More Studer work today. I tested the voltages that IC1 on the Tape Transport Control board outputs (pins 1 and 7) and they seem fine according to my Studer friend. These are DC voltages and they measured around -2 to -3 V when the appropriate little roller is DOWN and about 11.34V when the little roller was held UP.
Also, following my friend’s recommendation, I checked that the motors are getting enough juice (about 100VAC) from the power transformer’s secondaries. They are!
I think it’s some logic thing that’s messed up somewhere because it’s always only one motor that’s being responsive, and rolling the big, right-hand roller will cause the supply motor to stop running and turn on the take up motor.
Weird stuff. I need to really delve into the manual and understand what each voltage does.