04-05-2010, 10:59 PM
Appearances of the flat plastic capacitor packages can be deceiving. There were paper capacitors and mica capacitors that were in similar flat packages. The way to tell them apart is that the unreliable paper capacitors had capacitance values in the microfarad range. They should be replaced with modern caps. Mica or silver mica capacitors might look similar, but the capacitance would be in the micro-micro farad (picofarad) range. These are usually pretty reliable, though I have found a very few bad ones.
A couple of years ago, I posted a question about how to tell these two types of capacitors apart, and one of the moderators replied that Philco never used the molded plastic paper capacitor packages, so all caps of that type are actually mica. The capacitance value is also definitive. If all else fails, refer to the parts list. If it says "mica" or "silver mica" it most likely doesn't need replacement.
Philco used mica capacitors in other package types, typically large squares or rectangles covered in wax. Sometimes there were two different caps in one of these packages. A 42-355 I recently worked on had two of these big wax-covered mica capacitors in the FM circuit. On both, the wax was cracked and had begun to separate, so I figured they were on their way out and replaced them. When I had them out of the radio, I measured the capacitance and found they were both very close to their nominal value in spite of the appearance of the wax. Just for fun I chipped all the wax off one to see what was inside, and the capacitance hardly changed at all.
I usually replace micas if I have to desolder them anyway to change another component or wire, but otherwise I leave them alone. It is hard to measure picofarad caps unless they are out of the radio because capacitance of the meter leads can vary quite a bit depending on how you run them. I have a meter with little push terminals for the capacitor leads right on the body of the meter, and I get consistent readings from that. Once I have a mica cap all the way out of the radio to make the measurement, sometimes it just seems wiser to replace it, even if capacitance is pretty close to nominal.
A couple of years ago, I posted a question about how to tell these two types of capacitors apart, and one of the moderators replied that Philco never used the molded plastic paper capacitor packages, so all caps of that type are actually mica. The capacitance value is also definitive. If all else fails, refer to the parts list. If it says "mica" or "silver mica" it most likely doesn't need replacement.
Philco used mica capacitors in other package types, typically large squares or rectangles covered in wax. Sometimes there were two different caps in one of these packages. A 42-355 I recently worked on had two of these big wax-covered mica capacitors in the FM circuit. On both, the wax was cracked and had begun to separate, so I figured they were on their way out and replaced them. When I had them out of the radio, I measured the capacitance and found they were both very close to their nominal value in spite of the appearance of the wax. Just for fun I chipped all the wax off one to see what was inside, and the capacitance hardly changed at all.
I usually replace micas if I have to desolder them anyway to change another component or wire, but otherwise I leave them alone. It is hard to measure picofarad caps unless they are out of the radio because capacitance of the meter leads can vary quite a bit depending on how you run them. I have a meter with little push terminals for the capacitor leads right on the body of the meter, and I get consistent readings from that. Once I have a mica cap all the way out of the radio to make the measurement, sometimes it just seems wiser to replace it, even if capacitance is pretty close to nominal.
John Honeycutt
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