It hapenned a long time since they were produced, we know now some issues, caused by design mistakes of course. Some they appears only at one model. Other in several models across the same brand, showing a concept mistake.
I mean talk about the things that usually goes bad in some BBX, like power switches, idlers, etc that makes the technicians happy.

For example, I had 3 National Panasonic that has the same mecha. In the three an idler goes bad and auto-stop don't work properly. Even if I put a new tire in the idler, after some months of non-use, the problems appears again.
Always love Panasonic "keeping simple", but changing this tire is a real pain. So no auto-stop for 2 of my National (I've learned the lesson with the first one). Sometime work and sometime don't, like some humans

Almost every logic controlled box goes schizophrenic or stops working altogether in the cassette section it seems, no matter what the brand. All you can do is laugh because hey are so tricky to fix!

the buttons and sliders on the pc55 are prone to sketchy behvavoir usualy some contact spray helps them.
hmmm lets see the volume knob on the m9998k also i think was a weakling and needed replacing.

Is there any need to mention tape doors that flee en masse from their Conion C100F homes? Or slider knobs on GF-9000s and VZ series boxes that sometimes get an urge to follow those tape-deck doors?

quote:

Even if I put a new tire in the idler, after some months of non-use, the problems appears again.

if it doesn't work properly with a new tyre, it really does mean - the idler is not the single problem

Autostop is quite simple. The idler wich is moved by the capstan moves the function holder unlocking the play. No belts. No electronic.
The idler it's also used in the play function and works ok there.
The only thing is an spring forcing the idler against the cpastan. But I'm not sure of changing it's force

Aiwa CS-600 power cord socket breaking away from circuit board. I had to take mine back to the shop twice within the first year.

GF-9000 speaker switch tip going missing.

GF-777 family tape operation switch tip going AWOL.

Aiwa belts turning to slush.

GF-777/575 family weak tape door hinges.