Bad Unidisk 5.25 analog board.

1 post / 0 new
Lee Adamson's picture
Offline
Last seen: 15 hours 51 min ago
Joined: Oct 11 2018 - 22:53
Posts: 78
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