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SP-600 Rebuilding Experiences
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 |
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