RD54 Stopped Spinning

[tony duell]

I am going to try to answer several points together.

The mystery component sounds like a crystal or ceramic resonator. Does it connect directly to the Z8, if
so, to what pins? 1k is a very low DC resistance for such a component, though (was this tested out-of-circuit)?

The RD54 is a Maxtor XT1140 or something similar. The spindle motor is a 3 phase electronically-commutated
thing (no brushes -- does _any_ small hard disk have brushes in the spindle motor [1]). It is entirely within the 
HDA, there must be some hall effect sensors that are part of it (inside the HDA too) for position feedback.
Alas I have never seen a schematic for this drive (it's not on bitsavers that I can find) so I don't know many
details.

Assuming the power transistors are good, then the most likely fault is a failed hall effect sensor, inside the HDA. 
If we could find a pinout of the motor cable, or a schematic for the drive, it would be possible to see what is 
going on there.


[1] Some larger hard drives, the Diablo 30 springs to mind, had brush-type DC permanent magnet spindle 
motors (and in the case of the Diablo 30, positioner motors, but that is another saga). Many full-height 
5.25" floppy drives have permananet magnet brush-type spindle motors (belt driven to the actual spindle.
But I have never seen an 8" or 5.25" winchester drive that does.


> Interestingly at one point it started working again. Then after I put it all back together again, it stopped working 
> once more. In one case, it seemed to start spinning when I changed the disk's orientation (on its side rather 
> than flat), almost as if it just needed a mechanical encouragement to get it moving. But now, no matter how 
> many times I try, and what different orientations I try, it doesn't work. Does any of that help with pointing the 
> finger at all?

Maybe if one hall sensor or drive to one of the motor phases has failed then turning the drive moved the spindle
just enough to get another motor phase energised and it started running (possibly not on all 3 phases properly), 

-tony