PDP 11 gear finally moved

Christian Corti cc at informatik.uni-stuttgart.de
Tue Jul 21 03:30:00 CDT 2015

On Mon, 20 Jul 2015, Rich Alderson wrote:
> 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.

Please excuse me, but this is utter nonsense.

Most electrolytics in our machines are 30 years and older, and they just 
work. Those caps that I checked (mostly large filter/smoothing caps), e.g. 
those from the LGP-30 (nearly 60 years old) or Mincal 523 (44 years old), 
are just fine. Smaller ones don't even have to be bothered with. OTOH foil 
caps from the 50s/60s (e.g. ERO/EROFOL/EROID/Wima) tend to lose a bit of 
their isolation and become resistive (several MOhm). This can be a problem 
with AC coupling in tube circuits. Also problematic are more modern foil 
caps in line filters (e.g. X/Y caps), or even oil filled MP caps in power 
supply (magnetic constanters, filters) or motor applications (phase 
But admittedly I don't know what crappy electrolytics you have encountered 
in your "industry grade" machines. Or are we talking of modern 
machines (<30 years) ?

> [1] NB:  I am not now, nor have I ever claimed to be, a hardware 
> engineer of any stripe, and more particularly not an electronics 
> specialist.  I am, nonetheless, capable of reading and understanding 
> research papers with statistics that back up the claims being made even 
> if I could not devise the experiment to test them.  I rely on my 
> colleagues who are experts to assure me that the writers are not smoking 
> crack.

Statistics... don't believe any statistic that you haven't faked yourself.
Honestly, IMO this doesn't really qualify you as expert in capacitors.
I think those statistics are based on running the caps 24/7 at their 
nominal ratings, but surley they don't apply to moderate museum usage.


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