Products > Test Equipment
Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)
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bdunham7:

--- Quote from: SilverSolder on January 30, 2021, 05:20:24 pm ---I have successfully swapped modules between 8502A, 8505A, and 8506A.   Of course, just because the modules "work" doesn't mean the parts can't be made to different tolerances etc. -  I haven't tested them that thoroughly (yet!... now that I had the idea...)  -  but some cards, like the GPIB card etc., should be a no-brainer.

The transformers hum loudly in every single unit I've ever laid hands on - it is safe to say this is a "feature", not a bug...   I guess it might be fixable by rubber mounting it - I have a theory that it is perhaps  too stiffly coupled with the chassis, mechanically? - but the hum is hardly the worst noise offender in the old test gear collection...  :D

--- End quote ---

Yes, 8505/06 I'd expect, and I hadn't really considered GPIB or isolator, but with the 8500A I can tell you the ohms, current and AC modules don't swap and I think it seem obvious that the controller and A2D won't either.  Not sure on the DC filter or active filter parts.

I tried tightening the bolts on the transformer and had it out on a bench and it doesn't seem to be a mounting issue, just something moving inside.  I had this idea of vacuum-potting it with thin epoxy, but I have better things to do--as if that has ever mattered before!  The 8500A has a different type of transformer and it is completely quiet, but it just won't work.

joeqsmith:
For DCV (where the bias spec I copied came from),  it routes to the DC signal conditioner, then to the filter and finally to the ADC.   Not sure what peaks you are expecting to see.   
SilverSolder:

--- Quote from: joeqsmith on January 31, 2021, 12:55:13 am ---For DCV (where the bias spec I copied came from),  it routes to the DC signal conditioner, then to the filter and finally to the ADC.   Not sure what peaks you are expecting to see.

--- End quote ---

We can't rule out that the CPU switches the signal conditioner in and out of the high quality bus for each measurement, which could affect things at the margins?

Time to take a look...

garrettm:
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
SilverSolder:

That looks very interesting!

What manual are you finding that diagram in, I don't seem to be able to find that diagram in any of the ones I have?  (One found online, another purchased from Artek).

I guess it's time to open up the DC conditioners and see what revision level they are...   do you happen to know the rev letter on yours?
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