Changing ADC in a Keithley 2000 - what realibration is needed? - Page 1
Forum section: electronics / testgear

author: mimmus78
author_rank: Unknown
timestamp: August 29, 2013, 12:37:38 pm
content: I have a broken Keithey 2000 multimeter, it seems the ADC is fried (strobe signal from ADC does not arrive to U130).I already replaced U130 and checked with bad luck for shorts on the path from U130 to the ADC pin.What happens if I change the ADC?Do you think I just need to a re-calibrate (and adjust) it or it will require some sort of factory calibration procedure?PS: I discovered also that by touching strobe pin on U130 with my hands, the signal will reappear and the meter begin to work (spot on) for some minutes (or days).

author: Nermash
author_rank: Frequent Contributor
timestamp: August 29, 2013, 01:21:20 pm
content: I would rather think that there is cold solder joint on that pin or around it, than believing that your touch has magic ADC healing properties

author: Unknown
author_rank: Unknown
timestamp: August 29, 2013, 01:46:47 pm
content: Looking for bad joins, fractured traces or plated through holes (multilayer boards) is certainly worth a shot. It could also be a broken bond wire inside the IC, though.I think you might need to go through the manufacturing calibration in addition to the normal comprehensive calibration. Doesn't seem like a big deal since it doesn't require any additional equipment compared to the comprehensive calibration.

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 29, 2013, 01:57:02 pm
content: If you change something inside te analogue part, you need to recalibrate.If the strobe pin is sensitive to capacitive or resistive loads (i.e. your touching finger), and if it's an input, the problem lied´s perhaps in the sending output.Perhaps the output tranistor is weak, and the wave form is not ok anymore.Before replacing anything else, you should oscilloscope that line, how the digital waveform looks like.Do you have the schematics you could share?Frank

author: mimmus78
author_rank: Unknown
timestamp: August 29, 2013, 02:39:49 pm
content: @Dr. FrankI already put this pin on an oscilloscope and signal or is there (in good shape) or is completely missing since the ADC pin.Yes this pin on the ADC is output only (it's only connected on the U130 strobe input pin).Unfortunately I have no schematics, but being a two layer PCB it's easy to follow traces.@alm/@NermashIt's not multilayer PCB, anyway I checked already for short on this line during "not working phase".Signal is missing since ADC pin, so I'm "almost" sure it's not a cold joint.Almost convinced to try an ADC transplant from another K2000 ...

author: mimmus78
author_rank: Unknown
timestamp: August 29, 2013, 08:52:33 pm
content: So with one 1M pull-up resistor it was mostly working. With this setup was still going crazy (but able to recover by it self without the magic touch).I'm running now with 300K resistor since 4 hours and it seems this has fixed the problem (but I cannot be 100% sure without testing it for at least a week).Wondering how much will last as fix? Well at lest will be good excuse (with my wife) to not resell it now that it's fixed :-)Wondering also if there is any way to stop collecting all equipment you fix?Thanks to DiligentMinds.com

author: calmtron
author_rank: Regular Contributor
timestamp: August 29, 2013, 09:51:51 pm
content: No, there is no way to stop collecting test gear, unfortunately

author: EEVblog
author_rank: Unknown
timestamp: August 29, 2013, 10:32:26 pm
content: The ADC would be using an external reference which would not have changed. So it's likely you would not have to recalibrate for basic FS accuracy.

author: mimmus78
author_rank: Unknown
timestamp: September 02, 2013, 09:54:58 pm
content: Well after some days of testing it seems it does not go crazy anymore.Don't know if afflicted with the precision bug; most precise stuff I have is (just) a 7.5 digits multimeter (keithley 2001) and (just) a GPSDO (Z3805A).I'm searching for something like a voltage reference ... but I cannot find one at reasonable price.I think I should be safe for now, don't you think?

author: mimmus78
author_rank: Unknown
timestamp: October 25, 2013, 11:20:30 am
content: The meter worked up to the last week and than went crazy again.So today i feel enough brave to replace the ADC. I used ChipQuick to desolder both ADC and this did a good and "clean" job. Soldering it again with iron was more easy than expected.So the end of the story is that at 10V range keithley 2000 is spot on (0 to 2 digits) with the keithley 2001 and this confirm Dave's statement that changing ADC does not affect calibration.Now I have another big problem to deal.An EDC MV116 is on the way to my home, and I decided to build a voltage reference by scrapping the LM399 on the not repairable keithley 2000.I think I must join some "Volt Nuts Anonymous" club before it's too late!!!

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: October 29, 2013, 12:54:37 am
content: Where'd you get the ADC from? The sacrificial K2K?

author: mimmus78
author_rank: Unknown
timestamp: October 29, 2013, 12:29:13 pm
content: Yes this sacrificial from where I got the LM399AH.I tried to use also original K2K AD706 in my voltage reference but one of two op amp channel was unstable ...