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. Aiyman Mourad says:

    can anyone explain to me how it achieves it’s maximum output impedance?

  2. Michael says:

    Hello and thank you for the video.

    My triaxial cable broke and I started using the “output common” and “guard” outputs in the same way I used the triaxial clips/leads. This worked (created a current) but I’m wondering if it is completely wrong to use. I’m not exactly sure what the “output common” does, the manual just says it produces the output common and I assumed it was producing the programmed current. As far as the guard, I think the guard is some sort of voltage source that outputs the V-limit voltage. I’ve connected these across a resistor but it usually reaches the v-limit (v-limit blinks) and burns up the resistor.

    In general, I’m not really sure how the current source works; I assumed the output was outputting the programmed current but the “guard” is a bit more obscure. Is it acting like a current and voltage source?

    • KISS says:

      The removable strap between and OUTPUT COMMON and GROUND allows one to insert a voltage source, so you can bias the instrument above ground.

      GUARD is the same voltage that’s present on the center terminal of the Triax connection. It surrounds the center terminal at the same potential, so that there is no leakage. The Triax connector gives you the return path and ground, guard and the center conductor.

      Current measurements need ground and the center pin, Low current measurements place the DUT on a plate or in a box at guard potential surrounded by a box at ground,

  3. 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 🙂

  4. 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.

  5. Tiro says:

    Best electronic blog ever!

  6. 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! 🙂

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

  • Social Media

  • Administrator

  • Archives