Jaap - you were of cource right, it made a difference!! I got the belt yesterday, very fast delivery. I put it in and tested. Adjusted for the lowest speed it made 32 rpm and very close to 33 1/3 at the higher setting. Now I know that I will be able to get it running with the correct speed. Right now I don´t think that I will do much work on it during the summer. But I can prepare myself.
Part of that preparation is to find out about the pick up. It is a Shure M75MB Type2 in a Thorens mesh headshell. That headshell is not the original for this player. Is it ok to just do that replacement? The one who did this did not have the right screws so he glued them on to the headshell! I will look for some on ebay. And I can not balance the arm by the counterweight. I have to push it as far forwards as possible and then use the adjustment for the setdown force. The weight of the headshell with pick up is 17 grams. Can the original set up be so much heavier?