PDP 11 gear finally moved

Tothwolf tothwolf at concentric.net
Sun Jul 19 21:55:40 CDT 2015

On Fri, 17 Jul 2015, tony duell wrote:

>> Perhaps.  But not all of it, certainly.  I'm currently four for four 
>> fixing dead flatscreens by re-capping their power supplies; I imagine 
>> others have similar experiences.  It's not a huge stretch to imagine
> This could be taken to show that modern capacitors are not reliable, and 
> given that there are plenty of 40-year-old ones still in use in various 
> classic computers here it would be better to leave them as-is

It really depends. If I'm having to replace filter capacitors in a faulty 
PSU or monitor, even if I've isolated the problem to one or two 
capacitors, I would be a fool not to replace them all because the next 
part that fails will be one of those which I didn't replace (been there, 
done that, own the tee shirt). A standard 85C or 65C rated 20-30 year old 
aluminum electrolytic is simply past its useful service life.

> More seriously, a lot of modern consumer stuff seems to have 
> marginally-rated capacitors (and the use of 85 degree ones doesn't 
> help). Possibly on those it is a good idea to replace them. But the ones 
> in PDP11s were good quality at the start and were over-spec'd in 
> general.

With modern electronics, there are high quality parts and there are really 
cheap parts. Even though the high quality parts are about the same price 
as the really cheap parts when purchased in small quantity (in the one-off 
to a few 1000 quantity), profit-driven consumer electronics manufacturers 
are still going to use the cheaper parts when they can save even $0.01 or 
$0.005 per component or even a few cents on the overall cost of the 
complete widget.

Many of these manufacturers also design their widgets to last for the 
warranty term and no more. If the consumer gets 2-3 years or more out of 
said widget, those manufacturers consider it a loss in terms of potential 
profits. In these cases, wholesale replacement of really cheap capacitors 
with high quality versions from top tier manufacturers (Panasonic, 
Nichicon, Rubycon, etc) can be a /really/ good idea. That said, even the 
top-tier capacitor manufacturers have lower end lines of parts, so it pays 
to do your homework and choose a longer life (usually lower ESR, meaning 
less internal heating and thus longer service life) part when sourcing 

Even vintage Mallory and Sprague parts are not without their faults. I 
recently replaced a bunch of early 1980s era Mallory capacitors which 
tested good, but when desoldered from the board, had brown crusty stuff 
around the safety vent in their bottom rubber seals. They might have 
continued to work "ok" for another year or even 10 years, but the high 
quality replacement parts I put in will be good for at least another 20-30 
years. (The replacement parts also worked better as the DC rails had lower 
ripple after they were installed).

>> that other power supplies may have similar issues; even if it turns out 
>> to not be the case, there is probably at least a little "can't hurt 
>> anything, right?" running around.
> Ah but it can hurt. Damage to the PCB (unlikely, sure), the new part 
> might be faulty and thus introduce more faults, you might make an error 
> fitting it, and so on. I prefer to only replace that which needs 
> replacing.

It really is a case by case basis, however in PSUs, CRT monitors, and 
similar where the components are exposed to higher temperatures, it really 
makes sense to replace 20-30 year old aluminum electrolytics wholesale.

I guess another way to look at it is that an electronic device might still 
be functioning with old aluminum electrolytic capacitors, but is it still 
functioning as good as it was when it was new? A vintage device might have 
used really high quality aluminum electrolytics when it was made, but even 
high quality parts have a finite service life. What you have to decide is 
where to draw the when considering wholesale replacement of aluminum 
electrolytics, which with vintage electronics, I've found tends to be 
somewhere around the 20-30 year point.

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