HP-97 decimal point with ghost segment d

[teenix]

(07-21-2020 03:43 PM)AndiGer Wrote:  Segment d lit does not lead to a ghost digit to the decimal point (segment h). Displays are ok as the phenomen appears on the other calculator as well - I only transferred the main board. Display PCB and printer PCB both left inside.
So my conclusion is it is surely caused by (something on) the main board. But no idea what.
BTW: Card reader is still to be tested and adjusted (gum wheel is already repaired) as soon as I have time and patience to do.

It would appear to be a ROM 0 problem as that is where the LED data is generated from so could be an internal chip issue. Maybe a noisy power supply, although I would suspect the display would get worse as more LEDs are lit up.

An oscilloscope would be the best way to track the problem down.

cheers

Tony