They may need some cleaning.
Most cassette recorders use rubber belts. Those may be worn out, become to loose or even broken. In this case, they need to be replaced. As those are no longer made, you may need to make some replacement yourself. The replacement don't absolutely need to be made out of rubber, as long as you get enough traction without needing to much tention.
There may be some corrosion on some/all contacts: Press all buttons, one at the time, several times while unpowered to rub any corrosion off the contacts.
Electro, July 2014
