A browse of the schematic for the 68 am/fm 7303241 has confirmed that the plug [no cable attached] in the socket on the underside of the radio is in fact a jumper plug to link a few pins together to complete the audio signal path when the radio does not have the stereo multiplexer connected. I have since found various other references to this plug on the net.
Here's a pic of the pinout. The schematic does not say what view this is, but I would assume they are following valve convention, which would make this view as seen from the male pins side of the plug.
In any event, you can always work it out as pin 9 is the only pin with +12 volts on it [68 only, 69 is different - yet to work it out] and there should be 0 ohms continuity from the radio power connector [yellow wire] to pin 9 in the socket when the radio switch is 'on'
The 69 schematic of the stereo radio 91BFM2 also makes it clear how they were able to use the radios controls and maintain the stereo signal path. The volume and tone was now a monster concentric shaft 4 gang pot, two gangs for stereo volume and the other two gangs for stereo tone + the on/off switch at the back.
The fader is also a dual gang for stereo operation.
What still has me foxed is how the balance worked. There is just an arrow pointing to the region of the volume/tone controls, for both 69 and 70.
Did you push the tone knob to de-clutch tone and engage balance?? 69 stereo radio owners, please help me with this one!
