PDP 11 gear finally moved

devin davison lyokoboy0 at gmail.com
Fri Jul 17 14:50:20 CDT 2015

I had to do some cap replacement on some older Motorola tube radios,I have
some basic soldering skills. I was under the impression that the capacitors
in computer equipment this big from this year would have been of better
quality and it would not be an issue.

I have someone scheduled to come out tonight after i get off work and get
it out of the rack.

On Fri, Jul 17, 2015 at 3:45 PM, Brent Hilpert <hilpert at cs.ubc.ca> wrote:

> 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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