Electrolytic capacitors and other picture postcards.

From: Allison J Parent <allisonp_at_world.std.com>
Date: Mon Feb 7 22:29:11 2000

<started to get flakey and now won't come up at all. Is my
<understanding of ecaps correct? That is, as long as the
<capacitance is correct, the voltage is greater or equal to
<the required load and it is installed "right way around"
<it should be OK? There is so much logic on this board, a

They should not exceed 3x the wrking voltage plus any spikes
that may occur. Electrolytic caps run at .25 or less the rated working
voltage will not form and the actual capacitance will diminish. The
later does take a bit of time usually. Likely the shorted caps are
of the "tantalytic" tantalum electrolytic variety and they are know
for that behavour. In any case the newly installed caps should have the
following:

 -Short leads.
 -Working voltage of not more than 15-20V for a 5V rail, 50V for a 12-16V
  rail.
 -of course the replacement should be known good.

The flakeyness is likely due to some other factor that may be due to the
power cycling. I'd check the PS for a tired cap (a big one) or maybe a
fried rectifier. It's conceiveable the output voltage is at the lower
limit and causing flakies.

<lot of it 8Txx, I hope I haven't blown a chip. It would
<take me a month to find it and who knows if I could get a
<replacement.

Most of them are common enough and have TTL 74xx replacements that should
work just fine.

Allison
Received on Mon Feb 07 2000 - 22:29:11 GMT