There is an enormous capacitor running from the triode plate (pin 2) to ground on the XXFM tube on my 42-350 radio. It matches the type 1 mica cap on Chuck Schwark's mica condenser page
http://www.philcorepairbench.com/micacap.htm . I can't read the part number because it is covered with wax, but the schematic says the value is 100pf. On my capacitance meter after compensating for the lead capacitance, it reads close to the 100 pf nominal value. I wonder why this cap is so much larger than the other type 6 100pf micas I have seen in my 1940 and '41 radios. (Two or three times the size.) My question is should I replace it with an modern silver mica cap of the same value. Does the large size mean I need to use a different type with higher voltage or current capacity?
I know there are mixed opinions on whether micas need replacement. I probably will replace it, and my question is mainly about whether the huge size means my modern silver micas will work or not.
This cap is part 65 on the 41-350 schematic.
Hi John
Replace it with a mica or ceramic cap, 100 pF. As this is in the audio section, either mica or ceramic will be OK.
Interesting...I looked at the pictorial drawing of the underside of this set, and the drawing shows a small mica cap. Could be the cap in your set is a later replacement?
Thanks for the quick response, Ron. I did replace the cap with a mica I had on hand, as you suggested.
I am inclined to think that the type 1 cap is original, partly because the whole chassis looks untouched. Even the filter caps and the AC cord look original.
Just to have a little fun with "philco phorensics," here are some other reasons I think it might be original.
--The location would make it difficult to get to without disturbing several other wires and components, so I would think a repair person might cut it out and locate it more accessibly.
--One lead wire from the capacitor runs around pin 4 of the tube socket, around the center pin of the socket, and over to the ground pin of the nearby terminal strip. I think a repair person would be less likely to unsolder and replace that whole lead wire than to cut the cap out and replace it on pin 4.
--Finally, the capacitor leads are the lower ones (with the chassis upside down) on all the terminals they go around and tie to, as if they were put there first.
By the way, Ron, I can't say enough how much I appreciate that you take so much trouble to look all this stuff up when you reply to all of our many questions. You and Chuck, both.
I have been told by several "oldtimer" radio techs that sometimes, on assembly lines, different parts were subbed due to availability in many radio sets.Physical sizes dont matter as much as close uufs/voltages in most cases. Sometimes, the techs on the assembly lines would "chunk in" a suitable useable component to complete a factory run, even though it didnt match the orig schematic. I have found that most of the time, those old sealed type mica caps are still very stable/useable in most cases in recapping vintage radios. Just my .02 ol TR