how can fix my proform 750cs model#299560?

The end cap may be cracked. Look closely at the tension adjustment screws, there may be cracks around them. The screws may be tightened as far as they can go. If so and the end cap is cracked or broken, you will need to replace it. It costs about $30 dollars for the part and whatever they are charging for labor. You can always buy the part, undo the screws (while the treadmill is locked in the up position) then replace the end cap and start tightening the belt again. Pay special attention the the alignment from side to side of the belts tension. Do not overtighten. Check the slippage by starting the treadmill then stepping on it, pressing forward. You will feel if it is slipping or not. if this is too stressful for you to try alone, call a service technician. Good luck!

CAW
January 2007