Author Topic: Keithley 2000 repair  (Read 1148 times)

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Offline Signal Simon

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Keithley 2000 repair
« on: October 12, 2017, 01:52:28 pm »
I've just bought a faulty Keithley 2000 with self test fault codes 101.3 and 400.2

It seems to work OK on the higher voltage ranges, but shows OVR.FLW when switched to the 10V or lower ranges.

Ebay listing said that U129 and U163 had been replaced in an attempt to repair, but that didn't solve the problem.

Initial testing showed that the 1.03V test signal at U129 (DG211) pin 15 was not being switched through to pin 14 (and on to the ADC input TP105), even though the control pin 16 was low.
But that "low" didn't look quite right - if I measured with my Keithley 2015, I see it is not true ground, there is a slight voltage there, about 0.25V and fluctuating.
Probing with the scope, the voltage is zero. And weirdly enough, the ADC input at TP105 is 1.03V now....
Running test 101.3 manually, the test is passed when the scope probe is connected to U129 pin 16.

I'm thinking the low-side FET in the Q1 output of U130 (MC14094) is not driving the line low properly, it seems to be floating.
I've got a 4094 on order, hopefully that's all that is wrong with this meter (apart from a well worn display).
The manual says it is no good chasing the 400.2 issue unless the 101.3 is fixed first.

Will update soon as I can.

Simon.
 
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Offline Signal Simon

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Re: Keithley 2000 repair
« Reply #1 on: October 13, 2017, 05:25:33 pm »
 :) I'm happy my diagnosis was correct.
Replacing U130 has cured fault 101.3 and the instrument will now happily work on the lower Volts ranges.

Now on to the 400.2
 
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Offline Signal Simon

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Re: Keithley 2000 repair
« Reply #2 on: October 14, 2017, 02:45:17 pm »
Traced the 400.2 fault to the R133 9K/1K thick film SOT-23 divider (Keithley TF-246-2), which measured 10K and 1K33, reducing the stage gain of U112.

Thanks to info in the post below, will be ordering the Vishay parts to replace this and R110.
https://www.eevblog.com/forum/repair/keithley-2000-self-diag-fault-402-2-resistor-id/

For now, a couple of 0805s tagged in should do.

Will check a few others while I'm at it...

Simon.
 
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