I have a rev 2 unidisk analog board (it's actually out of an Apple Drive 5.25, but I guess they used the same board as the later-rev unidisk).
The disk head won't move. I replaced the board with the same rev board out of an actual Unidisk to fix the problem, but now I want to try to fix this one, as there's nothing wrong with the mechanism of the Unidisk that I stole the working board from. The enclosure is just really yellowed.
I have a sneaking suspicion that the problem is either transistor Q1 or Q2, as the 74-series ICs seem to be alright (but the reality is I may be too dumb to safely make that assumption too). Is there any good way to test the transistors in-circuit? What voltages and/or scope waveforms should I be reading on their various leads? Any other suggestions as to what to look for?
Thanks, folks! :D