I have removed the heat sinks from IC1 and IC2 on the SYSCON servo
board. IC1 is the cylinder driver chip (AN3815K), and IC2 is the
capstan motor drive chip (BA6435S). Because neither the cylinder nor
the capstan move, it is difficult to know whether there is a problem
with one chip, both chips, or neither chip (if the problem is upstream
of both chips). I think this is as far as I am going to get without a
service manual, so any more thoughts (JR?) would be welcomed.