Thomson UD90-070 disk drive repair

[schombi]

My Thomson UD90-070/CD90-015 bundle was delivered. I´m waiting for the boot disk to arrive, but since I was nosy, I connected it to my TO7/70. The drive gets recognized, the disk icon shows up, but when I press "2", the drive makes a sound for 2-3 seconds, then it stops with an error. Sure, I only had an empty 5.25" disk, but when I removed the top cover, I could see that the disk inside the sleeve did not spin. The drive head moves in a vertical position. When I openend the flap/lever, I could see that the motor starts and turns the part, which normally turns the disk. This, to me, means that the motor is fine, but the belt, I can see from underneath (when the controller is removed), has lengthened and has the wrong tension. Now, where to get a suitable belt and how the heck can it be accessed???