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