On July 28, 2010: My two 15 year old Aquaclear 500s were chattering and stopping. The problem was that both the center shaft of the motor unit and the impeller were so badly worn that the impellers were moving and jamming in the motor unit.
This motor unit plus a matching impeller fixed this in 5 minutes, at about 1/2 the cost of replacing the filter. I assume they are now good for another decade. Great product.