I have a twenty year old sander but there is no number on it. It is blue and has an orange button to keep the machine switched on without needing to keep the start lever depressed, My problem is that the belt keeps on coming off. I loosen the sander usng a thumb operated black lever and then place the belt in position. But it comes almost immediately when I start uisng the sander. there is a knurled knob which I imagine must serve a function but what is it. How do I get my belt to stay on? KCB Wilkie