Hi all,=0A=
=0A=
I have a 9830A that I haven't powered up in a while.=A0 Previously, it woul=
d take a few seconds to get to the lazy-T prompt, but now on powerup, the s=
creen remains blank.=A0 I got Tony Duell's phenomenal schematics, and was a=
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...
ll set to start debugging, starting at the clock, but
I checked the user ma=
nual first, and following the suggestion in the setup section, I hit "STOP"=
, which brought me to the prompt, and from there, it appears to be function=
ing normally.=A0=A0 However, the machine powers up this way every time--nev=
er to a prompt.=A0=A0=0A=
OK, first question. Is STOP the only key to do this? WHat happens if you
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 --
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