I purchased a very nice Canon EOS 1N that came with a free lens. The lens has perfectly clear optics but the USM focus motor was making sounds like it was trying to work but clearly was slipping. The manual focus ring was also very loose. So I dived in to see if this was repairable. Turns out it was a simple problem of the glue that holds the locking ring in place had failed and this meant the locking ring was rotating and had taken the required pressure off the USM motor. I used Blue-Tak instead of the glue with the Blue-Tak in a high enough blob that when the circuit board mounting plate screwed down it connected with the Blue-Tak and seems to be holding the ring in place nicely as the auto focus is now working perfectly. When I first put it together I found the focus not working but this was because I had moved the zoom and uncoupled the plastic pin from the zoom fork. Here is the workflow of photos if your willing to have a crack at this fix and save another old lens from the no-go bin. Good Luck.