The belt must be in perfect conditions in order for the Thorens turntable to function well. Any adjustment is pointless if the belt is not in shape. Better if it is a branded Thorens belt, or one with the correct size and materials. The correct belt ensures the correct rotation speed, bounce, stability, accuracy of sound reproduction.
Suspension tuning may affect the platter height and 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 from the inner platter itself. It could also refuse to switch to 45 RPM at all. You can check this behavior turning the platter, the mat and the record upside down like in the picture below.
Reversing the outer plate with an old record on is useful to see how the belt works on the pulley
The main reason for this problem could be an old or not original Thorens belt. But also the motor pulley’s tilt can be adjusted by a screw between the pulley and the inner plate (below). Once the correct pulley tilt is achieved, the belt will no longer slip away. A number of attempts must be carried out, removing the platter every time you need to adjust the screw. You need to start the turntable again and switch the speed control 33 to 45 and back a number of times before you can be sure the belt won’t slip again. This may take a while. Therefore, it is advisable to install an original Thorens belt, easily available on the Internet (this link to eBay is the current best bargain from Germany – note that original Thorens-branded belts will worked fine 100%).
Thorens pulley tilt-adjustment screw
The original standard Thorens belt for TD160/165 and the likes is #6800574 and measures 496x4x0.9 mm. The thickness is important since it affects rotating speed.
Some say the Thakker replacement belts are as good as the original Thorens, made according to Thorens specs.
Origin Live also offers upgrades to the original spec Thorens belt, but with different length (520 mm) and thickness, according to their own interpretation of best belt performances.
Audiosilente, in Rome, Italy, is said to make the best non-original replacements at the cheapest price.