Unfortunately, there are no stock faults listed by Toshiba for the problem you're having. I've seen similar myself in the past and usually it will be a spring off, a brake lever in wrong postion. May be due to slight mechanism phase misalignment.
You may have dislodged a tension spring on one of the brakes (brake pads press against reel tables), or the reel brakes are stuck on during FF/REW mode. Trouble is you'll probably need a dummy service cassette to see this clearly.
If you had removed the rear loading drive assembly to fit the small belt, on reassembly the phase position of the mechanism is likely to be out, affecting the brake levers. There are alignment holes in the main chassis, the main cam and gears on the bottom of chassis and they must be perfectly aligned, along with the mode switch. You'd need the service manual for phase alignment details as it requires the reference diagrams.
It can be fiddly enough with a full alignment manual, without one it's going to be very difficult to realign it, is that's the case.
Cobweb, January 2005
