I own an Atlas HO model train set that I recently took out of storage and set up in my home, and it still runs! The problem is that the speed dial on the powerpack doesn't work. When I turn it on, the train takes off at full speed, and I can't do anything about it. The powerpack doesn't seem to come apart at all (there are no visible screws). Is there anything I can do short of buying a new one? Is there some mistake I could have made when setting up the track?