HP-97 decimal point with ghost segment d

[teenix]

(07-22-2020 04:17 PM)AndiGer Wrote:  @Tony: What would you suggest to examin with the oscilloscope? I can do that as I have one handy.

Attached is an image of power supply noise on the RA line of the Card Reader. It is the only one I have available where I am at the moment, but might give an idea of what to expect.

The supply may not be the issue as the calculator functions normally otherwise and the fault moves when you change the CPU board.

Does the problem seem to change if the CPU board is wiggled in situ? That may suggest a connection problem.

I would try to look at the data coming from the D LED segment from the ROM 0 chip. (pin 6) with a few 1's or 7's on the display (no D LED lit) and check the D and H LED data being output on the DP only digit. ROM 0 chip pin 7 is the H LED (DecPt). These two pins are next to each other which made me think, if there doesn't appear to be a dirty board problem, it might be an internal chip issue.

Isolating these pins from the display to verify this might be a bit difficult. Please don't try bending the connecting pins to break the circuit between the chip and display board, or they will most likely snap off and are not easily replaceable. (Very brittle) A tiny bit of cling wrap over the pin before joining the boards might do the trick.

cheers

Tony