I used to have a REVOX B77 1/2 track stereo 7.5 / 15 ips back in the late 70's early 80's.
In the early 90's it required some attention and I found a place nr Manchester Airport who was the local repair agent for Studer / Revox. I took the unit over to him and he did an excellent job at what I did not consider an excessive cost.
I found a google hit for him just now at
Brian@RevoxService.co.uk.
Might be worth a try for info' or more if all else fails.
I should say that I have no connection with his business, apart from using his services back in the early 90's