Learned a bit more-
* the drive with the "stuck stepper" isn't stuck at all, it just only seems to go in one direction. The coils buzz out ok
* turns out I can rotate the steppers easily when de-powered. Both drives move easily.
* If I rotate all the spindles counter clockwise, both drives rotate the spindles clockwise till they bottom out.
* then the "bad" one just rattles around at the bottom while the "good" one swings the arms quickly rail to rail back and forth.
I have also swapped the controller boards between the 2 drives. no difference. so the issue with the stepper tracks the mechanical.
Strange, unless one coil is shorted to chassis.
I have also tried grounding the DRIVE READY signal to see if that changes anything. it does not.
Anyhow I am close to stumped as to things I can fiddle with to try and get one of these MFM drives to be READY.
I am sort-of convinced that the MFM controller is OK, and working.
Since the drives spin up and spin down on power up, and neither signal "READY" it seems reasonable to assume they are simply not ready and not functional.
As to why, who knows.
If I focus on the drive that seems to have a working bi-directional stepper function, I have no idea why that is not working.
There must be some kind of internal test happening that fails the drive.
none of the videos online of either a KC-20B or KC-30B show an unconnected drive that remains spinning up. What I see are drives that spin up and spin down again. Any working KC-20B drive seems to stay spinning during power up, but it is the boot drive so that makes sense.