Question on "mica" caps in Philco 40-155
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Looking at the parts list for the 40-155 I see that it lists 7 "mica" caps and two "silver mica" caps. First question is why list two of them as "silver mica"? Second, and more importantly, does anyone know if the 7 listed as "mica" are all mica? I think four probably are since they reange in value from 5 to 250 mmfd. The other three "mica" caps have values of 1250, 1500, and 5300 mmfd. I'm thinking these are paper in the postage stamp cases. Two of this big ones are hooked to the band switch and the remaining one is tied into the loop antenna.
Anyone have past experience on these? They all look the same except the "silver mica" are a light tan case and the other dark brown with the color code.
Johnny
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is there a makers name on those?
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I know I am not directly answering the question, but I would simply use Mica caps from today's catalog.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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The only caps to be concerned about are the larger mica caps covered in wax in some models, the molded ones in Philcos usually hold up fine.
Regards
Arran
(This post was last modified: 03-24-2021, 03:36 PM by Arran.)
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Everything looks untouched in the radio, but the makers of the mica caps vary as Corneill-Dublier, Aerovax, and Micamold versions are present. There are also two that have no manufacturer ID on the side with the color code and I can't see the back of those to see if they are marked.
The color codes don't align with the schematic. For example, what should be a 5mmfd has a code of Yel-Grn-Yel, a 110 mmfd has colors of Or-Yel-Or, and a 1250mmfd is Grn-Or-Grn. Only one of the mica color codes agree with the schematic...that one of a 5300 mmfd with Grn-Or-Red.
The large value "micas" (1250 and 5300 mmfd) are both Cornell-Dublier.
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if they are defective i would think that a silver/mica would be fine. the large values can be had in silver/mica. i did have some micamolds in a HALLICRAFTERS that were almost black around the bandswitch that indeed looked like mica ''domino's'' but were paper foil as i broke them in half to make sure. they were called out as paper in the i.p.b. i dont think that philco would have made a mistake but anything is possible i guess. i realize its apples and oranges but i thought i'd throw it out there. i looked in an online 1938 philco catalog and couldnt find your p/n for the 5300 pf,but i also couldnt find any paper/foil in a domino either of any value.
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The 5300 mmfd "mica" is part number 30-1134 on the parts list for the 40-155. I think the list is dated 1939.
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inquiring minds need to know, assuming mica caps are not SILVER/MICA, what other material was used?
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i'll answer my own question as i just found it on the web. there were two types of mica capacitors, silver mica and "clamped". clamped was two sheets of mica as it says ,clamped together and encased. they are reported to be not as stable or as accurate with regard to tolerance. just looking at it, if i was doing the set i probably just replace them with modern silver micas. mostly, as i dont know how well they hold up over the years as opposed to silver mica. seems like its prudent to eliminate any failure related item if indeed they are
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Thanks for the info on the difference on mica and silver mica caps. I'm suspecting that the "mica" caps might be part of the issue with the SW band not working. The band switch looks like it cuts in different caps and the high SW band doesn't hear a thing. The Police band that goes up to 3.5 MHz does pick up a station or two as well as some amateur stuff. Tracking is off on that since the band ends at 3.5 and the amateur stuff I hear is between 3.5 and probably just above 3.6 MHz.
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let us know if those caps are the problem.im working on a 48-482 thats loaded with them and they will be miserable to change. there are lots of them too.
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I have concluded that the so called "micas" may be the problem with the SW band since everything else checks out. Coil resistances, band switch cleaned and tested for continuity at the various positions, voltages within 10 volts or less from schematic, new RF and oscillator tubes ....resulted in nada. If everything measures according to the schematic and the micas are original, I don't know where else to try to fix the SW problem. I'm going to switch out 4 of them and l'll lpost the results. I simply don't know what else to try to get the SW band to oscillate.
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keep us posted.im in the midst of the recap a resistor replacement but im out of some caps that will take weeks to get here. it looks like my set has a lot of values that arent made anymore. the philco folks also didnt identify the type of cap whether mica or silver mica. at first i thought the prefix "60" was the key but only to find that they used 60 and 61 for silver mica as well.
(This post was last modified: 04-23-2021, 08:15 AM by falcon123.)
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I would check the grounds around the tuning cap including the rotor shaft. Be sure the that the brass forks that make the gnd connection are good and clean. Wiring on tuning cap for shorts do to failed rubber.
Check the heater voltage at the 7J7 socket. If it's a little low use a variac to increase the line till you get to 6.3v. Use a counter or digital communication rx to check the osc and track it across the band 455kc about the dial setting. Couple it with a few turn of wire around the 7J7 glass.
Happy Hunting!
de N3GTE
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
(This post was last modified: 04-23-2021, 05:58 PM by Radioroslyn.)
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