PDP 11 gear finally moved

Brent Hilpert hilpert at cs.ubc.ca
Fri Jul 17 14:45:27 CDT 2015

On 2015-Jul-17, at 11:42 AM, tony duell wrote:
>>> It is generally a good idea to re-form electrolytic capacitors in power
>>> supplies, and to bench check the power supplies (under some kind of
>>> load) before actually applying power to the whole unit.
>> It is always a good idea to replace electrolytic capacitors in power supplies.
> Could you, please, explain why? And how often should this be done? Every
> week, every month, every year, or what?
> FWIW, the number PSU elecrtrolytics I have replaced can be counted on the fingers of
> one hand -- in unary. Well, perhaps both hands. But it's <1% of all the PSU electrolytic
> capacitors I own. 
> Only 2 cases spring to mind :
> The PSU in my 11/44 had a high ESR capacitor on the +36V rail (all other caps in the machine
> were fine)
> I changed the 2 mains smoothing capacitors in my HP120 not because they were electrically 
> defective (they tested fine) but because one was bulging a little on top and had it exploded it would
> have hit the neck of the CRT with all the problems that would be likely to cause.
> I do find this witch-hunt against capacitors to be curious, given how few I've found to have 
> failed. I suspect a lot of it comes from audiophools who think this is the way to fix anything...

This is something Tony and I are quite in agreement on.

Similar to Tony, (and as mentioned in discussion on this topic a couple of months ago): in the solid-state category, of the many pieces of 1960s & 70s and later equipment I have or have serviced, the vast majority are running with their original capacitors.

If you're dealing with a 1936 or 1952 tube radio, a knee-jerk "replace the capacitors" is warranted.
If you're dealing with a 1970s computer, it isn't (IMHO). Esp. when they're screw-terminal 'computer-grade' caps.

My own perception of the concern is that it has been perpetuated over the years from the vacuum tube / antique radio arena. The issue of capacitors "drying out" dates from the days (1920s,early 30s) when electrolytics actually were filled with an active liquid which actually did dry up.
"Dry electrolytics" were developed in the 1930s, and while early dry electrolytics also warrant replacement, the chemistry and techniques have seen a few improvements in the many intervening years, and solid-state equipment is not placing the same stresses on caps as tube equipment.

In other arenas it's a real issue, in a modern arena it is largely lore.

The point of electrolytic caps is to form an oxide to be the dielectric, formed (in part) out of the electrolyte, and while I'm no expert on the chemistry, I will point out the oxidised state is 'the' or 'a' low energy state, and hence relatively stable. Rust doesn't normally undo itself.

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