PDP 11 gear finally moved

tony duell ard at p850ug1.demon.co.uk
Mon Jul 20 09:11:46 CDT 2015

> Replace - yes, *especially* if you don't have a big budget. Aluminum
> electrolytic capacitors are CHEAP and easy to obtain. Replacement
> semiconductors by comparison are expensive and can be quite difficult to
> find.

Err, have you priced the screw-terminal 'computer grade' electolytic capacitors
that were used in these PDP11 power supplies. They are not cheap, if you can
find them at all. And of course NOS ones might be as good or bad as the one
that's already there. Conversely when I had a major disaster in a DEC power 
regulator brick some years ago (blew almost all the transistors and the 723) the
replacement parts were easy to get (exact replacements, not just equivalents) and
were not expensive.


> Ironically, 20-30 years ago this same mindset used to persist with people
> who collected vacuum tube (valve) based radios and television, however
> that attitude no longer seems to be present in those communities today
> (not worth risking an irreplaceable transformer or inductor over
> $5.00-$10.00 worth of aluminum electrolytics).

Odd... I know plenty of people who restore old valve radios and audio stuff and
not one will blanket-replace all the aluminium electrolytics. There is a capacitor that
I (and they) would check very carefully, but that's not an electrolytic. I refer of course 
to the coupling capacitor to output valve grid. In a lot of radios this is connected to the 
anode (plate) of the audio ampilfier triode so if it leaks it puts a +ve voltage on the
output valve causing far too high an anode current there.

But even then I (and everyone else I know) would test it, not just replace it. Some of those
capacitors are very reliable and the replacements you get not any better.

I probably would replace certain safety-related capacitors in live chassis sets, like ones that
isolate external sockets, using class Y replacements. But that;s about it.

Incidentally, do you shotgun-replace 7805s and other 3 terminal regulators? If not, why not?
They can fail, and if they do they do a lot more damage than a failed capacitor.


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