Suggestions for repairing HP tape drive capstans?

It is a closed-loop system (with feedback), but unfortunaetely the feedback is not taken off the tape. The drive motor is a permanent-magnet DC motor. The capstan roller is directly fitted (glued :-() to the spindle of that motor, and below it is a slotted disk (I have no idea how mant slots, but it's a lot, possibly around 200). There's a slotted opto-switch fitted to the motor housing that straddles this disk. The electronics varies with the model of drive. But it's somewhat similar in all cases -- there's a 6805 microcontroller that takes the input from this sensor (in the 9144 for some unknown reason, the esnsor feeds a 2917 F->V chip, the output of that is fed into am ADC circuit, the 9142 does things more coventionally, and I don't know about the 9145 -- yet). The microcontroller drives a DAC which then drives a full-H driver that feeds this motor. So the spindle speed is regulated, but the tape speed also depends on the diameter of the capstan roller. I;'d have to measure a couple of rollers in the tape cartridge too (but that would be easy enough). I could easily measure the rotational speed of the spindle. All I need to find out is the correct tape speed for these drives. IOt may be in HP Journal or soemthing like that... -tony