One of the problems with old Selectrics is eventually
the neoprene drive
belt deteriorates and breaks. I'm no Selectric repairman, but a peek
inside a nice wide-carriage Selectric III that I've got stashed away looks
like replacement of said belt is a major operation and probably not worth
the trouble.
I have never seen inside a Selectric, but I've heard that to change one
of the belts you have to remove the main shaft, which is a major
operation.
I was told that if you do this, slip 2 belts over the shaft and tape one
to the side of the chassis out of the way. Next time it breaks, work that
spare one onto the pulleys.
-tony