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Keithley 6485/6487 teardown

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razberik:
Hello. My boss gave me more time on it. I made detailed pictures of it and they are being uploaded to xdevs. Check it out TiN ;-)

I was lurking around the preamp section. I was trying to reverse the schematic. There is quite small amount of components around, however I was doing it in work with board installed, so I was unable to track the traces under relays precisely. So I have some garbage schematic, attached. What is interesting for me is using positive inputs of opamps, but hey, they did it the same way in Kei 485. Anyway, no feedback loop is closed now in my schematic.

What I have so far found, that AD829 video amplifier is outputing rail-to-rail (+/-15V) swing in 100Hz frequency. There is no corresponding input, however I dont know the feedback network of it.
If this was some non-precise circuit I would definitely give it a try to desolder it, but I have to think twice at this delicate instrument. Any tip ? I have strong belief that this AD829 is the driver of the feedback loop.

AD) plesa - yes, I am absolutely sure that 500V was not exceeded. It happened while sourcing about 300V from internal supply and suddenly relays were clicking like crazy and then OV.FLOW.

TiN:
Thanks for photos, nice to know that same voltage source board used in 2501 too.
Did you check inputs on AD829, what voltage is there?

Here are all photos from razberik, moved at permanent location:







Kleinstein:
Seeing a 100 Hz signal makes me think about the supply: could be a capacitor problem and thus too much ripple. At least check the supply voltages with a scope.

razberik:
Here you go. The preamp schematic.



It was hard to track some tracks. Some of them I didnt find out.
I wonder how the U25 LMC662 is supposed to work.  :o

Need some time to rearrange the schematic into more readable form.

plesa:
Did you check the power supply for AD829 and also check the output of LMC6081 by scope or DMM.
If you needs some measurement of functional meter I can help.

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