HP-10 disassembly/reassembly

[teenix]

Just trying to summarise things...

As you mention, the A B C and E lines from the keyboard are common and must be working with the CPU or other keys wouldn't work. Also, the K12 line must be working with the CPU or the display digit 12 wouldn't work.

The display is not flickering when those two keys are pressed, suggesting that the CPU is either not seeing those keys, or is not processing them.

The PCB tracks have been tested as being ok on the key board for the 2 and 5 keys all the way to the connectors for the CPU board.

The PCB track from the CPU board K12 connector pin to the CPU pin has been tested and is ok.

Best case, if it is assumed that the CPU works, then the obvious weak link here is the connector pin that connects the K12 line between the two boards. Have you inspected the pin and the mating hole closely. Sometimes, the plating inside the hole corrodes, or erodes away, or the two prong pin is bent inward a tiny bit and will not make proper contact.

A word of caution with these pins. They are brittle and do not like being bent very much at all or they will snap.

You could try shorting the CPU board to keyboard connection for K12.

To simulate key presses, you could also try briefly shorting out the A line to K12, or the B line to K12. This would have be at the anode of the diode on the A (or) B lines to the K12 CPU pin.

If either of these tests work, it points to the connector again.

cheers

Tony