Correct.
Don't uou have at least one ST506/ST412-interfaced drive schematic?
Nope, unfortunately not :-( I've got various manuals for bridge boards,
which of course have the connector pin-outs in (and give some of the
theory), but nothing for a complete hard drive (I suppose I'mactually
surprised such schematics ever escaped the drive manufactuers to be
honest, as presumably ST506/412 units were never supposed to be field
servicable?)
THe HDAs were not field-repairable, but the PCBs certainly were/are.
The IBM PC series TechResf contain schematics of some such drives (which
ones depends on exactly which version fo the TechRef amd which updates
you have). I've traced out a couple more. In general these drives are not
particularly complicated, although there may well be ASICs for things
like head switching, motor control, etc.
I must get at least one of the latter diagrems to you ay some point.
-tony