PDP 11 gear finally moved

tony duell ard at p850ug1.demon.co.uk
Tue Jul 21 00:10:05 CDT 2015

> 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 thought that was the difference between class X (will fail in a safe way, but
may short) and class Y (will not short). The latter are to be used where
'failure of the capacitor may expose a person to electric shock' according
to the data sheets I've read.

In general class X go across the mains, class Y from mains to ground.

> 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!)


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