Time has come...
I was thinking a while which meter to use for the wild experimentation, and after all things considered, decision was made to use my rusty 3458A as a patient. 3458U2 will have replaced front terminal block and missing inguard A1 shield, but remain unchanged for now.
Let the game begin, starting with easy part - DCI function. Why easy? Because it's simple component swap.
Current shunt modificationsI have broken A1 board, from which I removed original R213, R212, R211, R207, R209, R206 resistors and tested their TCRs to establish our baseline expectations.
Test results:
* R212 1 ohm - +8.3 ppm/K
Datalog* R211 9 ohm - +4.7 ppm/K
Datalog* R207 40K "reference" VHP101 - +1.4 ppm/K
Datalog I don't have much better replacement for 40K VHP101, so decided to leave that one alone for now, as we as 90 ohm (1mA range) and 4.53K shunts (10uA range), for but replaced rest with better resistors:
* R213 replaced with VPG foil shunt 319790 0.1 ohm 9W, measured TCR
+0.55 ppm/K* R212 replaced with
VPG VPR221Z 1 ohm, measured TCR
-0.86...-1.47ppm/K* R211 replaced with AE foil resistor 9 ohm, measured TCR
+1.5…+0.94 ppm/K* R209 replaced with S102 foil resistor 634 ohm, measured TCR
-1.2…1.25 ppm/K* R206 replaced with
CADDOCK USF340 resistor 500Kohm, measured TCR
+0.78 ppm/K After installing A1 board back, no SELF-TEST errors detected, ACAL ALL passed clean too.
Running GPIB app now collecting same 100mA running thru both meters from HP 3245A (LTZ-powered).