Bad Unidisk 5.25 analog board.

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Lee Adamson's picture
Last seen: 3 years 7 months ago
Joined: Oct 11 2018 - 22:53
Posts: 92
Bad Unidisk 5.25 analog board.

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