Out of curiosity, I cracked open the R6581T and measured the R230 network resistor because I have a hunch that this component could be the culprit. The following resistances were roughly measured:
11.08 kΩ, 4.16 kΩ, 495 Ω (when connected the board, measured in both directions)
Does anyone else get these readings from the board? If so, does your Advantest have trouble reading high-ohms?
According to MickleT's schematics ("simplified diagram of the analogue board" and page 3/5 of "ADC"), the readings should be:
45 kΩ, 4.5 kΩ, and 500 Ω (when connected directly to the DMM)
What makes me suspicious of R230 is that the -1 V range was reported to be a problem by the calibration technician. He explained that the instrument needed to be calibrated 3 times before it was within tolerance and part of the reason was that the - 1 DCV was out on the 10 V range.
Using a Fluke 732A, I measured the following voltages with the R6581T a few days ago:
-0.9999998 V on the 10 V range (definitely an incorrect measurement and repeatable)
-1.000009528 on the 1000 mV range
+1.00000960 on the 10 V range
+1.00000985 on the 1000 mV range
Looking now at MickelT's "DCI & Ohms Artifact Calibration" flow diagrams, it appears that the 10 MΩ, 100 MΩ, and 1000 MΩ all require the -0.1 V, -1 V and -10 V reference voltages for calibration. Are these voltages directly related to the R230 voltage divider? If so, I may have found the problem.
Am I going crazy or could I be onto something?