On Mar 20, 2008, at 4:37 PM, Tony Duell wrote:
First
is an HP9810A. I just powered it up...Several LEDs turn on
immediately, but the display is blank and the keys are unresponsive.
Does anyone (Tony?) have any troubleshooting hints? Are schematics
for this machine floating around anywhere?
However did you guess that I've worked on this machine :-).
...
[huge pile of excellent troubleshooting information snipped]
Wow, thank you Tony! I hope to work on the 9810A within the next few
days. I'm very excited about getting it working again. I'll let you
know how it goes.
Be warned that's only the start. I've worked on a couple of 9810s, a 9820
and a 9830 (all of which use the same CPU boards and are basically the
same architecture). Some faults have been very easy to find, but I've had
at least one that's had me staring at the logic analyser for hours ;-).
If quick checks don;'t find the fualt, I would seriously recomend looking
at the CPU microcode address lines and asking me nicely for the source
listings. There's a web page (or there was a web page) from a guy in
Germany who had a logic fault on his 9810 that I talked to him about. He
connected a '688 comparator to the microcode address lines with the other
inputs coming from switches. He could then see what micorcode
instructions were being executed (although not their order) and quickly
found it was stuck in the I/O loop. That got him to the faulty section
quite quickly. Worth remembering if you don't have a logic analyser.
The logic fault in my 9820 (bought as 'non working' on E-bay, which was
an accurate description. It didn't work, but it was complete and not
messed about with) was evil. OK, the display was blank (no suprise
there). Doing the tests I suggested before showed that M(15)...M(12) were
toggling, all lower M (memory address) lines were stuck low. OK, I
thought, the shift register responsible for M11..M8 has failed, its
outputs are always low, so the lower half of the M register just gets 0s
shifted into it. I replaced that chip, only to still have a blank
display. And worse still, the M-line activity was unchanged. I looked at
the logic analyser more carefully and saw that the timing of the top 4 M
lines was crazy. There were not being latched. So the lower 3/4s of the M
register was, indeed always having a 0 on its input at the active edge of
the clock -- it seemed to be working correct. I rreplaced the top 4-bit
shfit reigster in the M regisater and the machine sprang to life.
Anyway, back to your machine. When you've got the logic working, that's
by no means the end of the restoration. The card reader roller will need
replacing, my merhod for doing this involves pulling the old hub off the
spindle and replacing it with a new one with a groove machined in it to
take an O-ring. Not hard if you have a lathe, but almost impossible if
you don't. I did have one machine where the hu was glued ot the spindle.
Trying to pull it off resulted in the spindle buckling, then shattering,
and a fright4ened cat ran from my workshop. I then had to calm the cat
down and make a new spidnle from metal rod.
The printer platten may have decyaed too. If you find sticky black gunge
on the mainoard under the printer then that's where it came from. 3M
'Cold Shrink' can be used to replace this (Thanks to the 2 people on the
MoHPC forum who suggested it). If you need to do it, I can talk you
through it.
You may have gueessed I like this series of HPs. Although the 9100 is
historically more interesting (it was the first) and a beautiful piece of
electronics, I find the 98x0 machinss more interesting in some ways. They
were a lot easier to interface and the HP interfaces are themselves
something to investigate.
-tony