HP-25 Co-luminous segments

[teenix]

The LEDs for digit number 4 are energized when key column 4 is selected from the Cathode Driver IC. This column corresponds to the keys [-] [7] [8] or [9]. Why this appears to blank to led display during the fault I'm not sure.

Does this happen for all displayed digits or just ones with segA lit (top horizontal LED segment)

For example, does it happen with only [1] or [4] shown in the display.

cheers

Tony