I believe there's an optical shaft encoder on
the spindle motor of the
TU58. I'd check to see that the disk (which is paper-thin sheet metal)
and the opto interruptor aren't clogged up with dust bunnies, then test
the opto interruptor itself.
What is the difference from a TU58 to a normal DC-600 drive? Can't be
used a normal DC-600 (e.g.: for PC) in place of a TU-58?
I don't have anything DEC, just out of curiosity :)