My guess they used local reference because it makes it completely independent.

The problem with it is that it adds more to ADC's drift and, as I see it, drift is the instrument's main source of noise (at least in 20V range).

Mostly because of the way they handle it (averaging for awhile, then dropping new constants in)

The regular zener as an additional local reference for the ADC is indeed odd, as this can add some noise and needs the measurement of the scale factor (e.g. the 10.5 Ref.) even for fast measurements.

They need to do scale measurement anyway because the compound ratio of R835+R846 to R840 to R841 is unknown.

Additionally, 6.9V measurement is the scale one (LM399's voltage is calibrated in). 10.5V is to determine R327 to R334 ratio. I.e. the input divider.

However the extra ref. does not effect the slope ratio part.

OK, that's true. As "zener reference" I meant entire +-10V local reference voltages.

But already you see that the ratio of final slope to "zero crossing" slope is dependent on two not ratio locked dividers, which in turn made of not ratio locked resistors.

In an instrument that heats up to 40+ degrees under the shield.

It (the ratio) drifts. And even when it finally reaches a stable point it will never be 8000, that point is also room temperature dependent...

Back to my original point: not auto-calibrating this ratio makes K2001 not a fully autocalibrated instrument and it shows up in readings.What, they didn't want it to compete with 2002?

The ratio of 8000 for the "red" and "blue" steps is odd, as the slope ration would be 4M/20K = 200

You're right.

I just crunched some numbers. Here's what I've got.

The granularity of that "fast" slope is 10 clocks (at 7.68MHz).

That got stretched by 200 times => i.e. it translates into 2000 clocks for the "slow" final slope.

But I get 8000...

Simply stuffing 2 bits on the right (i.e. multiply by 4) will result in even numbers, but I've looked into the data and it's not the case.

Looking closer I've found this: all measurements except payload indeed end with binary 00 (but they're all positive or close to zero)

All payload readings (for the final slope count) end with binary 01 for positive (on the panel, negative at ADC) voltages (left/first half of the graph) and 00 for negative (right half).

Thus they indeed shifted the count by two bits and used two lower bits for flags.

This explains why my overflow experiment didn't produce any results: they hide the polarity flag in there... Damn...