Hello everyone, I've recently repaired two separate Keithley 2400 SMUs, and I'm seeing some strange issues while performing my verification testing. As a preface, one unit needed repairs to the analog output board, but the other only needed a front panel repair. The second unit, of which I did not touch the analog board, was calibrated August of 2023, so it should still be within calibration. I have tested both 2400s with their self test programs and a 4-wire short, and both passed with no issues.
On both the units I've repaired, there seems to be a (relatively) large output impedance of roughly 0.27 Ohms, as observed by measuring the effects of loading the units to large current ranges. I've been using an Itech IT8512A+ to verify the maximum power output, and measuring the effect of loading using a Keithley 2000. By measuring the terminal of either 2400 in voltage source mode with the E-load disabled/disconnected, the voltage the 2400 displays is spot on. However, with a 1A load, the voltage measured at their output terminals drops by about 0.27 V (therefore has an output impedance of 0.27 Ohms), yet they still display near the programmed voltage. See attached images for demonstration.
I've contacted Tektronix regarding this issue, and they state that the output impedance should be magnitudes lower, and just claim that the units need calibration.
This impedance is observed while using a lower current range and using a purely resistive load, and furthermore the issue is solved by switching to a 4 wire measurement. I've additionally verified that the impedance of my cabling is not the source of the problem.
Is anyone able to replicate this with a functioning unit? I want to double check that it's actually a calibration issue before I drop the cash to calibrate them