On our 6620 serial # 455416 we are trying to set it for corn but for some reason cannot get the cylinder speed to go below 600 RPMs. We have tech and owners manual and set sheave gaps and belt tension accordingly... still will not adjust below 600. It doesn't have the 2 speed cylinder that are on later models. It appears both sheaves are opening and closing all the way...belt is at the top of one sheave and the bottom of the other when when use the hydraulic lever in cab to adjust speed. It worked fine in June during wheat harvest after replacing the tach in the cab and the cylinder indermediate belt. We haven't checked it with a hand tach to verify the tach in cab is accurate... just assume it is. Any ideas that we can try!!!