Capacitor Life Span
Q. Do mylar tubular capacitors and
ceramics deteriorate with age? If so, is
the deterioration gradual or is there
usually a sudden failure? If gradual,
what is the effect upon power amplifier
performance? Is there an average life
span for these components, beyond
which replacement is advisable?
Similar advice re electrolytic capac-
itors will be appreciated.-Walter Diehl,
Great Neck, N.Y.
A. Tubular paper capacitors and
ceramic units do not usually fail; their
life span is indefinite. When they do
fail, however, they will fail suddenly
and completely. They will either com-
pletely short or they will open.
Electrolytics will fail after a time. I
do not know if we can specify a defi-
nite life span for them, but perhaps
ten years is about what can be ex-
pected of most of them. Some will last
longer and others a shorter length of
time. Electrolytic capacitors fail grad-
ually, losing their capacitance little by
little, Depending upon their location
in a circuit this gradual decrease in
capacitance can lead to loss of bass
response, crosstalk between channels,
leakage of signal even with the gain
control turned down fully, motor -
boating, and hum.
As is true of paper and ceramic
capacitors, electrolytics are also sub-
ject to catastrophic failure. Sometimes,
too, the internal connections between
the lugs and the foil can become de-
fective, leading to intermittent opera-
tion of the capacitor.
If you have electrolytic capacitors
in the "junk box," you may find that
they deteriorated in another respect.
Their breakdown voltage may become
lower than their nominal rating. If
you plan to use a capacitor which has
been stored for long periods, you can
take precautions to see that it does not
fail when placed in service. You can
reform the electrolytic coating. This
requires the use of a variable voltage
power supply. Connect the capacitor
in series with a resistor whose value
is in the order of 50K or 100K ohms,
ten watts. Connect this series com-
bination across the power supply,
being careful of polarity. If the capac-
itor is rated at 450 V, start with ap-
proximately 200 volts. Over a period
of several hours, gradually bring up
the voltage to the rated value. You
should allow at least 12 hours for this
operation.
It is possible that the capacitor
will short out during this reforming
process. Of course, it must then be
discarded. The resistor, therefore, is
used to limit the current flowing in
the circuit, thereby protecting the
power supply from possible damage.