After suspension tuning, that may vary the platter height, the belt may slip away from the motor pulley while playing. This can happen more often when switching from 33 to 45 rpm ad back: the belt may slip out from the top when the speed selector moves it from the pulley’s larger section (45 rpm) to the narrower (33 rpm). If the platter rides too high, the belt may slip from the bottom of the pulley or of the inner platter itself. It could also refuse to swithch to 45 RPM at all. You can check this behavior turning the platter, the mat and the record upside down like in the picture at left.
The usual forum suggested the main reason for this problem could be an old or not original Thorens belt. But also the motor pulley’s inclination can be adjusted by a screw between the pulley and the inner plate (at right). Once the correct inclination is achieved, the belt will no longer slip away. A number of attempts must be carried out, removing the platter every time the adjusting screw is to be turned. The rotation must be restarted and the speed control should be switched from 33 to 45 and back a number of times before we can be sure the belt won’t slip again. It may take a while and it is advisable to purchase an original Thorens belt, easily available on the Internet.