Ok, the DAC vs ADC dilemma... Let's go a little further.
In the DPI510 user manual, in the calibration section (very poorly explained, just for day-to-day calibration), they mention an automatic DAC calibration routine, during the actual calibration (which I did), the display shows something like "Calibrating DAC" and the + and - values of the voltage references (~2.5V, as expected for a bipolar conversion and a 5V processor).
As for the IC itself, the part number is really faint, unfortunately, I could not take a good picture of it, I use an analog Zeiss stereoscope, no camera on it, and the cell phone trick didn't work in this case, the image was cropped... but what I can see clearly is the AD logo, and some parts of the code: AD42xx7, the xx looks like a 2 and 9.... in this case, we would have AD42297.
I did an extensive search on this part, I found it for sale, and I saw pictures of it but not a single mention in the databook (I already checked the ones you sent me Fzabkar).
Right now I did another search, with no luck with the datasheet again, but I found an AD document (Product/Process Change Notice - PCN 07_0024 Rev. E) that linked the AD569 to AD42297. Checking a few pins of the AD569 to the circuit of the DPI510, I believe it's the actual part number, and it's a DAC.
About my belief in a mechanical failure: I bought this unit second hand, it was totally disassembled, lacking the power supply, and was stored in a beach region (salt water)... and the packing was far from good, so, summing it to the apparent aleatory faults, probably it's a trace broken, or a bad solder joint, a corroded trace under some parts, tantalum capacitor dying...... the list is extensive, with a schematic in hand, which I'm now sure I'll not have unless I do it, the fix would be faster... lack of time is my main problem, I need this tool to work on some actual works.
Besides, maybe there are some configurations (like the range and type of sensors) that were stored in the factory, in the eeprom, which was lost, in this case, a service manual would be great.