I'm not enough of a sparky to figure out resistor specs and such. I'm not enough of a craftsman to fabricate a control thumbwheel connected to rheostats of pots. I took apart an early 140 PBU but the thumbwheel doesn't turn a mechanical device like I envision.

Besides spinning freely, check for axial play in the motor.

Squeaking in turns can indicate worn motor bearings. I experienced a metal-on-metal squeal as you described, not a plastic cage on plastic housing scuffing. The replacement motor had a wobbly cage that scuffed. I trimmed the housing to correct that.

Sixto
87 300D