03-08-2013, 12:41 PM
You probably won't be able to make a good measurement on the mica caps, unless you have the kind of meter that you plug the leads directly into. Otherwise, your test leads have so much capacitance that you can't get a good reading of the micas, certainly not with the cap in the radio. Sometimes you can connect the test leads together and subtract the number you get from the one you get when you measure the caps, but it isn't very accurate with caps in the picofarad range.
I don't like to do it, but when I remove a mica cap from a radio for testing, I just replace it with a modern one. They are OK more often than not, but it is as much trouble to test them as to replace them. And if you don't have the right kind of meter it is really not possible to test them accurately.
I'm not sure, reading this thread, that you have injected your 455 KHz modulated signal into the mixer output. It sounds as if your IF stages are all OK, but I'd like to hear that you've successfully injected a modulated 455 KHz signal through a (.1 uf capacitor to protect your signal generator from the high voltage) at the input to the first IF coil. That would be point "E" in section 5 of the schematic.
Some of these 40s Philcos used a type of mica cap that looks like a large, thin, wax covered rectangle. I've mostly seen these are in the oscillator/converter circuit. Sometimes these were dual caps, two caps in one package. I don't know if your radio has any of these caps or not, but if you see any, they might be a likely culprit. I've heard that these are more prone to failure than the "domino" style of mica caps that you see elsewhere in the radio.
I do like the suggestion that you clean the band switch exhaustively. I've seen an otherwise restorable band switch make a radio silent. There are lots of switches shown in the schematic. You might try measuring the voltage on both sides of each switch if you can get to them, to see if the switch has continuity or not. Measuring resistance across the switches would tell you much the same thing.
I have a couple of these radios on my "to do" list, but I haven't opened them up to work on them yet.
I don't like to do it, but when I remove a mica cap from a radio for testing, I just replace it with a modern one. They are OK more often than not, but it is as much trouble to test them as to replace them. And if you don't have the right kind of meter it is really not possible to test them accurately.
I'm not sure, reading this thread, that you have injected your 455 KHz modulated signal into the mixer output. It sounds as if your IF stages are all OK, but I'd like to hear that you've successfully injected a modulated 455 KHz signal through a (.1 uf capacitor to protect your signal generator from the high voltage) at the input to the first IF coil. That would be point "E" in section 5 of the schematic.
Some of these 40s Philcos used a type of mica cap that looks like a large, thin, wax covered rectangle. I've mostly seen these are in the oscillator/converter circuit. Sometimes these were dual caps, two caps in one package. I don't know if your radio has any of these caps or not, but if you see any, they might be a likely culprit. I've heard that these are more prone to failure than the "domino" style of mica caps that you see elsewhere in the radio.
I do like the suggestion that you clean the band switch exhaustively. I've seen an otherwise restorable band switch make a radio silent. There are lots of switches shown in the schematic. You might try measuring the voltage on both sides of each switch if you can get to them, to see if the switch has continuity or not. Measuring resistance across the switches would tell you much the same thing.
I have a couple of these radios on my "to do" list, but I haven't opened them up to work on them yet.
John Honeycutt
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