Electronics > Repair

Keithley 2400 DAC Fault

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derZockerMP:
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 |O. 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

derZockerMP:
I think I will measure the temperature of it.
Some internal photos are attached.

derZockerMP:
Nice to see it use a LM199/399 Refrence instead of the one in the diode Package

MadTux:
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?

derZockerMP:
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 => IDAC
202.2 202.4 202.5 202.11 202.12 => 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/4W
301.1 301.3 => 200mV Range
302.2 302.4 => 2V Range
303.2 303.4 => 20V Range
303.4 => 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 Range
404.1 404.3 => 10mA Range
405.1 405.3 => 100mA Range
406.1 406.3 => 1A Range
500.1 500.2 => Volt Clamps
501.1 501.2 => I Clamps 502.1
502.2 502.3 502.4 502.5 502.6 => 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? |O  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 Regards

Stefan

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