I recently picked up a Keithley 2001M (advertised as 2001), which came with firmware B16, and decent-looking Nichicon capacitors without any sign of leakage. Thanks to the information on TiN's website, I upgraded the unit to firmware B17, and added the MEM2 option. Everything seems to function correctly, including the self-test, but when I perform the open circuit step of user-level comprehensive calibration, I'm getting error 363, which is described as "200uA gain out of spec". The closest corresponding self-test seems to be 309.1, which routes the 89uA ohms source through the 200uA current range and checks for 89mV at A/D input. Now, I'm a bit stumped where the problem is, as I'd like to avoid desoldering any of the precision resistors, e.g. R344 (900, 90, 9) and R215 (24.5k, 500), the ADC board is mounted upside down over the A/D amplifier/ranging area, and I haven't found a reference locator for the layout (e.g. where is A/D IN?).
Currently, I suspect one of the analog switches for the current ranging, and I'm thinking about replacing all of the DG211BDY/DG411DY parts in this area (U323, U317, U320). I don't think the problem is with the main opamp U322 (LT1007) or the gain selection resistor R215 for 1x/50x, because the unit works fine otherwise, and I haven't received any errors about DC gain, which appears to be calibrated first during open circuit calibration. Rather, while probing the 200uA and ACA switch inputs for current ranging, I see the switches actuate, but there's also a slight ~2mV offset between the input and output of one of the analog switches. Does this seem reasonable?