Keithley 2400 DAC Fault - Page 1
Forum section: electronics / repair

author: derZockerMP
author_rank: Regular Contributor
timestamp: October 18, 2024, 05:46:58 am
content: Hello, I recently picked up an SMU from around 2006 that appeared to be quite new. I've been using it for about three months and noticed that the output voltage should be more stable than it is. Additionally, the ADC readings aren't very accurate, but this seems to only affect the voltage path. The current appears to be fine. I calibrated the unit myself with my also calibrated 2010 and 2001. I then attempted the self-test in the secret menu shown on xdevs, and most hardware tests failed except for the ADC ones. I’ll provide more details in a following post. The first failing test is the DAC test. Could this be causing the problem? Marco Reps experienced a similar issue with his SMU heated up the DAC. Are there any error code descriptions for the K2400 like there are for the K2001? I haven't been able to find any. Does anyone have an idea about what could be causing the failure?Stefan

author: derZockerMP
author_rank: Regular Contributor
timestamp: October 18, 2024, 08:45:22 am
content: I think I will measure the temperature of it.Some internal photos are attached.

author: derZockerMP
author_rank: Regular Contributor
timestamp: October 18, 2024, 09:04:52 am
content: Nice to see it use a LM199/399 Refrence instead of the one in the diode Package

author: MadTux
author_rank: Frequent Contributor
timestamp: October 18, 2024, 07:54:26 pm
content: What's that tiny chip, they replaced that huge Altera EPM7160 with?Seems like a more recent model, did they run out of old Altera EPM7160 parts?

author: derZockerMP
author_rank: Regular Contributor
timestamp: November 25, 2024, 06:47:27 pm
content: Hello, here are the error codes that the self-test throws. I think that’s nearly all except for the AD-Converter ones. Does anyone have an idea what could be possibly wrong or what I just can probe? The SMU itself luckily works just fine but the accuracy is not there as it should be even though it is freshly calibrated with my Keithley 2001. Below are some questions about the error codes too.200.1 200.2 200.3 200.4 200.5 200.6 200.7 200.8 => AVM DAC (What exactly is an AVM DAC?)201.2 201.4 201.5 201.11 201.12 => IDAC202.2 202.4 202.5 202.11 202.12 => VDAC For example, error code 202.5 says -4.916392, but expected is -5.51000 +/- 0.12500. It is always slightly off the nominal value. That’s for nearly all tests.300.1 => F/R 2W/4W301.1 301.3 => 200mV Range302.2 302.4 => 2V Range303.2 303.4 => 20V Range303.4 => 9.863552 Expected 11.056 +/- 0.25000 For more values, please comment 304.2 304.4 => 200V Range 400.1 400.3 => 1µA Range 401.1 401.3 => 10µA Range 402.1 402.3 => 100µA Range 403.1 403.3 => 1mA Range404.1 404.3 => 10mA Range405.1 405.3 => 100mA Range406.1 406.3 => 1A Range500.1 500.2 => Volt Clamps501.1 501.2 => I Clamps 502.1502.2 502.3 502.4 502.5 502.6 => AVM Clamps (What exactly is an AVM Clamp?)600.1 => AFB Signal (What exactly is an AFB Signal?)My question is what could be wrong in this unit throwing so many error codes and still outputting voltage and current?Any suggestions for the fault help are briefly appreciated.Did some thermal imaging and some probing of the common test points but nothing found so far.Best RegardsStefan

author: openloop
author_rank: Regular Contributor
timestamp: November 25, 2024, 10:26:09 pm
content: It's not clear what the symptoms are."voltage not stable": does this mean that voltage wanders around, or changes with the load? Are you talking about the actual voltage or instrument's voltage reading?"not very accurate": again, are readings off, or unstable?Anyways, from the list of errors, I would guess that something's wrong with the ADC. Or, maybe, communication between the ADC and the digital board.Somethings to check:U229 is ADC's input multiplexer.U229's output is buffered by U228.U229's input pin 4 is 7V reference.See if all those guys are working alright.Also check optocouplers and connections.

author: derZockerMP
author_rank: Regular Contributor
timestamp: November 26, 2024, 11:55:59 am
content: Thanks, the voltage and current is stable in all situations, also the readback of the ADC is stable and nothing changes. I think only the voltage ranges are a bid off but I had to check it agiainst the specs in the manual. I will check the MUX of the ADC.

author: derZockerMP
author_rank: Regular Contributor
timestamp: November 27, 2024, 10:29:35 am
content: Checked the mux and everything looks normal

author: openloop
author_rank: Regular Contributor
timestamp: November 27, 2024, 05:13:19 pm
content: OK,Let's check if voltage gets to the ADC unmolested.During normal operation: while sourcing voltage to a load (resistor, LED, what have you) and measuring it, check that measured voltage (TP230) shows up at the ADC input (TP233).Basically, check that voltages at TP230 and TP233 are the same.You'll need to use an oscilloscope: ADC input (TP233) is being constantly switched by U229 between current measurement and voltage measurement (I think).

author: derZockerMP
author_rank: Regular Contributor
timestamp: November 29, 2024, 07:04:20 am
content: Looks all ok

author: openloop
author_rank: Regular Contributor
timestamp: November 29, 2024, 04:37:50 pm
content: OK,I'm starting to suspect that hardware is fine and something went wrong during calibration instead.I recommend doing another calibration, but using different instrument, some good old 34401A should work just fine, even with 10 year old calibration.

author: derZockerMP
author_rank: Regular Contributor
timestamp: November 29, 2024, 09:14:22 pm
content: Could it be that the calibration has an influence on the error codes? I will redo the full calibration with another freshly calibrated meter.Thanks