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Keithley 617 Electrometer faulty behavior
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matthuszagh:
I purchased a Keithley 617 electrometer listed as used on Ebay. The power-on self test passes fine. But, the instrument is acting a bit erratically and doesn't appear to be doing anything sensible. Currently, when I perform a resistance measurement (according to "2.7.7 Resistance Measurements" in the manual) it display OL Gohms (zero check off). That's as expected, but previously it showed about 330ohms. The current measurement seems consistent. Previously in the 100s of nA and now in the low pA range. I don't yet have a proper 2-lug triaxial cable, but when I connect multimeter leads to the shield and force (outermost and innermost conductors) of the triaxial connector, I don't see anything sensible. For instance, with the range on auto, shorting the force to the shield with measurement set to ohms it recently showed about 80 kohms. Now it shows about 1Mohm, but it bounces around a lot. Either way, that's way off.

This unit seems faulty, but before I send it back I'd like to make sure I'm not just using it wrong and missing something obvious. Any thoughts? Happy to provide more detail and/or perform various checks if people want. Thanks.
HighVoltage:
Start with a few simple tests...

Does the output voltage work in the back?
You can set it from 0 to 100 V and measure with a multimeter

You need to get a proper 2-lug Triax cable.
When you short the triax input cable, do you get a stable zero reading?
What does it show on the display, when shorted?

alm:
What is the position of the V/Ohm guard switch? Is there any connection between the binding posts on the back, like COM and ground?

With guard disabled measurements are between the center conductor of the triax connector and the inner shield. With guard enabled measurements are between center and COM binding post, which might in turn be connected to the shield / ground via a link between binding posts.

See figure 2-3 from the manual (see attachment).

What happens if you supply a small voltage with the 617 in current mode on the right input high and low nodes, supply a current (e. g. DMM measuring resistance) with the 617 set to current, and measure current with your dmm while the 617 is set to Ohms? Note the values on both the 617 and the DMM. Try both guarded and unguarded configuration (observing where input low should go in either case) for volts and Ohms.
matthuszagh:

--- Quote from: HighVoltage on July 28, 2022, 07:14:53 am ---Start with a few simple tests...

Does the output voltage work in the back?
You can set it from 0 to 100 V and measure with a multimeter

--- End quote ---

This works.


--- Quote from: HighVoltage on July 28, 2022, 07:14:53 am ---You need to get a proper 2-lug Triax cable.
When you short the triax input cable, do you get a stable zero reading?
What does it show on the display, when shorted?

--- End quote ---

When I set the V, \$\Omega\$ guard switch to off and short the triax connector force to guard (input high and low) I see a reading that is roughly 10s of ohms but moves around a bit.

When I set the V, \$\Omega\$ guard switch to on and short the triax connector force (input high) to COM (connected to input low through 100ohm resistor), I get a reading in the negative Gohms that's very unstable.
matthuszagh:

--- Quote from: alm on July 28, 2022, 07:28:19 am ---What is the position of the V/Ohm guard switch? Is there any connection between the binding posts on the back, like COM and ground?

With guard disabled measurements are between the center conductor of the triax connector and the inner shield. With guard enabled measurements are between center and COM binding post, which might in turn be connected to the shield / ground via a link between binding posts.

See figure 2-3 from the manual (see attachment).

--- End quote ---

Thanks for pointing this out, I'd been getting the connections somewhat wrong. I redid some basic measurements using these correct connections (see https://www.eevblog.com/forum/testgear/keithley-617-electrometer-faulty-behavior/msg4325992/#msg4325992). COM has a shorting bar to ground.


--- Quote from: alm on July 28, 2022, 07:28:19 am ---What happens if you supply a small voltage with the 617 in current mode on the right input high and low nodes, supply a current (e. g. DMM measuring resistance) with the 617 set to current, and measure current with your dmm while the 617 is set to Ohms? Note the values on both the 617 and the DMM. Try both guarded and unguarded configuration (observing where input low should go in either case) for volts and Ohms.

--- End quote ---

In all of the tests below, when guard is off, high input is made to triax force (center conductor) and low input is made to triax guard (middle conductor). When guard is on, high input is made to triax force and low input is made to COM.


DMM set to ohms, 617 set to current with guard on.

| range (DMM ohms) | DMM     | 617    |
|------------------+---------+--------|
| 10 (10mA)        | OV.LD   | 12u    |
| 100 (1mA)        | OVL.D   | 8.4u   |
| 1k (1mA)         | O.VLD k | 08.4u  |
| 10k (100uA)      | OV.LD k | 08.4u  |
| 100k (50uA)      | OVL.D k | 08.5u  |
| 1M (5uA)         | 1.1M    | 04.96u |
| 10M (500nA)      | 2.4M    | 00.5u  |
| 100M (500nA)     | 2.7M    | 00.4u  |
| 1G (500nA)       | 0.0027G | 00.4u  |

DMM set to ohms, 617 set to current with guard off.

| range (DMM ohms) | DMM          | 617    |
|------------------+--------------+--------|
| 10 (10mA)        | 2            | 9.911m |
| 100 (1mA)        | 2            | .9903m |
| 1k (1mA)         | 0.002k       | .9904m |
| 10k (100uA)      | 00.003k      | 99.12u |
| 100k (50uA)      | 000.003k     | 49.57u |
| 1M (5uA)         | 0.000017M    | 4.959u |
| 10M (500nA)      | 00.00016M    | .4946u |
| 100M (500nA)     | 000.00017M   | .4946u |
| 1G (500nA)       | 0.0000002G   | .4946u |

DMM set to current, 617 set to ohms with guard on.

| range (617 ohms) | DMM     | 617                              |
|------------------+---------+----------------------------------|
| 2k               | 100.03u | . OL                             |
| 20k              | 100.03u | .OL                              |
| 200k             | 9.997u  | O.L                              |
| 2M               | 0.998u  | 200k initial, then 2M (unstable) |
| 20M              | 100.3n  | .OL                              |
| 200M             | 10.0n   | 170M (unstable)                  |
| 2G               | 1n      | .1G (bit unstable)               |
| 20G              | 1n      | 0.1G                             |
| 200G             | 1n      | 00.14G                           |

DMM set to current, 617 set to ohms with guard off.

| range (617 ohms) | DMM     | 617    |
|------------------+---------+--------|
| 2k               | 100.03u | .94k   |
| 20k              | 100.03u | 0.942k |
| 200k             | 9.997u  | 05.46k |
| 2M               | 0.998u  | .0054M |
| 20M              | 100.3n  | 0.045M |
| 200M             | 10.0n   | 00.54M |
| 2G               | 1n      | .0005G |
| 20G              | 1n      | 0.000G |
| 200G             | 1n      | 00.00G |


Function with guard off seems ok. With guard on, current sourced is still ok but readings seem nonsensical. Thoughts?
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