PDP 11 gear finally moved

Tothwolf tothwolf at concentric.net
Tue Jul 21 21:45:44 CDT 2015

On Tue, 21 Jul 2015, tony duell wrote:
> On Mon, 20 Jul 2015, Tothwolf wrote:
>> 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.

IMO an alignment is simply part of the restoration process. When I service 
a set, I do so expecting that it is going to be used and thus needs to 
have an accurate dial vs just sitting on a shelf. Simply installing 
replacement aluminum electrolytics and wax-paper capacitors is not likely 
to affect alignment. It is extremely common however to find sets where 
someone else has previously mucked up the original alignment in an attempt 
to work around electrically leaky wax-paper capacitors which have caused 
the band the drift.

>> 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?

Why is that inconsistent? If I test a carbon comp resistor and it measures 
within spec, there isn't much reason to replace it. Unlike an aluminum 
electrolytic capacitor, a carbon comp resistor is very stable chemically. 
Carbon comp resistors tend to drift due to absorption of moisture, and 
while it is possible to dry one out in a toaster oven at a controlled 
temperature, the resistor will again drift out again over time, so if one 
is out of spec, replacement is the best option.

>>> 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.

Except that the chassis in modern equipment is /expected/ to be connected 
to ground, unlike a floating or hot chassis in a vintage radio. Both class 
X and class Y can fail short. A class Y tends to have a thicker dielectric 
and/or a lower voltage rating, which means it is less likely to fail 
short, not that is cannot fail short.

>> 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.

Possibly. Radio repair shops of the AA5 era also had a vested interest in 
turning a set around as quickly (and as cheaply) as possible, and a set 
back in again in the same year for another repair was also good for their 
business. Back then, consumers expected their radios to need "routine" 
service, so people were less likely to even question it. I've come to this 
conclusion based on the types and quality of radio shop repairs I've seen 
in these old sets. I have a radio in my to-do queue right now (an AA5) 
which was owned by my grandparents, where a shop needlessly hacked the 
leads off a Centralab hybrid module and replaced about half of its 
functionality with some really cheap wax-paper capacitors and a handful of 
resistors (after searching for a number of years, I actually managed to 
find a NOS module for it, so that part of the circuit will be restored to 
its original condition when I eventually get to that project).

>> 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....

Cap kits or counterfeits? ;)

Best way to avoid counterfeits...do not buy modern name brand aluminum 
electrolytic capacitors on eBay. When I've looked, it tended to be about 9 
out of 10 capacitors were counterfeits when I searched for Nichicon or 
Rubycon parts.

I wouldn't even really mind high quality cap kits for some projects, but 
those selling them usually make them from the absolute cheapest parts they 
can source, including the fake junk found on eBay, which includes 
counterfeit semiconductors (transistors and voltage regulators).

>> 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!)

They fail in the worst possible way, a rotten garlic smell that seems to 
completely permeate a room, which can take days, and sometimes weeks for 
the smell to completely clear. Not to mention how poisonous selenium 
itself is, which is best not released into an indoor environment. Since 
the selenium rectifier has basically zero reuse value, unless it is burned 
up, I usually just leave the old rectifier stack attached to the chassis 
and disconnect it.

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