6 Jun
2004
6 Jun
'04
10:09 a.m.
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.
What's irritating about this is that I had only
powered it up to boot an HP64000 to copy some disks.
Failing that, does anyone have a comparable CS/80 unit
they might be willing to part with ? Or parts from
another 7908 ?
Thanks,
Dave
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6 Jun
6 Jun
3:58 p.m.
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
8 Jun
8 Jun
3:35 a.m.
--- Tony Duell <ard(a)p850ug1.demon.co.uk> wrote:
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.
-tony
By pulling boards I was able to isolate the problem to
the regulator board which contains three 12V supply
circuits, one each for the tape, logic and drive
spindle. I was able to identify high power thyristors
and op-amps on each 12V supply suggesting some sort of
crowbar arrangement for overvoltage protection (no
schematics available and this is a four-layer PCB
which makes it hard to derive the actual circuit). I
haven't checked the thyristors for shorts yet but I
believe the problem is either a defective thyristor or
some defect in one of the voltage comparators, maybe a
dead zener or something.
I need to keep the fuse from blowing long enough to
figure out why the overvoltage protection is
triggering. What would be the risk in trying to defeat
the overvoltage protection without any load connected
to the regulated 12V lines ? I assume I could do this
by disconnecting the gate of each thyristor.
Thanks,
Dave
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11:03 a.m.
By pulling boards I was able to isolate the problem
to
the regulator board which contains three 12V supply
circuits, one each for the tape, logic and drive
I assume it's one common fuse at the input to all 3 regulators.
spindle. I was able to identify high power thyristors
and op-amps on each 12V supply suggesting some sort of
crowbar arrangement for overvoltage protection (no
Very likely. HP were good about providing crowbars on power rails (well,
apart from in the 9815 and 9825 calculator, but I digress). Your problem
could well be due to one of the crowbars firing.
schematics available and this is a four-layer PCB
which makes it hard to derive the actual circuit). I
Wimp ! (:-)). 4 layes is hardly difficult. Now 8 layers, with all the
signal tracks inside, that's a little harder (and I've done that and
survived).
haven't checked the thyristors for shorts yet but
I
believe the problem is either a defective thyristor or
some defect in one of the voltage comparators, maybe a
dead zener or something.
Or a shorted pass transistor (I asusme this is a linear regulator, are
there any fat inductors on the board?). Any chips -- watch out for 10 lead
metal can devices in HP power supples. These are nearly always 723
regulators (and might be used in linear or switching circuits!). If the
chips have HP numbers (1820-xxxx, 1826-xxxx), post them and I'll look
them up for you.
A shorted pass transistor sounds likely, if only because I'd expect
current limiting in a supply like this. If the supply overcurrents
(possibly due to the crowbar firing), then the thing should remove the
base drive to the pass transistor, shutting the supply down. Of course if
the transistor is shorted, this doesn't do a lot, hence the blown fuse.
I need to keep the fuse from blowing long enough to
figure out why the overvoltage protection is
triggering. What would be the risk in trying to defeat
the overvoltage protection without any load connected
If it's a linear supply, there should be no danger at all. A switcher
could be 'interesting' under such conditions (you might blow other
components in the supply), but there are problems with it anyway, so it
probably wouldn't do that much harm in any case.
But I'd check the power transistors for dead shorts first.
to the regulated 12V lines ? I assume I could do this
by disconnecting the gate of each thyristor.
I'd just desolder the thyristors completely. And remove _all_ other
boards from the instrument, unplug the drive power cables, etc. Don't
trust that they don't use the 12V line, get them out of the way.
Pity I don't have this device, or I could probably be a lot more help...
-tony
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