02-25-2015, 02:21 PM
Thanks for the link Ron. I just learned something new that is old
Since paper foil caps rarely go bad (in my experience) the best thing is to leave the IF trap alone or pull it and sweep it.
The chance of 2 of the old caps having the same C and self L would be rare. A modern one would seem remote.
Anyone built a new one?
A SWAG of procedure to get a satisfactory replacement would be to get new 0.2 MFD wrap some turns around it hook one end of the coil to the ? inside foil
? end of the cap and the other end of the coil is the other end of the series resonate circuit. Not sure if the new caps have a band to mark the outside foil end. 
The one in klondike's picture is either hooked to the outside foil side hiding the mark or that cap does not have an outside foil stripe.
Guess to bench test / tune to IF frequency add a film or carbon resistor like 50 or 100 ohms in series. Monitor the voltage across the resistor and sweep with a signal generator to find the initial resonant frequency (peak voltage across the resistor). If close to IF frequency squeeze the coil to raise L or pull apart to lower L.
If the resonate frequency is too far off, change the number of turns.
Alternatively sweep the replacement cap to find self resonance. Then grab a copy of an old "Allied Data Handbook" off the web and use the "Single-Layer Would Coil Chart" for air wound coils to get the initial coil in the ball park. Not sure how much the Aluminum foil of the cap will effect resonate frequency.
As Ryder says in the link it would be a broad resonance.
Probably a bit of hot glue to hold the coils spacing when finished.
Since paper foil caps rarely go bad (in my experience) the best thing is to leave the IF trap alone or pull it and sweep it.
The chance of 2 of the old caps having the same C and self L would be rare. A modern one would seem remote.
Anyone built a new one?
A SWAG of procedure to get a satisfactory replacement would be to get new 0.2 MFD wrap some turns around it hook one end of the coil to the ? inside foil
? end of the cap and the other end of the coil is the other end of the series resonate circuit. Not sure if the new caps have a band to mark the outside foil end. The one in klondike's picture is either hooked to the outside foil side hiding the mark or that cap does not have an outside foil stripe.Guess to bench test / tune to IF frequency add a film or carbon resistor like 50 or 100 ohms in series. Monitor the voltage across the resistor and sweep with a signal generator to find the initial resonant frequency (peak voltage across the resistor). If close to IF frequency squeeze the coil to raise L or pull apart to lower L.
If the resonate frequency is too far off, change the number of turns.
Alternatively sweep the replacement cap to find self resonance. Then grab a copy of an old "Allied Data Handbook" off the web and use the "Single-Layer Would Coil Chart" for air wound coils to get the initial coil in the ball park. Not sure how much the Aluminum foil of the cap will effect resonate frequency.
As Ryder says in the link it would be a broad resonance.
Probably a bit of hot glue to hold the coils spacing when finished.
