PDP 11 gear finally moved

Tothwolf tothwolf at concentric.net
Mon Jul 20 20:52:20 CDT 2015

On Mon, 20 Jul 2015, tony duell wrote:

>> 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.

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.

> [...]
>> 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.

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).

> 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.

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