Straight to the point: The casing of the take up motor’s power transistor was making contact with the top chassis. I uninstalled it off, and there was a piece of fuzz on one of the transistor’s legs going all the way up to the casing. I cleaned it up, reinstalled the transistor, tested that there’s no continuity between the transistor’s casing and the chassis, and turned the machine on. The motor wasn’t spinning anymore! Can’t say if it was the fuzz that caused it, but reseating the transistor fixed it.
So how did I arrive at that? I’m not yet fully proficient in electronics, so what’s following here might be incorrect, but my prodding and testing was guided by my friend who’s a Studer tech, so maybe not!
Anyway, in my last post I detailed the measurements I took on both motor control boards. The readings on the supply board are normal, the readings on the take up board weren’t, particularly, the voltage between pins 5 and 10. On the other hand, the voltage on pins 7 and 10 was within the tolerances of what would come from the transport, so that reduced the problem to a short that happens somewhere around the motor (as opposed to the transport). I started checking for continuity at the transistor socket, and I noticed something peculiar. On the take up board, I had perfect continuity between pins 5 and both the transistor’s casing and the chassis. Same thing with pin 6. Pin 5 is the collector and 6 is the emitter, and it didn’t make sense that they should be at the same voltage. Also, that’s not what I got for those pins on the supply motor’s board. My friend suggested to check if the casing of the transistor is touching the chassis, and that was it.
I should mention, however, that now there’s a different anomaly. I turned the machine on and followed the manual’s initial checkout procedure (p. 2/17, section 2.6). When I pressed play, and only the take up motor was responsive. I turned off the machine, tested some connections on the supply board, turned the machine on, and now the supply was responsive but not the take up. Turned it off again, tested some connections on the take up, turned it on, and now the take up is running and not the supply. What the hell? Hopefully this one will be easier to chase down.