Prophylactic replacement of electrolytic capacitors?

From: Pete Turnbull <pete_at_dunnington.u-net.com>
Date: Mon Jan 14 11:29:14 2002

On Jan 14, 9:45, Jerome Fine wrote:
> >Pete Turnbull wrote:
>
> > >On Jan 13, 21:27, Louis Schulman wrote:
> > > Old computer power supplies generally have big old electrolytic
> > > capacitors. When these go bad, they can
> > > cause real problems, and damage other components.
> > > The literature indicates that many of these only have a working life
of
> > > 2000 hrs., or a shelf life of ten years.
> > > Obviously, this will be exceeded in old computers.
> > I think you may have lost a digit off the working life, Louis, at least
if
> > you're referring to the sort of electrolytics found in PSUs :-)
>
> Jerome Fine replies:
>
> I was hoping you would answer this question. Thank you! Of all the
> hobby users around here, it seems that you have the most experience
> with repairs.

Oh, I don't know about that, particularly for older equipment there are a
few others here who can outdo me, I think .

> Now, that lost digit for the working life, I presume you mean that 20,000
> hours is more reasonable? Would that vary if the usage was 24/7 (on all
> the time for the whole 20,000 hours) as opposed to being on for 10 hours
> continuous once a week on average, i.e. about 50 times a year or 500
> hours a year?

20,000 hours is the top end of the range, really. 2000 is very low,
though. No, 24/7 versus several hours on different occasions won't make
much difference. Temperature makes a difference, becasue high temperatures
make a cap dry out faster, as does the ripple current, because of internal
heating (high current or high ESR).

> The other MUCH BIGGER question for the stuff most of us tend to work
> with is shelf life. That was estimated at 10 years. What is a
reasonable
> estimate? Also, more specifically, I have a number of BA23/BA123 boxes
> which are about the only PSU I am concerned about for the long term.
> Most are probably more than ten years old already - as far as shelf life
> is concerned. Is a PSU (in general an very specifically a BA23/BA123
> box) more likely to have a longer shelf life of used every so often - any
> if so, how often - once a week, month, year, decade?

That will make a difference, as electrolytics tend to degrade over time.
 The insulator is really a thin layer of oxide on the metal foil inside,
and if left sitting unused, that may dissolve. Hence the low shelf life.
 That's why you run long-unused electrolytics at a low voltage for a while,
to reform the oxide layer. Running the PSU every so often -- once or twice
a year -- will prevent that happening. With a linear supply, running it at
low voltage is not too difficult -- if there's no variac handy to reduce
the voltage, a low wattage (but mains voltage!) light bulb in series is a
good idea. That's not a good idea with switchers, because they just try to
draw more current instead, which is often worse. The ideal solution is to
remove the caps and run them on a current-limited bench supply, gradually
winding it up from a low voltage to the rated voltage of the capacitor.
 The PSUs in BA23/123 (almost all QBus machines, in fact) are switchers,
BTW.

> Could shelf life of some PSU also be as long as 30 years?

Yes, it could, if stored under optimum conditions (but I confess I'm not
sure what "optimum" would mean -- probably not too hot and dry, but not
damp either) and started up periodically to make sure the caps reform.
  However, a blown electrolytic is easy to replace and often not hard to
spot, because there's often a bulge or even a hole in the end!

Or in extreme cases, an empty can where the capacitor used to be. A few
years ago, I needed a non-polar electrolytic to fix an Atari monitor. The
nearest equivalent I could find was the same value and voltage but
physically much smaller than the original. The ripple current rating
wasn't very high, either, but I didn't know what the original was rated
for, so I tried it anyway. The monitor ran fine with the case off for
about ten minutes, then there was a loud BANG! and small pieces of fluff
floated down from the ceiling. Apparently the ripple current rating wasn't
high enough :-) I replaced that one with a polyester cap in the end.


-- 
Pete						Peter Turnbull
						Network Manager
						University of York
Received on Mon Jan 14 2002 - 11:29:14 GMT

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