Unfortunately, that defect likely is not repairable on that particular model. Teac used very small DC motors to drive the reel hubs (via a belt). Those motors proved to be very under-powered and very unreliable. Almost a 100% failure-rate. The only real fix was a mod kit (sold by Teac) which replaced those DC motors with large, direct-drive AC motors. It was a major modification to the transport which also included circuit boards for the new motors.
Teac stopped selling those repair kits decades ago, so odds for finding a NOS kit for sale on eBay will be very low.
Sometimes a technician can "sort of" fix the original transport, but it won't last long. It's a bad design.