Friday, January 13, 2012
re-winding an RF coil in a Colin B. Kennedy model 20B
Even after re-stuffing the decoupling caps my CBK 20B was still not working properly.
It turns out that one of the primary windings was broken in one of the 3 high-Q RF resonator transformers.
Removing the transformer was difficult (see photos above). I had to remove a bottom panel then bend-out small metal tabs, and break-free the glue that glued the top of the transformer to its shield (very unusual construction techniques are used in this radio compared to most old radios that i've worked on).
I un-wound this coil and found the break about half-way in, probably due to the bad decoupling capacitors.
After re-winding it the radio still did not work well. It turns out that the wiring in the schematic did not match the factory wiring in my radio. No surprise, as my AF output tube is different (class A as opposed to push-pull) and a few other things.
I touched the grid cap on the first RF amplifier and the radio came to life. I then traced the circuit from the primary of the first RF transformer to a potentiometer on the front panel. It turns out that the RF input of this radio goes directly through a potentiometer which is ganged with the control grid potentiometer which controls the volume. This was not a bad repair job, it was factory wiring cable laced in, etc etc.
I removed this wiring and tied the primary to the antenna input and radio came to life. Very sensitive as expected, but a new problem was noticed.
The control grid rheostat is not working well. I think there are open windings or the bushing is not making electrical contact with the wiper.
Stay tuned for more....