On those Tandon 848s, note that the on-board regulator that drops 24V
to 12 is given to overheating. If your drive doesn't have a parallel
pass resistor across the 7812 regulator, it might be a good idea to
install one. Part of the field change was also to include an
electrolytic cap after the regulator--about 100 uF at 50V. Often,
these go toes-up, so if an 848's not working, start there.
--Chuck