HP-97S trouble: shown 0. 00 instead of 0.00

[Sergei Frolov]

The HP97 does not show 0.00 when turned on, instead it shows 0. 00 (exponent)
Every time the buttons are pressed it blinks the display and lights up 0. 00 again
What could be the problem? In the PRGM mode shown 000 at each key press.

[teenix]

It looks like the HP microcode is not operating properly.

It might be a ROM or RAM failure, or another problem could the one or more capacitors in the power supply need replacing.

I can recreate the display problem by failing RAM register $3E which controls the display mode, but it does not account for the other issues.

If I fail RAM $30 to $3F completely, I get very similar results to you, but not quite the same.

cheers

Tony

[Sergei Frolov]

Maybe one of the ROM chips is not working as it should. It would be good if there was an emulator where you could replace one of the ROM chips, for example, with zeros and see what happens.

Unfortunately, ROM chips in the calculator are chipselected by a digital code rather than a CS signal, and it's hard to understand which one is not working.

[teenix]

(07-31-2023 05:50 PM)Sergei Frolov Wrote:  Maybe one of the ROM chips is not working as it should. It would be good if there was an emulator where you could replace one of the ROM chips, for example, with zeros and see what happens.

The HP-97 emulator at teenix.org can do that and with RAM as well. You can set a single location or a range of addresses to fail. Failed ROM or RAM will read as zero's.

cheers

Tony

[Sergei Frolov]

Thank you for the information



I have disabled RAM $30-$3F and ROM range 0x300-0x3FF.
Display shown like my calculator. Hovewer, key pressed not same (may me incorrect stack address differently response in the emulator).

But I am not sure that ROM3 (?) range 0x300-0x3FF correspond to RAM 0x30-0x3F.
I have not found manuals for this ROM-RAM chips.

[teenix]

That's the ROM0 chip - 1818-0267, address range $0000 - $03FF. This 18 pin IC also helps drive the LEDs.

What happens if you press the SIN or Yx buttons. Does it take a second or so before the display lights up again?

If so, maybe it is just the display formatting that is at fault. No great help, but may narrow things a bit.


With the emulator, you can also fail any ROM instruction you like.

It has an in-built assembler, so you can replace any instruction with [no operation] and the code will automatically re-compile when emulation starts.

There is also a "Restore" button that will restore the original code.

One problem with finding a bad RAM or ROM location, is that the created fault might mimic the real fault but might not be the same. The best way would be to analyze the code flow and see where it is going astray. I have done this in the past to find faults, however this is difficult without proper gear.

cheers

Tony

[Trevor Gale]

BY coincidence, I went to use my HP-97 today and at first it worked just fine, including programming and writing / reading the cards.

Then it started to show exactly the same issues as you show in your video; at first I assumed it might be a low battery but I checked in my workshop that and it was not the case.

Sometimes it will return to "normal" operation, then if I do a line-feed on the printer button it will revert to the same problem.

I opened up the calculator to check for anything obvious and I lifted and replaced the logic PCB board a few times (moving the multiple gold connector pins in the plated-thru holes in the PCB) to maybe clean them a bit.

For a while after that it worked fine again; so I know that the various ROM's must be working as well. I am thinking that those connector pins onto the holes at the bottom of the logic board may be the source of the trouble. One CANNOT solder these in any effort to ensure good permanent contacts, because then the logic board would never be able to be removed again.

Perhaps it might be an intermittent "dry joint" on the ROM concerned, but I will have to examine this in my workshop tomorrow or later - if so I can easily re-solder these pins, but I fear that it is those connector pins to the bottom of the logic board... Has anyone any ideas of making sure they make good contact???

[teenix]

I wonder if a power supply capacitor has failed or partially failed.

Maybe even the power on reset circuit. The ACT might not be resetting properly.

cheers

Tony

[Trevor Gale]

I don't think a power supply capacitor has failed - as a precaution I had already replaced such capacitors...

I will look today at the reset process with a 'scope and verify; I'll report here again once I have had the time to do this.