[sdiy] Lifespan of Electrolytics?

[Byron G. Jacquot]

>    I recently got ahold of a Tapco 4400 reverb for cheap because one channel
>was dead.  After chasing down a handful of 4136s and systematically replacing
>each one, I came to find that the actual problem was a bad electrolytic in the
>signal path.  The circuit board is dated 2/17/77.  I'm guessing that several
>more of the caps in this board are marginal.  What is the expected lifespan of
>electrolytics, and what are the effects of old and aging ones?  Is it worth the
>trouble going through any old gear including synths that are past, say, about
>twenty years and giving them a electrolytic overhaul?  Anyone have firsthand
>(or secondhand) testimonials?  Thanks.

I've got a mixing console that's about 15 years old, I've owned it for about
3 of them.  I knew it was old enough that the 'lytics were going to be
questionable, but just how important recapping can be hadn't really hit home
yet.  The first time I tried to do serious work on it (more than just firing
it up for 15 minutes to make sure things were wired right), it started
making this "tik-tik pop tizzz.." kinda noise.  Just loud enough that I
couldn't get any work done.

I pulled out the oscilloscope, to see if I could figure out where the noise
was coming from.  It seemed to be everywhere...every insert point, every
output...

I started unhooking channels, one or two at a time.  When I unhooked a
channel, the noise would disappear for a little while, then come back,
insistent as ever...even with large portions of the thing unhooked, the
noise was still there.  With the thing opened up, I started checking for
some "common sense" things, like power & ground & solidity of the multipin
connectors...

Lo & behold, the positive rail would "dance" in time to the ticking noises,
jumping between 16.75 V and 17 V...at that point, I got out the Digikey
catalog, and ordered a whole slew of Panasonic caps.  I got on with the
recapping I'd anticipated.

The noise is gone now...and I'm sititng here with something like a pound of
used electrolytic caps that were pulled from the board.  One of those caps
is bad, and only shows it when it heats up!

Byron Jacquot