Keithley 2400 Repair Help with Current - Page 1
Forum section: electronics / repair

author: RoadRunner
author_rank: Frequent Contributor
timestamp: March 30, 2025, 11:40:11 am
content: Hello,I some how manage to get my hands on a somewhat broken 2400. It has one problem. even with nothing connected it shows current reading of around +705uA.Even With open terminal instrument try to compensate for this current by driving output to as negative as possible which make output to be very big voltage of ~ around -230 V.Any voltage setting with any output current less than +705uA will cause output to swing to this huge negative potential.If i set current to more than +705A for example 800uA , Voltage become normal.Instrument works well, can regulate current but this some how internal load/offset/error of 705uA is always visible.is there any help available may be schematic or partial schematic. Description of current regulationI have done some investigation. I have overlayed my measurement on the image from xDev.

author: voltsandjolts
author_rank: Unknown
timestamp: March 30, 2025, 12:12:05 pm
content: Try manually selecting each current range in turn (1uA, 10uA, 100uA, 1mA, 10mA, 100mA, 1A).Do you see an offset problem on all ranges?There is no schematic for this unit.

author: RoadRunner
author_rank: Frequent Contributor
timestamp: March 30, 2025, 12:31:44 pm
content: Quote from: voltsandjolts on March 30, 2025, 12:12:05 pmYes its visible in all ranges. Its just there.

author: voltsandjolts
author_rank: Unknown
timestamp: March 30, 2025, 12:46:19 pm
content: I presume you mean it's the same value on all current ranges, 705uA or thereabouts.Are the 1/10/100uA ranges max'd out, or what is displayed?How are you manually selecting the current range?There are effectively two calibrations on the unit, one for sourcing and one for measuring.Have you tried a calibration? Needs just a decent dmm, python script and serial or gpib adapter.The K2400 stores the new cal in ram, so you can test it, before overwiting the previous cal in eeprom.However, the readings you give are quite a large error, on the low ranges at least, and suggest a deeper problem.

author: RoadRunner
author_rank: Frequent Contributor
timestamp: March 30, 2025, 12:53:05 pm
content: Sorry for Confusion.Yes Same offset of additional 705uA is always there on every range.I have checked all range manually. Ranges 1uA to 100uA are useless as the always hit over range.I think its not a calibration issue. I Have tired to calibrate i think it allowed only 75uA to 150uA values on range of 100uA. and my instrument measure 705uA.Self test return no errors.

author: voltsandjolts
author_rank: Unknown
timestamp: March 30, 2025, 01:07:23 pm
content: Yeh, not a calibration issue.Try connecting an ammeter to the K2400 output, so effectively shorting the output.If it measures a somewhat similar offset current to what the K2400 reports, that may indicate the 'measurement side' of the K2400 is OK.Otherwise, the fault may be in the measurement circuitry, which might be simpler to fix.Check for leaky capacitors of course.A known failure mode of older units is thecurrent and voltage setpoint DACswhich get hot and need replaced.Other than that, check all K2400 threads on the forum. For example, this post mentions an offset issue on the 10uA range due to a bad fet Q400.https://www.eevblog.com/forum/testgear/keithley-2400-sourcemeter-review-and-teardown/msg2963884/#msg2963884That whole thread is worth reading.youtube.com/watch?v=8XfAHlx5iUk

author: derZockerMP
author_rank: Regular Contributor
timestamp: March 30, 2025, 11:28:58 pm
content: Also check the self test in the secret menu. On xdefs is a video how to enter it. Might give an rough indication there the problem could be.Best Regards

author: voltsandjolts
author_rank: Unknown
timestamp: March 31, 2025, 09:15:52 am
content: Is there a self-test in the secret menu?https://xdevs.com/guide/keithley_secret/#kei2400Options shown are:Code:Menu itemDetailNoteMODEL#Override model IDAllow to force ID to 2400/2410/2420/2430/2426/xxxx. Warning! Calibration will be LOST!FB-BURN-INBurn-in all VFD pixelsDIGITAL-REVWrite digital board revision ID into ROMStores digital board revision in calibration ROMANALOG-REVWrite analog board revision ID into ROMStores digital board revision in calibration ROMCONTACTEnable contact option 24xx-C(need hw)Allow to enable CONTACT option hardware

author: derZockerMP
author_rank: Regular Contributor
timestamp: March 31, 2025, 09:24:43 am
content: In my unit is a self test in the secret menu. But its a very new one around 2010. Will take a photo today.

author: RoadRunner
author_rank: Frequent Contributor
timestamp: March 31, 2025, 05:33:08 pm
content: Thank you all of. I think because of your indication i am onto something.Q403 Q406 Q407 Q408 are JFET All of them have only ~4Ohms between Source and Drain. While one have ~4Ohme between source drain and also Gate.This does not look right. This appears to be SST109 but its already obsolete. I have ordered SMPJ108 as replacement. I think it should perform as good. If some once could check and verify or have alternate part number then please let me know.I would write back if this fixes the issue.

author: RoadRunner
author_rank: Frequent Contributor
timestamp: March 31, 2025, 05:58:13 pm
content: I had tried to replace Q403 Q406 Q407 Q408 with a alternative JFET (before SMPJ108 arrive) without checking specs exactly. Does not really help.Issue is not in the Digital Measurement as Measurement always matches exactly the terminal. It just current regulation appears to seeing current error that is not really there.

author: voltsandjolts
author_rank: Unknown
timestamp: March 31, 2025, 06:05:44 pm
content: Those JFETs normally conduct S-D and a negative voltage is applied to the Gate to switch them off.https://www.allaboutcircuits.com/textbook/semiconductors/chpt-5/meter-check-transistor-jfet/