I have this "All American 6ix" tube radio circuit board I decided to install inside an RCA cabinet. It had been working quite well, and decided it needed a good home inside a nice radio cabinet.



Looking at schematics of such radio circuits, I was puzzled that the primary of the RF transformer (between the RF amp and the converter tube) seemed to have a fixed cap across it. Wouldn't this create a fixed frequency LC circuit? Anyway I foolishly removed this transformer off the board to look inside it. Then reinstalled it. Seems I damaged the cap connection (the transformer has a plastic piece that sandwiches the cap against metal bent pins that also serve to connect the transformer to the circuit board and the coils inside it. Seems this connection got flakey. I was chasing other possible faults (like cracks in the board, or bad resistors, caps or tube sockets. Decided to wade back into the transformer (constructed like a k-tran type.


I did manage to measure this cap. got 97pF. Tried forcing a hot soldering iron tip to try to make the plastic press on the cap. Managed to ruin the cap connection instead. Fine, I'll use a silver mica 100pF cap on the back of the board. I ended up removing the internal cap wafer so it can't come back to life... Reinstalled the transformer and the new cap. the trouble appears to be gone, dare I say... Been a month now and it's staying fixed.





More details here

This above radio has an RF amp stage and 3 gang tuning cap. the RF amp tube feeds an RF transformer T1 that has a fixed cap C1 (in the simulation schematic) across the primary, and the secondary has the 2nd tuning cap C3 on it. The primary L measures 1700uH and with the 100pF cap it would resonate at 386kHz. The secondary L forms an LC circuit with the 2nd tuning cap. As you can see in the schematic in my above first post. Seems a strange circuit, so I did simulations of this. The secondary LC circuit dominates, and produces a tunable resonance. The resonant peaks are about the same amplitude.

Simulation results:






I updated my web page that discusses my adventure with this radio