PDP 11 gear finally moved

tony duell ard at p850ug1.demon.co.uk
Wed Jul 22 08:48:38 CDT 2015

>    > From: Tothwolf
>    > How do you know those aluminum electrolytic capacitors are functioning
>    > just as good as they did when they were new? Unless you've tested them
>    > out of circuit ...
>     > ... aluminum electrolytic capacitors by their very electrochemical
>     > nature degrade as they age and as they are used.
> I am way out of my knowledge range in this discussion, but here's something I
> wanted to ask about: how do you reconcile this observation (assertion?) with
> the observations from several people (e.g. the PDP-1 people) that they _have_
> measured the electrolytics in their power supplies, and despite being N
> decades old (where N ~= 5), they are _still_ within specs? If the very nature
> of electrolytics mandates that they degrade, how are these still meeting
> specs?

Well, at least 2 possibilities...

Firstly, the tolerance of the capacitance of an electrolytic capacitor is very wide --
-20% to +80% is not uncommon. So it's quite possible they started off at the top
end of that range, have deteriorated over the years, and are still within spec. Of
course nobody can prove that (unless there are records of the values meaured
50 years ago) and nobody really knows how they will continue to change (if indeed
they do).

Secondly, I have no idea what was measured. The capacitance value is not the 
whole story by any means. In fact the most important thing most of the time is
ESR (Effective Series Resistance) which increases as the electrolyte dries up. 
The ESR of these components could well still be OK after 50 years, but again
nobody knows what it was when they were new.

That said, I keep on with the comment that the important thing is 'does the
circuit behave as required', If so, then the capacitor is almost by definition
OK in that circuit.


I'm very confused...


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