I recently picked up a cheap Sony wm-f77 that wasn't working in the hopes of being able to put it back on it's feet for a few more years. It ended up needing a new rubber, new caps, much TLC (expected for a 1986 player), but also a modification to the mechanism to even have it run. I've seen a few reports of this online and what happens is the player struggles to get torque from the idler wheel through to the "FWD Limiter" setting it into an auto-reverse loop courtesy of the complex mechanism. On my player, I put a thin layer of double sided-tape on the flywheel and the plastic disk that contacts the wheel which has sadly increased the w&f. A gear has cracked on the reverse take up reel which makes w&f on reverse playback intolerable so I'm out of luck on this guy. To me it seems clear that shrinking plastics like on DD center gears, wm-10/f10 reels, wm-7 gears, and other models is unavoidable and impossible to "cure" without side effects or actual replacement parts. I guess this is trivial but it's nice that the stereo2go forum keeps record of the "cures" that are available. Though if I plan to fix another of these buggers, I'll probably learn how to 3d model first.
My much newer Sony WM-GX670 (I think it was a 2003 release) struggles with the endless auto-reverse loops. I think it aged much quicker than your F77. I have replaced the belt but it only cured one of the problems ever so slightly. I am dreading the day when I will have to take it apart and do all the required maintenance. I suppose what I dread most is finding a cracked center gear...
Typically, what I've seen is the plastic hub of this idler gets a bit eccentric, but after a good cleaning of all surfaces and some running they work fine. Never had to employ this method, which indeed I would expect to increase the wow&flutter as it will incrase friction significantly. Gears that crack can be repaired by enlarging the hole with a drillbit (to make it identical to the shaft diameter) and then glue the gear to the shaft. Seen similar problems in quite a few walkmans and repaired them successfully without any side effects. This method is actually better than putting a new gear, because once the gear hole is enlarged, it's no longer in tension, so it can't crack again in the future. Those that don't have a hole that can be enlarged (like the WM-10) can be fixed with epoxy as shown in the picture attached. Yes, cracked gears are a problem on quite a few devices, but I disagree it's impossible to cure without side effects. WM-10 type reels can be repaired, I've done it before successfully and without any side effects. Besides, there are still many walkmans in the series which have the reel gears intact, it's not like on DD where all of them break. The problem creating this symptom is very different than on the WM-F77. On the GX unit, the auto-reverse is controlled via a solenoid (so electronic control, not mechanical) and the microcontroller needs 2 things in order to not do this: detect reel rotation and detect the FWD/REV position. A cracked clutch gear which prevents the mechanism from freely rotating can also cause this symptom. These EX/GX type units are pretty reliable apart from motor/capstan bearing lubrication issues and of course the potential problem with clutch gear (but new gears are available).
@Valentin Thank you for your feedback and expertise. I am just about to start working on it and hopefully can bring it back to life.
@Valentin You were right about cleaning the mechanism . . . The idler only grips when you have cleaned it extremely thoroughly with a microfiber cloth and alcohol, even with a new one by mihokm. I think this mechanism is too sensitive for my clumsiness (I lost a few clips and a spring and ended up getting a donor unit). As for the gear, I enlarged the hole with a needle file and glued it on which worked like a charm. Couldn't have done it without your wisdom
Glad you found my advice useful ! Generally it's a good idea to clean any rubber parts after installation with IPA, as they are very likely to be contaminated at that point. Even when installing belts it's best practice to clean them and wash your hands before installing them, especially when you have worked on the mechanism before and traces of oil or grease may be on your fingers. On WM-DD capstan rings for example, some oily residue always comes off them when I clean the surface and seen situations where a DD won't latch into play because of this. Same is true with rubber tires, those should always be cleaned after installation as the smallest amount of grease or oil can make them slip. The contact surfaces on which they grip on should also be cleaned.
