Keithley 2100 resistance measurement issue(s)

[giosif]

Hi all,

I've recently bought a Keithley 2100 off eBay and resistance measurement is not working properly: if I short the leads, it displays a varying resistance of a few kOhms.
Some additional details:
* I am talking about 2wire resistance measurement.
* The same issue is present on both the front and the back input connectors.
* If I switch to manual range, it looks like the 100 MOhms, 10 MOhms, and 1 MOhms ranges are ok; the problems are seen only for the 100 kOhms, 10 kOhms, 1 kOhms, and 100 Ohms ranges.
* I measured the current for all the above ranges and all are in spec (yes, including the ranges with wrong measurement values).
* The self test result is PASS now (initially, I would get error 607, but that seems to have gone now after I replaced U201 and U507).

With regards to 4wire resistance measurement, I tried that as well and, for this one, the behaviour is different between the front and the back input connectors:
- when using the front input connectors, I get OL for all ranges when measuring a given resistance.
- when using the back input connectors, I get the same behaviour as with the 2wire resistance measurements (i.e. works fine for 100/10/1 MOhms, doesn't work well for the rest of the ranges).

Based on the above observations, I would conclude the following:
* for 2wire measurements, I suspect a calibration issue
* for 4wire measurements, I suspect a problem with the sensing part of the front input

Do you agree, or would you have any different thoughts?

Thanks,
George

[FlyingHacker]

Open it up and and put contact cleaner in the front/back pushbutton gang switch. I had issues like this with current and this fixed mine.

[giosif]

Open it up and and put contact cleaner in the front/back pushbutton gang switch. I had issues like this with current and this fixed mine.

Thanks!
I did that, but there's no change.
I also checked the multiple contacts on the switch, both when in front and in back position, and all are good.
So, it's not the switch.

[giosif]

Hi all,

I'm getting a bit desperate with this troubleshooting, as I haven't managed to confidently narrow my search area for the issue(s).

The problem I decided to focus on at the moment is the fact that 4wire resistance measurement is behaving differently between the front and the back input connectors, even with all probes shorted: OL when using the front input connectors vs. varying/non-stable reading when using the back input connectors.
And I confirmed that, connection-wise, both rear and front input connectors reach the same points on the board in exactly the same way (i.e. it's not a switch/physical connectivity problem).
Digging a bit deeper, I see that, when using the front input connectors, the meter stops auto-ranging and stays at the highest range (i.e. 100 MOhms) - suggesting it doesn't sense anything.
OTOH, with the back input connectors, the meter doesn't always lock onto a specific range, but at least it keeps auto-ranging (so, it must be sensing something).
With that and since the currents generated for the different ranges are all correct, I suspect there is a problem on the sensing side of the meter.
Now, understanding where the sensing area is located on the board and how it works is making me 

Any suggestions on what to check next, please?

Thanks!

[MosherIV]

Hi, I do not have this meter or any of the service manuals.

If I switch to manual range, it looks like the 100 MOhms, 10 MOhms, and 1 MOhms ranges are ok; the problems are seen only for the 100 kOhms, 10 kOhms, 1 kOhms, and 100 Ohms ranges.

This would suggest to me that the CC for the Ohm 100K and below measurement is not working properly or at all.

Since the Volts is working, that would suggest the input protection circuitry is also working but would be worth checking.
That is check the PTCs (should be low - look it up to be sure) and MOVs (should be high).

[FlyingHacker]

I would start first by measuring the power rails AND checking their ripple with a scope. These meters are know for having crappy capacitors:

https://www.eevblog.com/forum/repair/keithley-2100-at-spec-mode-err-599-repair/msg1017585/#msg1017585

I wish they provided a service manual.

I would still harbor some suspicion for that stupid gang switch. Perhaps shorting the proper contact leads with a jumper is a good test. Or feed the signal (the probe input or similar) directly PAST the switch.

When you checked the switch you compared the resistance reading you got on the switch, right? ...rather than just using a continuity beeper that doesn't give you much detail.

So:

Does DC Volts work properly front and Back?

Does AC Volts work front and back?

Does DC Amps work front and back?

Does AC Amps work front and back?

Did you measure the voltage coming out of this meter in ohms mode with another meter set to volts? Front and back?


Hook up a resistor to measure and then push around on the board looking for a bad solder joint. Try the same with a heat source or some cold spray (or canned air).

[giosif]

Hi all,

Late reply/update, but had to deal with other priorities, before I could come back to this issue.

So, the news is I was able to sort out the problem with this meter and that was by performing a zero calibration (using a poor-man's equivalent of a 4 wire short), followed by an Ohms calibration only for the ranges that were not working properly (using my Time Electronics decade resistance box).
The meter is now working properly, albeit I'm sure it is out of spec due to my "un-orthodox" approach to calibration.
Good enough for me, though... 

Thank you all for your suggestions!

[FlyingHacker]

Sounds good. Glad you got it working.