22 Mar
2014
22 Mar
'14
6:58 p.m.
On Sat, Mar 22, 2014 at 2:45 PM, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
If you have a fualt, like nist spikes on a power line,
that could be due
to a faulty capcitor, then of course you _check_ the capacitors. And
replace (or reform) them if necessary. Capacitors can fail. But changing
parts for no logical reason will get you nowhere fast.
If old electrolytics fail for want of reformation, you'll know about it.
The failure mode is punch-through; a portion of the capacitor becomes a
dead short, though usually only briefly, and it can be spectacular. (At
least that's what I've observed for large ones with 16V and 25V ratings;
smaller, lower-voltage ones might not be obvious.)
With the PDP-1, we had the expectation that some capacitors would need
reformation. If we hadn't checked them, and reformed the ones that needed
it, some of them would have blown up and needed modern replacement. The
purpose of our checking and reformation was to avoid replacing any more
than absolutely necessary, due to the interest in preserving as much of the
historical artifact as possible.
We had to replace one high-voltage capacitor (which was not electrolytic),
and we were able to get an identical NOS capacitor from the original
manufacturer, even though it was custom-made for DEC and probably hadn't
been sold to them for more than 20 years.
Eric