Ok, so the capstan always spins if there’s no tape loaded unless all 3 speed buttons are depressed. I depressed them and the capstan stops spinning.
I keep trying to isolate the problem:
Swapped transistors. The problem persists (take up motor is spinning).
Swapped motor control boards. The problem persists.
Installed head stack (I don’t know, maybe because the tape sensors aren’t “functioning”?). The problem persists.
I also noticed, and don’t know if that’s much of anything, that when I press play, the supply motor just spins fast clockwise. Same thing when I press rewind or fast-forward. Not sure if that’s anything, yet. It also makes sense to focus on the take up right now.
Maybe the motor’s run capacitor shorted?