1 Apr
1997
1 Apr
'97
3:52 a.m.
Electrolytic Capacitors: They dry out, or will start
to ooze the
electrolyte. (I address this to some extent on the 'Repair Tips & Tricks'
section on my web pages) Average lifetime (of large caps such as found in
power supplies) is probably 5-10 years. On machines that I restore they
are (almost) always replaced before I do anything else. The very small
caps found on logic boards for timing and similar circuits do not seem to
be a prone to failure.
I think the average lifetime may be longer for electrolytics, especially
those made in the past thirty years. I have had decent luck getting 50
year old capacitors alive again - but only if they are treated right. The
best way to keep them happy is to reform them, treat them gently, and use
them.
Most of the following probably is better suited to guys with minis, as
often they have not been powered for 10 years and stored in a damp
warehouse...
Always disconnect the power supply(s) and check them out first! For
non-switching supplies, disconnect the output (no load condition) and use
a variac (variable transformer) on the input. Slowly ramp up the variac,
starting from 0. For really old electronics, I do this over a 30 minutes,
but that might be me being over-cautious. An ammeter on the input is also
handy, in case something starts to go (moisture in transformers - but
that is a different rant).
For switching supplies, connect a decent load, as some units _need_ a
load or they will break. Still ramp up the input voltage, but do it
quickly - perhaps over just a few seconds (the supplies will complain
about the low input voltage). I suppose the best thing to do is to power
the capacitors out of circuit - probably too much work for electonics of
this vintage.
The object of all of this is to prevent the capacitors from getting
whacked with full voltage from the rectifier. The longer the ramp-up on
the input, the better, as it allows the electroyte in the capacitor to
reform (I do not know the exact chemical reactions that take place during
decay and reformation, but I could probably dig them up).
Finally, the best way to keep the capacitors from decaying is to use them
from time to time - just like a vintage car engine.
Eventually they will
wear out, but I think that would be _far_ longer than 10 or 15
years.
Tantalum capacitors are pretty bulletproof, and do not need this treatment.
Card edge connectors: corrosion is common, especially
is stored in damp
places or contaminated by skin oils. Can be cleaned with the classic "Pink
Pearl" eraser. (Don't use the grey erasers - too abrasive!)
I would not do this, unless it is followed by a cleaning with a solvent
of some sort. Erasers do leave a residue, mainly in the microscopic pits
in the metal.
William Donzelli
william(a)ans.net