Microwave oven capacitor.. Dead short overnight?

[Doug Warner]

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.. =20

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)=20

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[Edwin Pawlowski]

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?

[Don Klipstein]

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 ( [e-mail])

[FDR]

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..

Nitpicking here, not really shorted, but appears to be acing like a resistor than a capacitor from what you describe.

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?

I am not familar with the schematics of microwave circuits, but is the cap failure the cause, or was it a result of another part failing? Are you sure the diode isn't trashed? How are you testing it? The breakdown voltage isn't your typical voltage considering the application. If I were tring to fix your oven I'd replace the diode too.

(The diode is OK)

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[Joseph Meehan]

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.

[HerHusband]

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

[Robert Mech]

In article , HerHusband writes: |> 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.

[Rich256]

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.

[Commodore Joe Redcloud]

At ten years, that microwave doesn't owe you anything. Get a new one. Really.

Commodore Joe Redcloud

[MrAppliance1]

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.

[Texas Joe]

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

TJ

[Commodore Joe Redcloud]

It's not a stupid comment at all. When your 17 year old microwave stops working, you would be a fool to even think about repairing it.

Commodore Joe Redcloud

[Texas Joe]

It is a commercial grade microwave, not a bic disposible lighter.

TJ

[Commodore Joe Redcloud]

Doesn't matter, dopey.

Commodore Joe Redcloud

[Texas Joe]

And I suppose i should toss out my 10 year old fridge, also? Just because your too clutzy to fix things doesn't mean everybody is.

TJ

[Commodore Joe Redcloud]

I didn't say I couldn't fix it. I have fixed hundreds of them back when it was still a reasonable thing to do. The problem is that it would be economically STUPID to fix a 10 or 17 year old microwave.

You seem to have the market cornered on "stupid"

Commodore Joe Redcloud

[maradcliff]

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....

[Texas Joe]

''economically STUPID''? I assume you think it is smarter to spend $500US on a new one instead of a few bucks for some parts.

TJ

[maradcliff]

So if the fuse blows or a prong breaks off the plug, you would just toss it and buy a new MW? Apparently you have too much money, and have no concern for our environment and natural resources. Everything wears out eventually and it becomes unrepairable due to cost or lack of parts, but lets be realistic. I fix anything that can be fixed, as long as I can get parts and not spend more than a new device.

[Commodore Joe Redcloud]

Uh, Joe... The original poster has a regular old microwave that can be replaced for less than $130.

Your 17 year old microwave is no longer worth $500, and you are not going to repair it for "a few bucks for some parts". If it is used in a place of business, you can't repair it yourself anyway. OSHA would crucify you, and any lawsuits brought by workers exposed to radiation would have a very easy time suing you.. After any repairs, regardless of what they are, the unit would have to be leak tested and certified in writing by a licensed technician using a certified and calibrated instrument.

Commodore Joe Redcloud