HP 66312A current incorrect display - calibration?

[Chefbastler]

Hello,

I assembled an HP 66312A laboratory power supply from leftovers from the scrap. And has a CPU board in it from another 66312A.

I found a blown fuse F309 on the power board and replaced it. After that, it basically works correctly.

When calibrating, however, it is noticed that when calibrating the current of P2, when entering the measured value, the message "out of range" appears and the calibration is aborted.
Measure there with HP34401A approx. 1.8A,
the HP66312A shows about 2.2A

Without calibration, at least the externally measured current that is set is very accurate.
But the internal measurement is about 1.5 times too high.

Am I doing something wrong with the input when calibrating or is something wrong from the amplifier to the ADC?

[Chefbastler]

Can someone please measure a functioning device on the CPU board between the red (OP27 output) and the tantalum blue(GND) with a set 2A and short circuit at the output?

I measure 3.8V at 2A here.

This is the place before it goes into the multiplexer and into the 16-bit ADC for the current display.

[strawberry]

current shunt resistor?
in schematic diff amp gain ~7.8125
next stage ~1.21

[wn1fju]

Not sure if this will help, but:

I have the HP 66311B which should be very close to your 66312A.  In fact, the chips on my control board look exactly the same as in your picture.  At 2 amps shorted, I measure 1.31V across the two points.

More importantly, when I first got my unit, it also threw calibration errors as I ran through the steps.  I kept doing the calibration procedure over and over and eventually the errors disappeared and the unit calibrated successfully.  It may be one of those cases where the procedure is calculating new values based upon a portion of the old value and the new user-entered value, but I'm only guessing....

It also threw a non-calibration message Error 4 when I first got it, but this was cleared by saving some configurations into the four memory locations.