HP 9815A

[Fabrizio]

Hi,
I've just recovered an HP 9815A, it's in very good condition, the problem is that when you switch it on the display shows only a roe of minus and nothing else.
Does someone has an idea of what can I try to repair it?
Thanks Fabrizio

[Tony Duell]

The row of dashes is generated in hardware (on the keyboard/display interface PCB) if the CPU board is not driving the display. So your problem could well be CPU/ROM/RAM related, it could be the power-on reset circuit, etc.
The CPU (amzingly for HP) is a standard part -- a Motorola 6800. The other 40 pin chip on the CPU board is a 6820 or 6821 PIA -- and it shouldn't be hard to tell which is which. I'd start by looking at the signals round the CPU to make sure it's clocking, that it's not stuck in the reset state, etc
To work on this maching you need to make up a 4 wire extender cable for the power cable to the CPU board. You can then work on the machine with hte keyboard/CPU folded forwards. Leave the 24 pin DIL plug plugged into the PSU board. Ignore the 2 26 war ribbon cables (unplug them at the CPU end and leave them disconnected) -- theyr'e just for the I/O slots and the machine will run without them.