philco: Make a new Special Inductive Capacitor - IF trap.

ID: 243083
This article refers to the model: 46-421 (Philco, Philadelphia Stg. Batt. Co.; USA)

philco: Make a new Special Inductive Capacitor - IF trap. 
21.Jan.11 07:10
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Paul E. Pinyot † 2013 (USA)
Articles: 187
Count of Thanks: 3
Paul E. Pinyot † 2013

 

How to make a Philco “Special” capacitor.
 
Philco uses a Special Capacitor that has an inductive series coil added. Philco also uses a special capacitor that has highly inductive qualities engineered in. It is designed to bypass IF to chassis ground. This Philco 46-421 uses one of these as C401. I had to fabricate a replacement. This is how I did it.
 
To build the coil:
  • First acquired a new polypropylene capacitor of 0.2uf 630volts. The high voltage is what I had in stock. 
  • Use enamel covered magnetic wire.  First soldered one end of the wire to one capacitor lead and clipped it short. 
  • Lay the wire up the middle of the capacitor lengthwise and wrap the coil over top of the magnet wire lead. This holds the wire from slipping.
  • Wrap about a 1cm long coil around the out side of the new capacitor. 
  • Secure the loose coil in place with sticky tape. 
 Make the capacitor
 
To adjust the coil:
  • Connect a 1 or 2 ohm resistor to the free capacitor lead wire. This resistor will generate a voltage to be measured by the oscilloscope.
  • Connect the ground lead of the o-scope and signal generator to the loose end of the resistor. 
  • Connect the o-scope probe to the junction of the resistor and capacitor. 
  • Connect the signal generator to the loose end of the new coil. 
  • Set the signal generator for 455 kHz IF frequency.
 Set Generator for IF frequency
 
Adjust the o-scope to display the signal and set an amplitude reference point. The o-scope has a percentage graticule. I set the amplitude for 100%.
 
 
Raise the frequency 
 
 
Then raised and lowered the frequency. Added or remove coil windings until the signal decreased on either side of 455kHz.   
 
Lower the frequency
 
Checked the top end and bottom end frequency at 90% attenuation. Once satisfied with the response replace the sticky tape with heat shrink tubing.
 
 
 
I tried a UL rated safety capacitor with a smaller value but birdies (harmonic interference) appeared across the band. I remade the filter with the original capacitance of 0.2 uf and all the birdies were gone. 
 
Other reference reading can be found on the “Philco Repair Bench” web site. It has a great functional description of Philco’s Special Capacitors.
 
Happy Fabricating,
Paul
 
 

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