HP-25 Co-luminous segments

[teenix]

Sorry, I should have asked in previous post, do the keys in that row work ok? (- 7 8 9)

Have you checked inside for corrosion?
While it is open, you might try wiggling the circuit boards and reassemble. Might be a dodgy connection pin.

Electrically, it is hard to see how all segments light. Being a multiplexed display and seeing as how they are dimly lit suggests that they are somehow strobed on and off through the faulty row. An oscilloscope would be the only way to confirm what is happening.

You could disconnect the keyboard and see if there are any shorts or partial shorts in the keyboard matrix, especially when one of those keys is pressed.

With a bit of fiddling, with the keyboard disconnected, you could short the faulty row to a key column line. If it works ok, then the keyboard would appear to be the culprit. If not, it is more likely in the logic somewhere.

Current through the LEDs for each digit should only go through the active switch in the cathode driver IC. This is also connected to the keyboard. In the absence of anything visual, maybe this has gone faulty.

cheers

Tony