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Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617

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Alex Nikitin:

--- Quote from: David Hess on May 08, 2024, 09:10:01 pm ---
If construction is on a substrate, then I think that level of leakage could be caused by circuit board hook, which is sort of like dielectric absorption.

--- End quote ---

In case of the 617, the high impedance part of the circuit is all wired point to point using PTFE stand-offs.

Cheers

Alex

ZhuraYuk:
Check out what I have scored on ebay - 35617EBS Dosimeter  https://www.ebay.com/itm/186416162087 there is one more for 228USD.  Zero information in the internet but inside it is the same K617 with additional bias board.  Cant figure out yet on which output this bias is applied and how.
They symptoms are the same as in my previous K617 - high input current 250fA with zero check disabled slowly going down to zero for 20-30 min. 

Alex Nikitin:
The way to look for the leakage source is to disconnect bits one by one, starting from the input connector (just unplug it from the board), than disconnect all the switching parts, starting with the K307 pin 4 and R355, leaving only the input JFET and bias compensation network . I've modified my 617 some years ago mostly to reduce the offset voltage drift however the input current is also very good with the mod. The Keithley is still on my bench, working mostly 24x7 for last 8+ years (though it did develop some faults and so has been repaired several times over that period).

Cheers

Alex

HighVoltage:

--- Quote from: ZhuraYuk on May 09, 2024, 01:12:29 pm ---Check out what I have scored on ebay - 35617EBS Dosimeter  https://www.ebay.com/itm/186416162087 there is one more for 228USD.  Zero information in the internet but inside it is the same K617 with additional bias board.  Cant figure out yet on which output this bias is applied and how.
They symptoms are the same as in my previous K617 - high input current 250fA with zero check disabled slowly going down to zero for 20-30 min.

--- End quote ---

Nice score!
Seems to be a very special 617.

ZhuraYuk:
In my both K617 the 250fA input current was caused by C307 in my case. In one unit it was  bloated, the foil was  visible from the sides. In second unit it was replaced by someone with incorrect value of 0.01uF instead of 1000pF and poor dielectric type.  Replacing C307 with  WIMA MKP 1000pF  1250V decreased bias current to 50fA  and it was impossible to compensate. So next step would be to buy and install original polystyrene capacitor of at least 600V as in  datasheet.

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