Interesting thread!
I've got one of the late model 8506As manufactured around 1992. My favorite feature is the 200% overrange on the 10V range and below as well as the permanently stored zero corrections for each range. My unit has the laser trimmed glass/ceramic resistor package in the DC Signal Conditioner module. So I'm curious how well mine performs now that everyone is sharing data from their units. If anyone is curious, I can take photos of the DC Signal Conditioner or other boards. My unit has the Current Shunts installed, rather than Ohms.
What software are people using to collect and process the data? I have MATLAB and could write my own code to collect and plot the data, but if there is a turn-key solution I wouldn't mind using that to start with.
The low input bias current, at least for the late model units is from using an Analog Devices OP97 (LT1012 equivalent) to buffer the input. The BOM doesn't list the part suffix for the OP97 as it was a selected part for <5pA Ib. I'm not sure if there is a better OPAMP available. The chopper stabilized OPAMP is an LT1052CN8, which looks like it could be upgraded to something a little more modern, possibly an OPA189?
I just finished replacing all the electrolytics in the power supply. Almost all of the tiny 5mm caps were dead. The larger diameter parts surprisingly looked okay. The beefy 15000uF snap-in part was a pain to unsolder and I ended up pulling a trace, but some Kapton tape and soldering the pad back to the plated through-hole tidied that up. When installing the large snap-in style replacement caps I had to bend the leads so they would fit into the holes, otherwise the repair went smoothly.
Fluke 850xA PSU Module Radial Aluminum Electrolytic Capacitors
Qt C WVDC DxHxP Part
1x 15000uF 16V 30x42x10mm (C8)
1x 3300uF 16V 22x26x10mm (C5)
4x 470uF 50V 16x26x8mm (C3,4,6,7)
2x 330uF 100V 16x26x8mm (C1,2)
4x 5.6uF 35V 5x11x2.5mm (C9,10,11,17)
3x 2.2uF 50V 5x11x2.5mm (C18,19,20)
Notes: D=diameter, H=height, P=lead pitch