HP-97 decimal point with ghost segment d

[AndiGer]

I have here an HP-97 that always shows the decimal point with the digit d dim lit no matter where the decimal point is shown. No other strange behaviour.

I put another working main board into the rest of the calculator and all works ok but when I drop the main board into another known good HP-97 the ghost digits switches together with the main board to the other calculator.

It is the newer layout of the main board not contained in the service manual I once posted here.

I replaced C1 (2.2µF tantal capacitor) but that didn't change anything.

Any ideas?

[Paul Berger (Canada)]

Do you see a ghost of the decimal point when segment D is lit? if that is true it would suggest a partial sort between the two signal lines. It looks like segments D and H (decimal point) are on separate anode drivers. The two signals are adjacent to one another on the connector between main board and display board but not on the display connector. I suppose it could even be a fault in the display or ROM 0.

Paul.

[AndiGer]

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.

[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

[Roland57]

Hi,

Several times I had problems with ghost-digits on hp 67 an hp92 - in all cases the reason was oxidation on the PCB. Residue from e.g. battery electrolyte caused a weak shortage over several conduvtive lines. Careful cleaning with ethanol solved the problem

Roland

[AndiGer]

Thank you for the valuable advice.

Cleaning the boards with IPA - especially the connectors and the holes didn't change anything. No real gunk found on the board.

What I just saw today: The battery low indicator LED is a little bit on although the battery is fully loaded (you can see on both images if you look carefully). With the same battery the power low LED isn't seen on the other fully working machine. Maybe by accident maybe both effects deal with each other.

@Tony: What would you suggest to examin with the oscilloscope? I can do that as I have one handy.

[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

[AndiGer]

Today I took the time to measure with the oscilloscope what Tony suggested.

Isolated SD and SH, i.e. pins 6 and 7 of ROM0 or pins 12 and 16 out of 18 of the golden forks (XA2P1 in the service manual where I found pin assignment shown in figure 4-10 isn't that it is in reality). Calculator switched on doesn't show neither DP nor segment d of the initial 0.00 display.
With 6 or 7 times 1 in the display I got the attached picture. So it seems that ROM0 generates not only segment h but also segment d for the decimal point. So - I don't care that much. This way I got a very special calculator (board), maybe unique ;-).

Next to try is the card reader.