a flopped around for what seemed like quite a while while I stood and watched. Afterward I said "Well imagine that". The TV worked before he started and it worked after he finished, so I don't know why he thought it needed to be worked on.
Another time he thought he would work on a light that didn't need to be fixed, and he asked if it would be alright with the switch off or should he turn off the breaker? I told him that as long as the switch was off it should be fine. He stuck a screwdriver in there and was practically doing a jig on the counter top. Again I said "Well Imagine that". The light worked fine before he started and after.
I still laugh when I think of those times. Old Geezer
And you

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Ah, reminds me of my dear old dad ;-)

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re: "Just make sure you don't handle the leads on the capacitor."
I don't know what size caps are in a MW, but I used to work on transmitters that had high-voltage oil filled capacitors, like this...and bigger:
http://www.recycledgoods.com/product_images/h/422/s_p_32920_1__51850_zoom.jpg
If you didn't put shorting straps across the taps as soon as you removed them from the equipment they would charge up to 5KV or more just sitting on the workbench in the transmitter building.
We used to use a hi-pot and a dead-man stick to show the newbies why they should stay out of the transmitter building unless they were accompanied by a journeyman tech. We'd hook the grounding strap of a dead-man stick to one tap, charge the cap up to 2 or 3 KV and then touch the hook of the dead-man stick to the other tap.
The resulting arc and *crack* would scare the crap out of the newbies. One time we charged the cap up too much and it blew the threaded rod of the dead-man stick right out of the handle. That one scared even us seasoned techs!
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On Sat, 03 Jul 2010 22:29:11 +0000, Chief Two Eagles wrote:

It's usually just a fuse or fusible link inside. Careful, don't fry your eyeballs.
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Ulysses wrote:

Not even remotely close. Magnetrons work at around five thousand volts after the doubler circuit; I don't know where you got the 170VDC figure from.
Read Sam's guide and then ask over at sci.electronics.repair to get some advice from guys that do this every day.
Sam's guide: http://www.repairfaq.org/sam/micfaq.htm
Note: Cautions about killing yourself by mucking around inside of a microwave oven are neither unfounded nor exaggerated.
Jon
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I've measured rectified 120 VAC in the past and seem to recall it read about 170 VDC.

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Does the word transformer mean anything to you except for the movie ?
The 120 volt line voltage is not rectified in the microwave. There is a transformer in the microwave that changes the voltage to over 1000 volts and then it is rectified. Most common meters will not measure voltages that high. The voltage can puncture the insulation of the meter and kill you.
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Thank you. I'll keep an eye on the beast. I have no intention of working on my microwave oven while it's plugged in or on. I did not need to discharge the capacitor because it is shorted. But I still don't trust it.
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How are you sure the capacitor is shorted??? Unless you have a very small capacitor, it will look like a short to most voltmeters.
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120V is the RMS value. Yes, the peak voltage is about 170V. V(RMS) * sqrt(2) = V(Peak).
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Wow, that site is a lot better than anything else I found. Thanks!
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