Re: Microwave oven capacitor.. Dead short overnight?

In article <svjjr1hcp5mkjdj6hl73pv35ferauj69se@xxxxxxx>, Doug Warner wrote:
>
>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)

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 (don@xxxxxxxxx)
.