HP 7908 woes

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Sun Jun 6 22:58:57 2004

>
> My HP 7908 (kindly donated last year by Steven Hirsch)
> failed this weekend. Investigation reveals that it has
> blown the 10A fuse in the +12V section of the power
> supply and when I replaced the fuse it blew again -
> but stayed up long enough for me to see that the
> on-board diagnostics do at least begin to run and that
> the disk itself begins to spin up. Rather than
> continue to stuff fuses in it I need to identify the
> root cause of the failure.
>
> Recognizing that there could be 1000 reasons why that
> 12V line is being overtaxed, has anyone encountered a
> similar issue ? Clutching at straws at this point, but
> any pointers would be helpful. The service manual that
> I have does not give any specific guidance on how to
> deal with this.


Is this a real service manual, or a boardswapper guide? I assume the
latter, alas :-(

Anyway, what I would do is figure out what runs off the 12V line
(probably some of the motor drivers, etc). And then disconnect the 12V
line from them (disconnect the power cable from the drive PCB, etc as
appropriate). Try to get the fuse to hold _and the 12V line correct_ with
no load or a dummy load.

Then you need to trace the circuits that it feeds to find what's shorted.
The fact that it holds for a short time suggests to me that it's not a
dead short across the power line. Maybe it's one transistor of a totem
pole pair across the 12V line that's shorted (perhaps part of a stepper
motor or voice coil driver). The fuse is OK until the other transistor
gets turned on, then you get a short across the 12V line.

-tony
Received on Sun Jun 06 2004 - 22:58:57 BST