The shematic shows that this amp used a 250K MN potentiometer, with 50% of the track metalized so that upon rotation the volume does not change over rotation on that range of the track. Take a close look at the wafers for the balance potentiometer. Note that the metalized ends of the tracks are on opposite ends of the travel of the respective channels. The metalized portion of the tracks looks silvery while the non metalized end looks like carbon grafite. It is very important to distinguish each lug of the balance potentiometer precisely to wire it correctly.
The volume potentiometer is also a 250K potentiometer, with a tap. Locate the volume potentiometer. For each channel there is a wafer with 4 lugs. Take a close look at the wafer for one of the channels. On one side of the wafer there are 3 lugs, one connected to GND (lets call this lug-1) On the manual, this is J403 on the tone control board. The next lug in the middle, is the wiper, and is going to the tone control (lug-2) and this corresponds to J401 on that board. Then there is lug-3 which also goes to the Loudness control (on a separate board). This "lug-3" of the volume potentiometer should be connected with a wire to the wiper of the balance potentiometer, which is the center lug on the balance pot wafer.
The lug on the balance potentiometer on the metalized end of the track goes to the mono/stereo switch.
The lug on the non-metalized end of the balance potentiometer goes to GND. If you did not cut the original wire it should still be hanging there.
The other channel is correspondingly similar.
I hope this helps.
