PDP 11 gear finally moved

Lyle Bickley lbickley at bickleywest.com
Tue Jul 21 10:04:49 CDT 2015

On Mon, 20 Jul 2015 23:14:36 -0600
Eric Smith <spacewar at gmail.com> wrote:

> 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.

As Eric, I'm a member of the PDP-1 Restoration Team. The PDP-1 restoration was completed in 2005 - and annually we check the power supplies for voltage, ripple, etc. Not one of the re-formed capacitors have failed in the ten years since the completion of the restoration.

I also re-formed all P/S capacitors in my PDP-8/S in September, 2013. Not one has failed since...

Same with my EAI TR-20 Analog computer. And so it is for all the systems in my collection...

IMHO, these "white papers" indicating that ALL aluminum electrolytic capacitors decay is obvious nonsense - based on real life experience - not someones theory...


73      AF6WS
Bickley Consulting West Inc.

"Black holes are where God is dividing by zero"

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