PS I have the
9825A Service manual, but this simply tells you how to
disassemble the machine. In fact the troubleshooting flowchart essentially
tells me that anything could be at fault.
I assume you also haev schematics from
http://www.hpmuseum.net/
Some things I didn't say last night... Assuming you don't want to repair
this as HP intended.
After the PSUs, my next check would be the clock. It's on the CPU board
(bottom board in the stack). Note that the CPU clock inputs are not at
TTL levels, and that the clock is jittered a bit to reduce RFI (look at
the circuit if you don't believe me!).
I assume you know there are 2 separate buses in the machine, on separate
ribbon cables. One goes from the CPU to the memory boards and ROM
backaplane, and is multiplexed 16 bit address/data. The other goes from
the CPU board to the I/O backplane, tape drive controller, and KDP
(Keyboard/Display/Printer) board. It, of course, is 16 bit I/O data, 4
peripheral address lines, etc.
There should certainly be activity on the memory bus (and on pins of the
DRAMs, etc). I _think_ there should be some on the I/O bus too, the KDP
board is pretty dumb, and I don't think it does display refresh on its own.
-tony
