PDP 11 gear finally moved

Eric Smith spacewar at gmail.com
Tue Jul 21 00:14:36 CDT 2015

On Mon, Jul 20, 2015 at 4:46 PM, Rich Alderson
<RichA at livingcomputermuseum.org> wrote:
> industry white papers with tables of decay rates for
> the aluminum electrolytics that indicate that, *no matter what*, they lose
> capacitance over time, until c. 14 years from manufacturer date they are at 10%
> of rating.

That's very interesting. I haven't seen those white papers, but the
"no matter what" must in fact depend on something, since on the PDP-1
Restoration Project we found that most of the 40 year old aluminum
electrolytic capacitors still met their original specifications,
including capacitance within rated tolerance. Of the few electrolytic
capacitors that had failed, the problem was a catastrophic failure,
not the capacitance being outside the rated tolerance.

In the PDP-1, we preferred to keep the original components as much as
possible. Had there been a capacitor, the failure which would have
caused extensive damage to other components, we would have given
serious consideration to replacing it. However, that was not the case
for any of the capacitors in the PDP-1.

Had our analysis indicated any expected benefit to replacing all of
the electrolytic capacitors, we would have done so, and bagged and
tagged the originals similar to what we did with failed components, so
that they could be replaced if it ever was desired to return the
artifact to its pre-restoration condition.

I'm not recommending against LCM's policy, but I also wouldn't
necessarily encourage anyone to adopt it, nor to adopt the practices
of the CHM PDP-1 Restoration Project, without studying the issue.


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