Yes it is mysterious. Was that drive known good then? Also what is the track sense mechanism? Optical shaft encoder? Could have been just a bunch of dust. A

Yes it is mysterious. Was that drive known good then? Also what is the track sense mechanism? Optical shaft encoder? Could have been just a bunch of dust. Also a lot of drives used a Hall sensor or even an opto-coupler with a slot to detect track zero. First thing was to calibrate the head with a track zero seek. Another thing. Those flat ribbon cables are prone to a lot of RFI over long distances, especially when you have them hanging around on the bench. Notice some of the other ribbon cables are twisted pair. A lot better! I've wire wrapped my own disk controllers back in the early 80's. This series is serious nostalgia! <3 Curious to know what the final problem will end up being diagnosed as. The smell of old hardware like that is literally burned in my brain!