Author Topic: Some help troubleshooting a Keithley 196 with bad ohms reading(s)  (Read 182 times)

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Offline staze

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Hi All,

I've got a Keithley 196 that isn't reading ohms right. They never settle (even when shorting the terminals together), and often go negative. I assume there's some leakage in the current source path, but I'm having a heck of a time figuring out what I should be looking at in the Schematic.

Anyone have any places to look? The manual talks a bit about how the ohms path works, but it's mainly "Protection on the ohms ranges is accomplisehd by RT1, Q9 and Q16. For input voltage applied to the  \$\Omega\$ terminals, Q9 and Q16 clamp the voltage to the reference resistors to a safe limit. RT1 limits the current to Q9 and Q16."

The equivalent circuit shown just shows the reference resistors, which I assume isn't the issue, but rather some leakage through a FET or BJT. The second image on 5-4 may be the answer...

I've also attached hi-res schematics. Any help would be greatly appreciated!

Also, hope everyone is staying healthy! I'll admit, I'm working on this because I'm stuck at home... nevermind I have other meters in perfect working condition. Just figured this would be a good opportunity to fix this one.
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Offline JKKDev

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Re: Some help troubleshooting a Keithley 196 with bad ohms reading(s)
« Reply #1 on: March 29, 2020, 06:55:16 am »
Disclaimer: I have never used this unit and have only looked at some pictures of it.

It appears it offers a 4 wire sensing for resistance. Are you sure it isn't required to use 4 leads? Have you tried it with 4 leads? That seems to often be the problem :)
 

Offline staze

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Re: Some help troubleshooting a Keithley 196 with bad ohms reading(s)
« Reply #2 on: March 29, 2020, 06:30:13 pm »
It doesn't need all 4 leads, it's only optional. I have another 196 that works fine with 2 leads. =)
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