20 Jul
2015
20 Jul
'15
10:10 p.m.
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.
I was quoted over \pounds 30.00 each for the ones for DEC power bricks. No
thanks. I've yet to have one fail in a couple of dozen of said regulator units.
[...]
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.
It is over here.
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
It depends a lot on the circuit. If replacing the capacitor is going to involve
major realignment and the original is probably OK and leakage is not going
to do further damage (likely in the case of a tuning component) then I will
leave it and only replace if it fails.
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).
This is inconistent. A capacitor which is failing (starting to leak, say) may get
worse. A resistor which is drifting may get worse. Either can do more damage
when it fails. Why replace the cap and not the resistor?
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 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.
I do wonder if this data is based on the cheaper components used in
consumer electronics (paticularly things like AA5s) and that the capacitors
used in computers were of a much higher quality and longer life.
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]
Oh don't get me started....
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.
In some cases you need a series resistor to compensate for the forward
resistance of the selenium rectifier or the output voltage goes too high.
Particularly in those mains/battery valve radios with 1.5V directly heated
valves that have filament burn-out if you look at them wrongly...
But yes, selenium rectifiers rarely work now (although there are exceptions)
and when they fail they can take out the mains transformer. And they smell
horrible (think of school dinner cabbage!)
-tony