PDP 11 gear finally moved

devin davison lyokoboy0 at gmail.com
Fri Jul 17 14:58:33 CDT 2015

I was operating the panel when i first got it, now the numbers do not light
up, panel is unresponsive, and run light stays lit.(just describing the
behavior, i will not start it back up till I work on the power supply)

On Fri, Jul 17, 2015 at 3:50 PM, devin davison <lyokoboy0 at gmail.com> wrote:

> 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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