2 Mar
2015
2 Mar
'15
10:39 a.m.
Hello,
I recently acquired an HP9100B which was working before shipping, but on arrival does not
respond. The fuses
are intact, and I can hear the power supply humming on power up, but there is no CRT
output, and I cannot
activate the error light (for example, by STOP-CLEAR-/ to get a divde by 0 error.) The
register label backlight
also doesn't light, but apparently did not light before shipping, when the calculator
otherwise worked.
OK, the first obvious thing to check is that the logic power supply is there. It's
-15V, the chassis can be taken as
0V. This is unlikely to be the problem, but you don't want to spend time working out
why nothing works, only
to find it's got no power.
The register label is lit by a couple of T1+3/4 bi-pin bulbs running directly off the -15V
supply. They might be
burnt out. The error lamp is a similar bulb running from a transistor driver on the left
sideboard, this
bulb may also be burnt out. With a machine as complex as the 9100, it is well worth making
sure trivial things
are right before diving into the logic.
I have Tony Duell's excellent schematics, which I'm grateful for. I also have a
service manual, and plan to go
through it, but it seems a bit short on theory and detail. I'm not in position to
swap out boards.
The lack of processor information in the manual is one reason I traced out the schematics
:-)
To be fair, the service manual is useful for the PSU and display sections. But if
that's not the problem, you
need the schematics. Have you read the interfacing manual? Read with a hackish eye it
gives away a few
of the 'secrets'.
One trap for the unwary, the flip-flops are current-operated (the J and K inputs are to
the bases of transistors
with the emitters held at -1.4V). So you won't see much there with a 'scope. The
outputs of the flip-flops (E and F)
do swing by almost 15V, and are easy to measure.
I haven't been able to check it out in depth yet, but this week I will be able to
spend some time on it. I have,
however, done some basic inspection, and reseated whatever boards are accessible without
a screwdriver.
This might be bad connections. It's worth removing the keyboard, card reader and
processor chassis, then taking
off the sideboards (3 screws each) and making sure they connect correctly to the gating
board and the ROM
assembly. Note that the machine will work without the card reader. It will not work if the
keyboard is not plugged
into the processor.
This didn't help anything. I didn't hear any
rattling from the CRT, but haven't yet pulled it out to inspect it.
My plan is to start out by checking the power supply voltages, clocks, and move on from
there. However, since
apparently something changed during shipping, I'd be interested in any suggestions
that would help inform a
smarter investigation. I am hopeful it's not the CRT, as I would expect a bad CRT to
not interfere with the error > light.
Well, you don't know the error lamp is good :-). Worth checking. The processor will
run and give errors
with a faulty CRT (or CRT driver circuits, EHT power supply, etc). But if the CRT is
internally shorted which
can happen if the electrode supports break in shipping, you can and up with a couple of
hundred volts on
the processor. This wipes out diodes and transistors...
-tony