HP-80 display problem.

[teenix]

Those connections are direct chip to chip so won't be an external issue unless there is corrosion or a bad connection at the pin header between the boards.

As the CPU board works in another calculator, and assuming there are no board faults, it would seem the anode driver has gone faulty. Pin #1 on the anode driver has a direct internal connection to an input on the NAND gate that drives segment C, maybe this is the culprit.

Also, the anode driver power is sourced directly from the battery supply. If the calculator has had a higher than normal voltage applied from different batteries, an external supply, a spike, or accidental reverse voltage then it might have partially failed. If it has a fault, it might feel warm to the tough after being turned on for awhile.

I believe the CPU circuitry is PMOS, not sure about the anode driver, could be the same or TTL maybe.

cheers

Tony