If your food processor stops dead in mid-chop, turns erratically or begins making a grinding noise, there's a good chance the drive belt or the drive gear is the cause. You can fix either problem with a pair of screwdrivers and, if need be, a replacement part or two.
Unscrew the machine's base, using a screwdriver. You may need to pry off the rubber feet to access some of the screws.
Inspect the drive belt (see A). If it is slack, loosen the screws holding the motor assembly (see B), and follow step 4 to increase the belt's tension. If the belt is broken, get a replacement from an appliance store or the manufacturer.
Install the new belt. Loosen the screws holding the motor assembly, and shift the drive gear toward the reduction gear so you can slip the belt over each gear.
Shift the drive gear away from the reduction gear to increase the belt's tension, then tighten the motor-assembly screws.
Inspect the drive gear. If the teeth are worn, you need to replace it. Remove the drive belt as described above to access the gear.
Remove the gear, using an Allen (hex) wrench to loosen the setscrew in the side. Then buy a replacement from an appliance store or the manufacturer.
Slide the new gear onto the drive shaft, and align it so the center is parallel to the center of the reduction gear. Tighten the setscrew with the Allen (hex) wrench.
Replace the belt, and increase the tension as directed in step 4, above.
Reinstall the base and any rubber feet you removed.