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Keithley 617 Electrometer faulty behavior
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matthuszagh:
I read in the manual that a dirty input connector can cause leakage current (makes sense). My unit, including the input connector is pretty dirty, including rust on some of the back screws and caked dust on the connectors. I figured it would be easy enough to test if this was causing leakage current by disconnecting the force (center conductor) from the input stage. That had no discernible effect on the leakage current.
r6502:
hello all,

hello matthuszagh

there is also an interesting thread regrding this instrument with a lot of details, especial about the input stage and how to repair:

https://www.eevblog.com/forum/testgear/fun-with-low-leakagebias-current-femtompere-electrometer-keithley-617/

Guido
r6502:
in my k617 I had to replace the following parts:

- all electrolytics
- defect -5V regulator U307
- doublle FET in inpt stage Q308 replaced with 2N5909
- OP U309, replaced with OPA177
- NPN Transistors in the output stage Q301 Q304 Q306 replaced with MJE340
- PNP Transistors in the output stage Q303 Q307 Q305 replaced with MJE350
- all high insulating relays K312 K311 K309 K306 K310 K307
- trimpot R348 and R314: after replacing these the output of the input stage was stable

- I added a Heat sink to U306, the +5V regulator for th digital supply in the analog part

Later I exchanged the input stage double FET Q308 and U309 with an ADA4530 as MIDI suggested in this post:
https://www.eevblog.com/forum/testgear/fun-with-low-leakagebias-current-femtompere-electrometer-keithley-617/msg2501178/#msg2501178

I'm just working on the replacement of the relays, if I have any progress here, I will poste the results.

Guido
matthuszagh:

--- Quote from: r6502 on July 31, 2022, 06:27:16 am ---in my k617 I had to replace the following parts:

- all electrolytics
- defect -5V regulator U307
- doublle FET in inpt stage Q308 replaced with 2N5909
- OP U309, replaced with OPA177
- NPN Transistors in the output stage Q301 Q304 Q306 replaced with MJE340
- PNP Transistors in the output stage Q303 Q307 Q305 replaced with MJE350
- all high insulating relays K312 K311 K309 K306 K310 K307
- trimpot R348 and R314: after replacing these the output of the input stage was stable

- I added a Heat sink to U306, the +5V regulator for th digital supply in the analog part

Later I exchanged the input stage double FET Q308 and U309 with an ADA4530 as MIDI suggested in this post:
https://www.eevblog.com/forum/testgear/fun-with-low-leakagebias-current-femtompere-electrometer-keithley-617/msg2501178/#msg2501178

I'm just working on the replacement of the relays, if I have any progress here, I will poste the results.

Guido

--- End quote ---

Wow that's quite a repair! Were you able to find suitable replacements for the high-insulation relays? Or, is that what you're working on finding now? Please do post your results when you get them, I'd be curious to see.
matthuszagh:
I attempted to clean the motherboard around the 100ohm COM to low input resistor where I did rework. I did this by soaking the affected area in 99.9% IPA, tilting the board and blowing it off toward the edge of the board. I think this was at least somewhat effective in removing flux because I could see somewhat shiny areas at the PCB edge where some IPA solution had pooled (I then cleaned this off too). Unfortunately I ran out of canned air before I could do the same area of the back side.

 The 617 has now been left on for another half day or so and is showing a pretty stable -500fA, which is marginally better than the -550fA seen previously. The short term noise is not so far off the 1.5fA p-p noise mentioned in the manual (7.4.9 Input Current Adjustment), but there is longer term drift. In the time I've been writing this the measured leakage has gone from about -510fA to about -490fA.

I'll probably give this one more round of cleaning to get the noise + drift down further. If it gets closer to the target noise I'll probably attempt the input offset followed by input current adjustment.
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