My gf recently picked up a Sony TCM-313 walkman for €8 secondhand. We wanted to record some random stuff on it so we could listen back to it when we would be older. We tried to play a cassette with it but noticed the reels weren't spinning, So I took it home and opened it up. Upon opening it up it immediately became clear why it wasn't playing the cassette. The belt that went from the capstan to the reels mechanism was disintegrating and already in a few pieces.

The belt that connected the motor to the capstan was also disconnected, but it was still in good condition.

I ordered a set of new belts to add a new belt from the capstan to the reels mechanism:

When the belts arrived, I replaced them and it worked like new!