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Problem solved. In this radio the audio tubes are on the chassis while all the resistor/capacitor parts for each output tube are mounted on left and right circuit boards. The tube sockets are then individually wired to the circuit boards. It looks like someone melted two wires together where they connect to the right circuit board at some tine in the radio's history. This effectively shorted the right side of the balance control to the top of the right side bass control which caused the signal path from the output of the preamp stage to bypass a 150K resistor which is in series with the input of the bass control. This skewed the balance control's action. With the short removed the control functions as I would have expected, mostly quieting one side or the other as the control is turned.
Why they designed the balance control to only work in the stereo mode is still a mystery to me.
Mike
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