Microwave oven capacitor.. Dead short overnight?

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It worked OK last night, I popped in a meal tonight, pressed the button.. Overload hum, blue flash from the back, then no display (10 year old Sharp model.)
Since my background is in electronic repair, I opened it up. put a new fuse in, tried again and saw the flash location, near the big cap in the rear.. (Unplugged, checked for charge, then metered it. 2 ohms!) Just to make sure it was really shorted, I connected it in series with a 1156 automotive bulb and a 12V source. The bulb lights. It's really shorted..
It's odd that it went from OK to shorted overnight, but the big question is: Is there typically any collateral damage when this happens?
(The diode is OK) -- Email reply: please remove one letter from each side of "@" Spammers are Scammers. Exterminate them.
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So, shouldn't you be telling us what the problem is?

I thought shorted was shorted. Did you expect it to be a little shorted at first? Sort of like being a little pregnant?
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It is normal for capacitors to work just fine and dandy and then totally short (or open) suddenly with no warning. Early warning, if any, is changes in leakage resistance or in dissipation factor or slight change in capacitance - and you have to take the capacitor out, discharge it, and measure these characteristics. Occaisionally the capacitor may run warmer when it is in its final months or days. But expect no warning.
Electrolytic capacitors (which the high voltage one in a microwave is not) sometimes give warning signs of deteriorating performance or overheating that may be noticed without removing and testing the capacitor. My experience is that electrolytics that obviously start going downhill fail (or become unusable or, in the case of CRT monitors, cause something else to fail) in a few months.
If the diode is still good, chances are nothing went bad except the capacitor.
- Don Klipstein ( snipped-for-privacy@misty.com)
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Spammers are Scammers. Exterminate them.
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Doug Warner wrote:

It's ten years old. Just replace it. You will like the newer one.
It has been my experience that if I repair something like that it is likely to need an even more expensive repair next week.

--
Joseph Meehan

Dia duit
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Doug,

I worked as a microwave tech for about six years. Be VERY careful working with those capacitors. They can store one heck of a charge, even weeks or months after the oven has been unplugged! We always discharged the caps with an insulated screwdriver across the terminals. One big "POP" and flash I never got used to! :)
Anyway, the caps often failed without any other damage to the machine. But, they sometimes took the diode with them too. I'd replace both parts if it were my machine.
And, I'm guessing you'll need a new fuse too... :)
Depending on the price of the parts, it may cheaper and easier to just buy a new oven... :) Seems to be the way of the world anymore...
Anthony
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|> Doug, |> |> > It worked OK last night, I popped in a meal tonight, pressed the |> > button.. Overload hum, blue flash from the back, then no display (10 |> > year old Sharp model.) |> |> > Since my background is in electronic repair, I opened it up. put a new |> > fuse in, tried again and saw the flash location, near the big cap in |> > the rear.. (Unplugged, checked for charge, then metered it. 2 ohms!) |> |> I worked as a microwave tech for about six years. Be VERY careful working |> with those capacitors. They can store one heck of a charge, even weeks or |> months after the oven has been unplugged! We always discharged the caps |> with an insulated screwdriver across the terminals. One big "POP" and flash |> I never got used to! :) A few years ago there was a news report from my hometown about someone working on his microwave oven. That capacitor electrocuted him.
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writes:

He probably had a bad heart or was working in that area with the power on. It would not electrocute a normally healthy person. More likely get killed by bumping your head when jumping back away from it. Kind of similar to the voltage on a TV tube. You don't accidentally touch it twice.
I was in a lab where a guy working on a very high powered radar system got hit by a string of large capacitors charged to 30,000 volts. It knocked him clear across the room. Fortunately the power was off.
Doing a "crowbar short" to that system you could draw an arc over a foot long.
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I agree. I got zapped off the old tube type tv sets many times as a child, and being a farmer, I get zapped off my electric fences at least 5 times a year. A person dying from this either has a weak heart, or as you said, the power was on.
Unplug the device (always do that with anything). Then place a screwdriver with a well insulated handle across the cap, and proceed. I learned to do the same thing with tv sets. There's no sense instilling fear in people. Besides, we have the Bush administration to instill fear in us now. Thats their job. They'll probably tell you the microwave repairman is a terrorist, if you ask them....
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snipped-for-privacy@UNLISTED.com wrote in writes:

The worst damage comes from pulling your arm back and ripping it on a sharp piece. MW oven caps are only around 2KV,and fairly small in capacitance,compared to a high power radar system,or broadcast transmitter.

I prefer a 1 Kohm 2W resistor glued on a plastic stick.(old tuning tool) (unit unplugged and OFF.)

Now that's nonsense.
The Bush administration is doing a FAR better job than Clinton did,or Kerry would have done.(and the terrrorism threat is REAL.)
--
Jim Yanik
jyanik
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I could see that happening

My electric fences are 5KV to 8KV. (some go to 10KV) I've had them knock me flat on the ground, but they are not fatal. Just hurt like hell. They always force all the 4 letter words out of my mouth too.

That sounds like a good idea, eliminated that snap. I guess either will do the job.

Believe what you want !!! I think the terrorism threat is far worse since Bush started the war than it was before.
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snipped-for-privacy@UNLISTED.com wrote in

I once got across the sparkplug of the gas engine of my mini-bike;magneto- generated,that's about 40KV,one heck of a pulse.I can guess what a TASER shot feels like.

Hardly;9-11,*the FIRST WTC bombing,*USS Cole,*two US Embassy bombings,*Khobar Towers(* all on Clinton's watch),all before the Iraq War,before Afghanistan. Terrorism was happening before Bush,and will until Al-Qa'ida,Hezbollah,and Hamas are soundly defeated.
What's happened since then? Nada. But the Feds did catch some terrorists BEFORE they could do their dirty deeds.
--
Jim Yanik
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wrote:

The megneto is a limited pulse. Doesn't quite approach what can be in a large capacitor. Both can hurt like hell.
I got "bit" just turning the flywheel by hand on a mower engine. Checking how close the magnets came. The plug wire was close to my hand. Just made me jump a bit.
I got my fingers across a 300 VDC source once. The current flowing from one finger to another. I had a hard time letting go. My army hurt for two weeks.
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I wonder why they have such a high voltage? I may be wrong, but I think cars only have about 5KV to 10KV going to sparkplugs.

Similar to an electric fence. They are basically a capacitor discharge into a coil that puts out a pulse about the same as an ignition coil.

test them. The shock is minimal when you turn by hand compared to when the engine is running.

DC is much worse than AC. When I was a kid I had gotten an old tube type army radio transmitter. I got about 1000VDC shock, in addition to the RF coming from the unit. I was grounded to a metal microphone in my other hand. The chair under me was a heavy oak chair. After that jolt, I was sitting on the concrete floor about 20 feet away, the chair was flipped over and 10 feet from my bench, and the microphone was all the way across the basement, with the plug pulled out of it, and it hit my friend as it flew. My friend said I was walking around in a daze for at least a half hour, and I kept drinking water. He said I was not making sense, but I did pull the plug on that transmitter while stumbling around. I vividly remember this happening, but I still to this day find this void or memory loss for a short time. I think I darn near electricuted myself that time. I know I touched a tuning capacitor that was directly across the high voltage output of the power supply with a huge electrolytic cap right ahead of it. I grabbed it bare handed, and with that mic in my other hand, I got the full load. Additionally, I did not have a decent load on the antenna (like I should have), and got the full load of the RF too. This thing transmitted in the AM band, and used to knock everyones AM radio stations off the air. After several complaints from neighbors to my parents, my father took that thing away from me. Probably the best thing he ever did. Even though back then I was really mad at him. The electronics devices in the 1950's and 60's were far more dangerous than those made today. Today, most circuits are harmless, except for capacitor discharges, and of course the 120VAC line cord.
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That is what I was brought up on. Put together my first Ham transmitter in the 40s. I remember something a professor stated in the mid 50s. It was something close to: "The transistor is an interesting laboratory novelty but will never achieve a useful purpose"
I worked with the first airborne digital computer. About 2000 peanut size tubes. Within four of five years we were upgrading to integrated circuits, bypassing discreet transistors.
The radar transmitter hat in the planes were the dangerous area. 30KV using tube diodes about the size of 1 lb coffee cans.. They lit up like big light bulbs so you knew to keep away.
In my life I got "hit" more times that I would like to say but was always really careful when working around those high voltages. Work with one hand and the other in the back of your belt was a safety procedure.
But not too many capacitors that will really hurt you. The string used in that transmitter were charged to 30KV and could hit pretty hard. Another I remember was about two feet square with a couple big balls sticking out. Even with a low voltage charge it could melt a screwdriver held across the terminals.
Reminds me of a guy working on an aircraft with a roll of solder in his hand. He got the solder between the 28 volts and ground the the solder melted in his hand.
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At ten years, that microwave doesn't owe you anything. Get a new one. Really.
Commodore Joe Redcloud
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it's true the price of the capacitor alone will be more than most table top microwavwes. And your time! Time is to short....Buy a new one.
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wrote:

Maybe he's living in poverty and can not afford a new one. It's so easy to spend other peoples money. Why dont you just give him a barnd new microwave for FREE. Just think how good you would feel inside, and you'd earn awards for heaven. Better yet, give a new MW to EVERYONE on this group for FREE. We'd all love you, and you'd have so many new friends to get Christmas cards from....
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New cap was $15.00 Oven works perfectly now. -- Email reply: please remove one letter from each side of "@" Spammers are Scammers. Exterminate them.
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Commodore Joe Redcloud wrote:

That is a stupid comment. My current microwave is 17 years old and works fine.
TJ
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