Atari 1050 diskette drive repair

Anyone have experience with these? I have a defective unit (purchased on eBay - sad story) that tries to bash the drive head to death against the Track 0 stop when turned on. I picked up the service manual and schematics and was able to verify that the Track 0 sensor was working and that that NTRACK00 (pin 34) on the 2793 FDC chip is pulled high at that point. It looks like the head-step pulses are generated in software by the onboard 6507 CPU. The four phases appear on PB2-5 of a 6532 IO interface and are presumably getting cycled in the correct sequence since the head moves quite smoothly until it hits the physical stop. I've already verified supply voltages and inspected the board closely for burn marks, broken traces, etc. and reseated all the socketed ICs. The unit is fitted with the 'US Doubler' enhancement, about which I know very little. Would appreciate any tips on troubleshooting. Steve --