14 Jan
2002
14 Jan
'02
11:29 a.m.
On Jan 14, 9:45, Jerome Fine wrote:
of
Oh, I don't know about that, particularly for older equipment there are a
few others here who can outdo me, I think .
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 > > 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. 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
14 Jan
14 Jan
12:18 p.m.
Pete Turnbull wrote:
On Jan 14,
9:45, Jerome Fine wrote:
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.
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