EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: cassiopeia on February 04, 2015, 02:09:09 pm
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Dear Keithley 2000 owner,
I repaired an old Keithley 2000, salvage from a trash at work (they said that repair was too much expensive versus the price of a new one...).
Anyway it is now finally repaired, all self-tests are OK, and measurement seems OK also, even if it needs to be calibrated.
But now, when the unit is powered on (DCV on auto range), without inputs, , the displays shows a voltage wich drift from -0.5 V to -1.2V in about 10 sec.
I have found only this : http://forum.keithley.com/phpBB3/viewtopic.php?f=32&t=25520 (http://forum.keithley.com/phpBB3/viewtopic.php?f=32&t=25520), but as this person, I am not convinved : the other keithley 2000 at work have not these behaviour, and, as far as I remember, the unit that I repaired displayed 0 V with open input before beeing broken.
If anybody can confirm/infirm this behaviour with your units, I would be more confident with my repair !
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It's a very normal effect of a high impedance input charging and discharging. My 2000 and 2015 meters both do it.
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As AndyC said, it is very normal. My 2015 goes to around 1.2V but my 2000 is a little less.
0V would be a little worrying as the impedance should be very high and there must be some intrinsic capacitance so 0V would imply leakage of some sort.
There has been several discussions on this on this forum, if you're still worried do a search.
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Thank you both for your answers !
I had unfortunately no luck with searching on previous post...
So I will close the unit and try to have it calibrated !
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So I will close the unit and try to have it calibrated !
Check it first against a known calibrated meter from work or friend, my experience is that these units are very stable.
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Thanks for your advice, I'll check it against one other. But as I have change 2 opamp in the ADC + the main MUX + the voltage buffer, I fear that calibration will be required. Althought against my MX24B (calibrated at factory 10 years ago ;)), the error is within 10 mV...
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I just checked multiple units, and they all drift on the 10V scale and lower
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Just checked my two units sitting on an anti static mat, they both have the same behaviour: start at 0.00VDC on power up, then drift down to around -1.2 VDC then switch to -0.35 VDC and continu this last iteration on and on
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Oh no, I too found that behaviour very odd and wildly soldered around in my K2000 when I should have just been checking for 101.2 test fail :palm: :-- :--
https://www.eevblog.com/forum/repair/keithley-2000-multimeter-voltage-drops-below-0-in-lower-ranges/ (https://www.eevblog.com/forum/repair/keithley-2000-multimeter-voltage-drops-below-0-in-lower-ranges/)
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Oh no, I too found that behaviour very odd and wildly soldered around in my K2000 when I should have just been checking for 101.2 test fail :palm: :-- :--
https://www.eevblog.com/forum/repair/keithley-2000-multimeter-voltage-drops-below-0-in-lower-ranges/ (https://www.eevblog.com/forum/repair/keithley-2000-multimeter-voltage-drops-below-0-in-lower-ranges/)
As long as you haven't damaged the unit you can now relax and enjoy it ;)
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I can also confirm this behavior from every KE2000 I ever encountered.
As AndyC already stated, this effect is due to the very high input impedance of the Instrument at the lower Ranges, which is >10Gohms at the 10, 1 and 100mV-Ranges.
At the 1kV and 100V-range the input impedance is 10Mohms (+/-1%), all according to the data sheet.
I like the Model 2000; it's a quite versatile little piece of kit :) Have fun with yours! :-DMM