[MrFixr55]

Edited 10/22/2026. You can do this with a DPDT switch. Connect one half of the switch as Rod described. The "common" of that half of the switch goes to the leg of the volume control where you broke the connection between the control and the radio's audio signal. The other leg of that switch connects to the "aux" input. You can use one or 2 RCA jacks or a 3.5 mm phone jack. If inputting a stereo signal, "merge" the signal using two 100 Ohm resistors joined. The "legs" of these resistors go to the 2 jacks. The "junction" goes to the "Aux in" terminal of the switch through the 0.05 uF cap that RodB recommends. I actually did this on an 89 years ago and it sounded great, much greater than the radio itself. It really rocked!!! The owner was not interested in the SW channel so I rigged it so when he turned the bandswitch, it hit the actuator of a SPDT microswitch. I did not have a crosstalk issue.

I made the above mod out of despiration before the 3rd attempt to rewind the oscillator coil and update to a 77 tube finally made the set work. It was interesting that the set used the 75 Detector / AVC / 1st Audio tube but not the 2A7 pentagrid convertor. The competing RCA R28 used the 2A7, used an RF amp, did not have an IF amp and used a biased detector, making the Philco actually the better set, the crappy autodyne convertor notwithstanding.

The other circuit of the DPDT switch is used to ground out the circuit not used. The "common" of that leg of the switch is grounded. Each leg of the switch goes to the opposite (not near) leg of the other circuit. In other words, if the top right contact is the Aux signal, run a jumper from that terminal to the bottom left contact of the switch. If the bottom right of the switch is the radio signal, run a jumper from that terminal to the top left of the switch. The middle left is ground. the middle right connects to the volume control. when the switch is in the "Aux" position, the radio signal should be grounded. When the switch is in the "Radio" position, the Aux input is grounded. This will solve the crosstalk issue.