Basically there's four things that usually go wrong with un-serviced, neglected bill validator heads...>>>
1). Clouded, scratched
used-to-be-clear plastic optic sensors, or burned out.
2). A belt or wheel becomes hard & brittle.
3). The motor dies out.
4). Broken or chewed up gear teeth.
They ARE old and PAST their expected lifetime usage.
If you have any technical skills at all, you can get one back up running like brand new.
Number one, you have to being extremely resourceful in obtaining 30 year old parts, and know how to service it.
These are complicated devices that require a lot of patience, AND time to work on them.
That's why it's NOT cheap to have them re-furbished.
See that garbage can in the corner?
Take careful aim, and see if you can get 2 points...
P.S. As a consolation prize, I gave you a + karma! lol