Teardown, Repair and Calibration of a Keithley 220 Programmable Current Source


In this episode Shahriar investigates the cause of failure of a Keithley 220 Programmable Current Source. Despite being manufactured in the mid-1980’s, the Keithley 220 is still a very popular programmable current source. Alongside the repair efforts, the schematic of various analog, digital and power supply boards are also presented. By interpreting the failure mechanism, the fault is traced and corrected. The Keithley 220 is then calibrated and the calibration procedure of the sub-100nA range is explained. Finally, the performance of the calibrated unit is verified using a Rigol DM3068 Multimeter.


  1. j.don says:

    Great blog! Since I’m not very proficient in electronics I have one question that’s a bit unclear: You said that the U315 opamp has a direct resistance path from the output to the inverted input, and then you said that this means the inverted and non-inverted inputs should have the same voltage. Can you elaborate why is that? It seems to me that the inverted input voltage should depend on the output voltage (isn’t that determined by the gain?) and the values of the resistors.

    Thanks and sorry if the question is too simple :)

  2. Raymond says:

    Thanks for yet another interesting video!
    Re. calibration of this instrument: I’m sure you have the equipment to measure your calibration resistors to within .1% or so (maybe hi-R resistors excluded).
    You may then calibrate the instrument’s currents to those values.

  3. Tiro says:

    Best electronic blog ever!

  4. Mateusz says:

    Thanks for another great video. It would be fun to see this current source working in some real-life measurement. However, teardowns & repairs are equally as interesting.

    I do occasional repairs of electronic devices. But I haven’t had any broken test equipment on my desk yet. Maybe one day I find myself courageous enough to give it a try;)

    B11 is certainly a firmware version, there are a stickers on EPROM chips with this label.

    Could you tell, do you have all these measurement equipment connected to some network? GPIB, ethernet, something else? Or do you use them as a standalone devices?

    • Shahriar says:


      Whichever equipment that has a USB port is connected to a HUB which is ultimately connected to my PC. Some other equipment are connected via GPIB and through a GPIB-USB to the PC. The rest are stand alone. You are the first to ask me this! :)

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