Hi, I am building up a de vere 507 and have a question about the chain in the base that is used to change the heads position. I find that the chain is quite loose, so when rotating the wheel it comes of the sprocket quite easily. (see pics) At the top there is a plastic thing that can be used to adjust the chain but even in the most left position the chain is loose. So i must have done something wrong here. Does anybody know if the chain is running wrong over the sprocket and plastics wheels??? thanks. Willie Jan.