fixing am radio?

Mopar offered Motorola AM radios, and my '62 Valiant has a Model 204, which was for the Valiant V-200. Motorola made 4 different types of AM radios for Plymouth and Lancer that year. They all used 3 tubes in the front end and 2 transistors in the audio stages, plus a transistorized power supply. I am referring to the Motorola service manuals, which are readily available.

The receiver didn't have any RF amp stage because I guess the AM stations were strong enough that it didn't need one. The antenna was fed right to the grid of a 12GA6, where it was converted to IF, then into a 12BL6 IF amp, and thence to a 12AE6A which did triple duty as a detector, AVC amp and AF amp. From there the weak audio signal went to a 2N573 germanium driver transistor and finally to a 2N176 germanium audio power amplifier and then to the speaker.

These radios used NPN-type transistors because a positive 12-volt power supply was available. (PNP transistors would have required a special negative power supply. In an NPN transistor, the voltage at the base controls the amount of current flow through the transistor, and it flows from the collector to the emitter. In a PNP transistor, the voltage at its base still controls amount of current flow, but it flows from the emitter to the collector.)

In my troubleshooting procedure above, I forgot to say you should check the "B+" power supply before going very far with the repair. In the case of my Model 204, after checking to make sure the transistorized power supply was producing plenty of voltage for the plates of the 3 tubes and that it was actually present at the plates, I would also check the power on the collectors of the 2N573 driver and the 2N176 audio power amp.