Success! A new IR emitter at the wormdrive encoder set it right. Now all functions seem to be working and it sounds great with just a used MMC3 cartridge. Hope to upgrade that soon.
Sonavar’s advise was extremely valuable. I doubt if I could have fixed this without his excellent posts. For anyone attempting to repair one of these 8002’s (and particularly starting as a novice such as myself) check out his incredible online instructions… concise and with great pictures showing exactly where to place probes for testing. I do have to add that after all the work with components open on the bench I thought I was never going to get everything crammed back into the cabinet! JM