|Re: HP 9820A|
Message #2 Posted by Tony Duell (UK) on 9 July 2002, 2:25 p.m.,
in response to message #1 by thibaut.be
I don't know how similar this machine is to the HP9810, but I think some boards (at least) are the same. I have a 9810 in bits on the bench at the moment, you see, and anything I say below relates to that machine.
Firstly, what test equipment do you have? The processor in the 98x0 series is a bit-serial design (mostly...) so it's hard to do any serious logic debugging without a logic analyser. The display on these machines is driven by the main CPU (which has to select each digit and turn on the right segments -- presumanbly in the 9820 it selects a column of dots and turns on the right dots). So unless the processor, RAM, ROM, and memory controllers are all working, you won't get a display.
The lights in the card reader are normal -- there should be 4 of them, 2 on top and 2 hidden inside. These are part of optical sensors to detect the card, etc. The fact that they're appearing indicates that there's something on the main +5V line, but you still need to check the PSU voltages. And that would be my first test. On the 9810 there are clearly marked testpoints on the PSU boards for all the voltages apart from the +5V rail. Check the lot. The machine will not work if any of the supplies are missing (OK, the +24V one is only for the printer, I think). The +/-12V lines are needed by the firmware ROMs, the +16V and +20V lines are needed by the RAM.
Then check the +5V rail at the power pins of the convenient chip -- on the display board or keyboard encoder, for example.
If all the power lines are OK< next unplug the keyboard connector from the main backplane, remove the printer (if fitted -- does the 9820 have this option?) and the card reader interface. None of those are needed to get a display, and one could be locking up the I/O bus.
Still noting? If you have a 'scope or logic analyser, look for activity on the pins of the test connector on the 09810-55613 board (CPU control/microcode). Those pins are bascially the microcode program counter, the microcode address change bits, and the condition flag. If you see changing logic levels there, then at least the CPU clock and microcode program counter are running.
After that, we need to get into _serious_ troubleshooting...