01-29-2006, 11:06 PM
First of all, I am confused by this talk of voltage measured on Pin 4 of the 7F8 converter (Pin 4 is the cathode). I assume you are actually talking about voltage on Pin 1 (oscillator grid)? If so, the -2.2v you are measuring on AM is normal, and you would expect to see somewhat less on FM, the -0.8v would not be unreasonable and I suspect the FM oscillator is actually running OK.
I am inclined to think that Chuck may be right here, look for any interruption in the signal path, most likely in the bandswitch wafers.
Also check mica capacitor C418 (100 pF) for open and/or leakage. And check resistor R409 (should be 15K).
Also make sure pins of 7F8 are clean and that it is making good connection in the socket. This was a common problem with postwar Philcos in FM mode.
Also, if all else fails, go back and look closely at all areas of the 6AG5 RF amp stage.
Regarding the four pieces of black "friction tape" over the rivets on the 7F8 socket, these have been present on every postwar Loctal-tube Philco with FM which I have ever serviced. They need to stay in place.
Poston
I am inclined to think that Chuck may be right here, look for any interruption in the signal path, most likely in the bandswitch wafers.
Also check mica capacitor C418 (100 pF) for open and/or leakage. And check resistor R409 (should be 15K).
Also make sure pins of 7F8 are clean and that it is making good connection in the socket. This was a common problem with postwar Philcos in FM mode.
Also, if all else fails, go back and look closely at all areas of the 6AG5 RF amp stage.
Regarding the four pieces of black "friction tape" over the rivets on the 7F8 socket, these have been present on every postwar Loctal-tube Philco with FM which I have ever serviced. They need to stay in place.
Poston
