20 Jul
2015
20 Jul
'15
6:52 p.m.
On Mon, 20 Jul 2015, tony duell wrote:
I've not replaced any in a PDP11 power supply, however I have replaced
them in other equipment with equally large PSUs. In the odd case where a
computer grade screw terminal capacitor is extremely expensive or
completely unobtainable (those which I've purchased were under $20-30) I
might be willing to leave an original part in place, *if* it can pass a
leakage test.
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. [...]
Testing each aluminum electrolytic and wax paper capacitor vs replacing
them all doesn't seem to be the prevailing norm in the vintage radio
communities today.
Yes, the grid cap would /usually/ be a non-polarized wax paper type, which
tend to be very unreliable. I've yet to find a wax paper type which will
pass a leak test and those are also on my replace on sight list.
Of course you wouldn't want to replace mica, ceramic, or plastic film
parts without good reason, but if a set is going to be more than just a
shelf queen, aluminum electrolytics and wax paper capacitors are a
must-replace item. Carbon film resistors in this sort of equipment should
also be tested, however I only replace those which are either bad or out
of tolerance (some brands held up better than others).
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.
That's a good idea, however something to keep in mind is that class Y
safety rated capacitors are not designed not to short (and not put say a
floating chassis at mains potential). Safety rated capacitors are instead
designed to blow clear while not catching on fire (or otherwise be
self-extinguishing), should they short out across mains potential. This
means should a class Y part short, a floating chassis could still be
placed at mains potential, and that capacitor is not going to blow clear.
That said, anything is going to be better than a wax paper capacitor, and
a class Y safety rated part is also much safer in this sort of application
than a ceramic disc.
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.
As a general rule, I do not make shotgun-repairs to electronic equipment.
While I may replace certain failure-prone parts outright on sight, I still
prefer to determine which part(s) are causing a malfunction.
I consider replacing aluminum electrolytics to be preventive maintenance.
One wouldn't drive a 20-50 year old car with original hoses, belts, and
tires, and IMO it is just common sense to replace electronic components
such as aluminum electrolytic capacitors which have extremely well
documented life expectancies and failure rates.
As far as shotgun-repairs go, one of my own pet peeves are those out there
selling "cap kits" (usually really low quality [sometimes counterfeit]
capacitors, too) to newbies which also include a bunch of semiconductors
(diodes, voltage regulators, and transistors) on the theory that those
parts fail because they run hot. I've gotten to the point where I will not
even attempt to service a board which has been botched up by a fat
fingered newbie who has attempted to install one of those kits.
There is however one component besides certain capacitors which I
absolutely will replace on sight, no exceptions, period, and those are
selenium rectifiers. There is nothing good that can be said of selenium
rectifiers, and it is absolutely trivial to solder in a silicon diode as a
modern replacement.