Hey, I have this Aiwa CS-J1 portable cassette player i tend to use a lot. It played fine after i changed the belt and cleaned it up inside. Suddenly though, it stopped working. The motor and everything spins but at such low speed it cannot spin the cassette. I'm suspecting it might be a mechanical problem. I've got here a video to demonstate the problem. As I'm no cassette player professional i have absolutely no clue of what may have caused this problem and how i could fix it. I'd really love to get it fixed up, so i thought of asking here. I hope someone can help me on this matter. - Kind regards
You have to check if the motor itself spins slowly or there is increased resistance in the mechanism or something slipping. More info is needed.
If the old belt turned to black goo the little chunks will slow down movement. Clean everything with IPA and Q-Tips until there is no dirt or black on the tips. If the new belt has been on the shelf a long time, it may have gotten hard and is slipping on the pulleys. Try using with a wall-wart and see if the problem is the same, the battery terminals may have corrosion.