If you can't adjust your VR1 and VR2 knobs, either it is doing a recurring hard grab which overrides the pots, or the pots need replacing. To stop a recurring hard grab, put coins through (not freeplay button) for a credit, and drop a prize down the chute at the right time. The potentiometers are on standard machines and they are used to adjust the VR1 and VR2 voltages.