PDP 11 gear finally moved

[Tothwolf]

On Wed, 22 Jul 2015, tony duell wrote:
> On Wed, 22 Jul 2015, Noel Chiappa wrote:
>
>> 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.

>From what I remember from an earlier discussion about that PDP-1, after 
reforming, those large capacitors were leak tested at or slightly above 
their rated voltage.

As for meeting spec, unless DEC documented what their original design 
criteria was, there really is no way to know with 100% certainty. The only 
thing we could know today is if the capacitor passes industry standard 
tests and if the power supply those capacitors are a part of functions 
correctly when fully loaded.