Is my Keithley 2400 faulty? - Page 1
Forum section: electronics / testgear

author: Sören_Marodören
author_rank: Contributor
timestamp: September 13, 2023, 07:51:24 pm
content: A year ago, I purchased my Keithley 2400 from Ebay with an error code indicating that the battery was dead. It was an easy fix and the instrument looked good both on the outside and inside.I “calibrated” it myself with the help of other newly calibrated instruments like an Keysight 34465A DMM etc.Since then I have mainly used it for sourcing voltage and current.But there are some strange behaviors. The measured values are way off but the sourced current or voltage are correct.(I must admit that SMUs are a bit new to me. But I use Keithley 2450 at work, and there I get it to work better. Maybe due to a better user interface or even an improved instrument. I don’t know.)I think there is something wrong with the measurement in the instrument.If I set up a PSU to 5V/50 mA and connect it to the Keithley 2400 as a multimeter (or Measure only, as they write it in User manual page 3-20) I get 3.56 V (very unstable figures).If I set it to measure the delivered current I get -47 mA.(If I change the PSU voltage or current, I get other values on the SMU. They are not linear and varies a lot.)A similar problem is if I try to measure resistance with the instrument. Some readings are too high and some are too low.Seems to be a problem related to the ADC or so.Any suggestions on what might be faulty?Anyone have had the same problem?Can it be related to lost calibration values from when the battery was dead? (I have not found any way to calibrate the ADC.)Cheers,/Sören

author: voltsandjolts
author_rank: Unknown
timestamp: September 14, 2023, 12:02:38 pm
content: You might be aware, but there is seperate calibrations for source and measure, cal commands :CAL:PROT:SOUR and :CAL:PROT:SENS.How did you calibrate? Manual cal or automated, or maybeTiN's python script?In any case though, the large discrepancy of 3.56V versus 5.00V applied sounds more like a hardware fault to me.

author: Sören_Marodören
author_rank: Contributor
timestamp: February 11, 2024, 04:34:28 pm
content: Hi again,I have solved it. It was a faulty op-amp.Long story short: I tired many different approaches to find the error. But it is not easy when there is no schematic. In pure desperation I set the SMU to generate 21 V to my Keysight 34465 DMM. The DMM showed 21.000 V and the SMU showed 22.8 V-ish (very unstable). Then I took my hot air gun and started blow hot air on the opened SMU. At first from a distance, to see if anything happened, and slowly moved closer and closer. I was particularly interested to see if anything happened when I warmed up components around the A/D conversion. But nothing happened. Not even when I blow hot air to the voltage reference generation or the Altera FPGA.There are three active components before the A/D-conversion and those are U229 (DG4080 – analog switch), U228 (LT1007 – Op-amp) and U226 (LT1097 – Op-amp).When blowing hot air on them the SMU immediately showed 21.000 V very stable. After cooling down a bit, the unstable reading of 22.8 V came back. I then took a smaller hot air pen and were able to locate the fault to LT1007.The LT1007 is buffering the signal from the analog switch. When looking at the output from it, it is very clear that the measurements is done by switching between the reference and the signal that shall be measured. Due to the faulty Op-Amp both these were corrupted, but still similar enough to give a measured value that is almost correct.After replacing U228 to a new LT1007 my SMU is working again.After calibration, I now have a fully functional Keithley 2400 SMU.Have used it for some time now and it works perfect.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 11, 2024, 08:32:04 pm
content: Great job, and thanks for returning to tell us the story!I've come across a surprising number of bad op-amps in old equipment -- an 8360 Microwave Signal Generator, an R3371a Microwave Spectrum Analyzer, and a PS350 High Voltage Power Supply -- and I wonder why this is. I would have thought that mass-market devices like op-amps would tend to be more reliable and trouble would tend to come from the low-volume components, but the evidence seems to show that op amps are trouble.+1 for "poke around with hot air" debugging. Between hot air, physical pokes, and "human finger as microwave frequency dielectric adjustment," this technique punches way above its weight

author: sdragosljvich@comcast.net
author_rank: Newbie
timestamp: March 21, 2024, 12:18:46 am
content: Sir:Hello, hope all is well. I need to replace the battery in my Keithley 2400. Can you please tell me where you bought the battery and a part number. I cannot find the Keithley BA-44 battery mentioned in the manual.Thankssteve