Have you got the handbook? If not, you can download one from the net - print off and read thoroughly.
The blade runs on a rubber tyred pulley - the tension of the blade is achieved using the knob on the very top of the machine. At the rear of the machine is a knob that controls the 'slant' of the top wheel - if this is on the 'slant' and not level, then it will throw the blade off the pulley wheels. Does this make sense so far?
Jim Martyn
May 2009