20 Jul
2013
20 Jul
'13
7:25 a.m.
Second try at wrestling with the yahoo mail formatting . . .
I would be most grateful for that.
I thought about that, and tried removing all the tape controller boards and, for
good measure, all the option ROMs. ?No change.
I will have to try that when I get back as well.
________________________________
From: Dave <dfnr2 at yahoo.com>
To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
Sent: Saturday, July 20, 2013 12:34 AM
Subject: Re: HP 9830A always turns on with blank display
Yes,
I actually picked up the HPCC DVD to get some of those. ?I have a
subscription to Datafile, but for some reason never received the first
2012 issue. ?I suppose I have to wait until the next DVD update. ?I
actually am much more comforted having the schematics, but I like the
articles because there is frequently some subtlety that's not obvious
without scoping, and I learn something by going through the circuit
description while looking at the schematics.
I am glad you like them... There were also some
theory-of-operation
articles for the HP9800 series in the HPCC jorunal over the last couple
of years. But I rather glossed over the power-on circuitry ans I felt it
was not likely to be much of a problemn. Oh well...
OK, first question. Is STOP the only key to do this?
WHat happens if you
Only the STOP key. ?I had pushed various other keys earlier with no effect.
?The only reason I went back an tried the STOP key is because it's in
the operator manual. ?I was surprised by this directive, as I would
expect only one behavior on power up. ?After than, the machine works as
normal.
power up and press any otehr key?
It is curiouse
that pressing a key has this effect. The power-on circuit
resets the microdoe program couter (OK, it actually sets all the address
lines, the first address of the microcode [1] is 1 111 1 111 or 1717 in
'split octal'). The STOP key clears a few latchies on the I/O interface,
not much more.
[1] Let me know if you want a commented source to the 256 word CPU microcode.
Ah.. Init/ (power-on intialise signal) also clears
the interrup flag
latches on the I/O interface PCB. Perhps for soem reason the interruppt
line (KeyDn/ on the schematics, as that's all it was usef for in the
HP9810, and that's the machine I worked on first) is being set at
power-on. That will cause interesting probkems with the machine tryign to
exectue the interrupt service route far to oarly.
A few quick things to try. Pull out the tape contoller boards. As others
have mentioned there is a power-on init circuit there too, The tape
controller boards are the 4 with one blue handle behind the display. You
may have noticed that the board handles give the last 2 digits of the PCB
part number using the obvious code. Anyway, the machien wil lrun without
the tape controlelr boards. So try it now, does it give a prompt at power-up.
Nest disconnect the keyboard (paddleboard in front of
the display PCB --
I will try that when I get back home next week.
leave the in-like AMP connetor pluged in, it
conencts the mains switch.
Power up again, do you get a dispal ynow. If so, suspect a problem on the
keybaord enocder.
Most likely neither of these is the problem, the
problem is the Init/
circuit on the PSU board. It's based roudn a 74121 monostabl IC. I would
suspect the pair of 6.8uF capacitors associated with this IC.
-tony
Thanks!
Dave