On Mon, 10 Nov 1997, Bill Yakowenko wrote: I don't have the Qume service manual, but I've worked on a number of other 8" drives. In general there's a slotted optoswitch (an LED and a photodiode mounted facing each other) that is triggered by a piece of opaque material on the head carriage. The light is blocked when the heads get to track 0. A few (a very few) drives used a microswitch instead, but it should be easy to find out what you have by looking at the drive. The output of this sensor is sent to the disk controller (the Track00 signal on the cable). Only a very few drives also use it to inhibit the stepper on the drive itself. But sometimes the Trk00 signal is gated with a signal that says that the stepper is in a particular phase (the one corresponding to track 0, track 3, track 6, etc - these drives nearly all used 3-phase steppers) so that provided the sensor trips somewhere between track 3 and track 0, it will give the right output. You problem could be the sensor (unlikely, since common failure modes would cause it to be 'triggered' all the time), the drive logic, or even a dead wire in the cable (if the trk00 signal is not getting back to the controller). It shouldn't be too hard to trace the signal from the phototransistor to the Trk00 pin through the logic chips, though. -tony