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Who made those? Some early advertising for Sprague Black Beauty caps said they were plasticized paper. The main problem with these is some sort of production problem they had. I have dissected a number of "bumble bee" caps, these are actually Sprague Black Beauty caps with striped value markigns. I found the windings were distorted. I suspect this happened during the encapsulation but don't know. Some of these caps, the higher voltage ones, were oil impregnated. The oil was introduced through a tube which formed one of the leads. It is common for all the oil to have leaked out due to a lack of sealing at the solder joint between the tube and the lead. That may have been due to excessive heat when the caps were installed. The most notorious of the BB caps are those in the Hammarlund SP-600, however, they were very widely used including in a lot of -hp- gear. However, in the receiver it is common to find cracked or split caps or even caps where part of the casing has come off. I have not seen that in any other make of equipment. Perhaps just a bad lot or perhaps some difference in installation. Nonetheless, ALL paper caps become degraded with age. The usual symptom is an increase in series resistance. Leakage, which is another term for parallel resistance, is sometimes a problem but not as often. I don't think any of these caps, even the real oil filled transmitting caps, was intended to have a lifetime of over about ten years. The life problem was known and described in some engineering texts. It was dependent partly on voltage; the closer the voltage to the rated voltage the shorter the life of the cap. Likely it was some sort of electrolytic degradation of the paper.
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?? In any case, paper caps should be replaced with modern plastic film caps. These for the most part will last forever and generally will improve performance perhaps beyond the original. There is a good amount of information on the web about the relative merits of various kinds of plastic dielectrics. Polypropylene seems to be about the best especially for RF use. On 11/4/2021 9:33 AM, Dave Wise wrote:
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Richard Knoppow dickburk@... WB6KBL |
