Damaged HP 9810A

[Tony Duell]

The HP9810 CPU has a 16 bit I/O register, when driving the display the bits are used as follows :

DO(0)-DO(3) : Select one of 15 digits in a particular line (binary coded)
DO(4) : Enable X register display (active low)
DO(5) : Enable Y register display (active low)
DO(6) : Enable Z register display (active low)
DO(7), SO(0)-SO(3),CO(0)-CO(2) : segment drives (7 segments + decimal point)

It is very likely the problem is on the display PCB itself. I had thought that damaged connections to it could cause this problem, but since TTL inputs float high, this would
not explain why all register displays were enabled together, a bad connection to DO(4) would prevent X from being displayed, I think.

Anyway, with a 'scope or logic probe, I'd start to make sure all of the DO(n) lines are toggling (changing state) on the display PCB edge connector. Then check round the 7404 inverter, U15, on the display PCB. This IC both handles the register enable lines and the high bit of the digit selection.

You can download my schematic for the HP9810 from the Australian museum site, the display PCB starts on page 60 of the .pdf