capacitor aging claim

Eric Smith eric at
Thu Oct 7 17:09:22 CDT 2010

William Donzelli wrote:
> I suspect that most big old electrolytics found in older computers
> mainly die from the seals breaking and moisture getting in, just as
> capacitors of the 1940s (even micas) are fond of doing. No amount of
> reforming will fix this problem.
I thought that when the seal failed, the electrolyte dried out?  Either 
way, though, you're correct; once the seal has failed the capacitor 
needs to be replaced.

I worked on the PDP-1 Restoration Project at the Computer History Museum 
in Mountain View, CA.  This equipment was manufactured in 1961.  We did 
about six months of inspection, maintenance, and repair of the PDP-1 and 
the Type 30G display before we first applied power.  Part of this 
process was to check all of the electrolytic capacitors, and reforming 
or replace them as necessary.  It appeared that all of the capacitors 
were original.  Many of them needed to be reformed, and we used an 
extremely over-engineered approach to doing that.  Only a very small 
percentage of the electrolytic capacitors had failed such that they 
could not be reformed.

I fully expect that all of those capacitors will fail eventually, but 
there isn't nearly enough data to predict a mean lifetime.


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