I've searched for and read a lot of the previous posts on mica capacitors. I didn't see any reference to the flat molded paper capacitors like Phillip Nelson describes on his site
http://www.antiqueradio.org/recap.htm . These look about the size and shape as some of the mica capacitors and are marked with 3 dots like the micas.
Did Philco use this type of paper capacitor? Should I assume any of the flat rectangular type are mica?
Thanks, Chuck,
The pictures on your site do resemble the little rectangular packages I'd thought were capacitors. The ones I have seem to be less elongated and closer to being square, but the marking seems right. Tonight I'll pull some off and see if they measure ohms or picofarads.
The radios I am working on are 1940 through 1942, so the time frame is correct for those resistors.
Chuck,
I spent some time rummaging around the component section of your site. I think the rectangular components I thought might be micamold paper caps are probably mica caps of body style 6 with pf values and RMA color code.
I traced a few of them down in the schematic as best I could, and they all corresponded to a cap with a pf value that matched the dot pattern.
Then I unsoldered a few and measured the capacitance. The results were not encouraging. Most had values very different from the dot pattern, so I think I'll be replacing them.
While I was looking, I looked over some caps that were similar but lighter colored, reddish instead of brown. Some have a six dot code and some have three. At least some are microfarad range caps that look similar to, but not exactly like, some of the other body types on your mica cap page. Two of them had "MFD" and "Type 2" stamped on the body near the 3 dots. I might need to puzzle the values out from your table or else find them on the schematic.
I had been hoping that I could leave the micas alone, but after I read some old posts on the subject, I guess I'll replace them, too.
The radio plays well and is sensitive on all bands, so I might take my time getting to this chore. With replacing rubber wires and figuring out what to do with an open push-button coil, I have plenty to keep me busy!
Thanks for your help.
Hi John,
Nothing wrong with replacing these with new silver mica caps of correct value.
But I would not put too much stock in reading value of these caps from the colored dot patterns. This can be very difficult for even those with a lot of experience, and choosing a replacement mica cap based on the dot color coding of the supposed original is a last resort. Rather, I would base my decision on the proper value as noted on the schematic and/or parts list.
Several reasons for this: Colors on the dots can fade over the years making them difficult to read, or cause one color to appear as another. Also, sometimes one dot color may fade out or peel off completely, leading you to assume that the dot is blank when in fact it may have originally been colored.
Regarding measuring capacitance of your caps, what equipment are you using to measure them with? For mica caps in pre-war Philco radios in general, I have not experienced many which vary much in capacitance from original values. Rather, the more common problem is leakage which can only be tested in a bridge-type capacitor tester at full rated voltage, such as a Sprague TelOhmike, EICO 950, etc. And the second-most common problem with them are the short leads which become brittle and limit flexibility due to expansion and contraction over the years and can cause internal "opens" which will give erroneous and/or varying readings when testing for either capacitance or leakage.
Poston