"Chuck Harris" wrote in
message ...
Steve wrote:
I'll second what Frank suggests about replacing ALL of
the black
beauty caps. I'm doing that now on the SP-600 I'm
restoring.
While none were a dead short, most showed some leakage,
about 100K ohms. In many circuits, that won't cause a
problem
but in some its a big deal. About 1/3 had cracked cases.

Steve

Two things make the Sprague Black Beauty caps a problem,
and both relate to their losing oil.

The cap is molded out of a black plastic, with a small
brass filler tube on the banded end. The capacitors were
filled with mineral oil through this tube, and the lead
was
placed in the hole, and crimped. They then sealed the
capacitor
by adding a little bit of solder.

So far, so good. Well, when the capacitors are installed,
very
often the banded lead is close to the lug that is being
soldered,
and it will carry the heat to the solder seal, and to the
oil in
the cap. The slight pressure increase in the cap will
softly
blow the solder out of the seal joint, and the oil will
begin to
seep out of the capacitor. Note: the oil is the
dielectric, not the
paper.

Also, most all plastics will shrink over time, and the
plastic the
Black Beauties are made from is no exception. When it
shrinks, the
leads, and the filler tube do not, and you get a small
crack, and a
leak.

If the oil is allowed to seep out, and the moisture is
allowed to
seep in, the capacitor will be compromised and become
leaky. Also,
without the oil, the WV of the capacitor is lowered,
because the oil
is the dielectric. Capacitors that are used near their WV
will often
arc over, and become shorted.... with catastrophic results
for the
rest of the circuit.

-Chuck

The BB's I found in the SP-600 are all of the dry Mylar
type. I have seen BB's with oil all over them in other
equipment. These were sold originally as highly stable, wide
temperature range, replacements for the conventional paper
type. Obviously something went wrong. The BB's I disected
had glazed looking paper and attachment ferrules at the
ends. They were not rolled up into a round tube but rather a
quite flattened one. I don't know if this is how they were
made or if its a result of the case shrinkage. In any case I
agree with those who recommend replacing them at sight with
disc ceramics.
BTW, Sangamo also sold Mylar-paper caps in Epoxy dipped
form. I wonder if these were any more reliable than the
molded ones.


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Richard Knoppow
Los Angeles, CA, USA