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Can these two instruments test the transistors on my printer PSU?

Discussion in 'Power Electronics' started by needsleep, Mar 13, 2018.

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  1. needsleep


    Aug 2, 2017
    Hi, I have a friend with two test instruments that he's going to lend me to figure out how to test some PSU transistors. But I'm afraid I'm pretty new to this, and hoping you can give me some direction. Will these two instruments help me figure out if my transistors are working? I am testing a PSU board out of a home computer printer. I've already determined that one diode needs to be replaced, for example.

    My friend is willing to let me borrow his Fluke 87V Industrial Multimeter, and an older Fluke Oscilliscope.

    Thanks for your input.
  2. Bluejets


    Oct 5, 2014
    Multimeter may be ok but one needs to know how to use it to begin with.
    Meter manual would help you there.
    Testing in and out of circuit can give quite differing results both for a shot device or a perfectly good one.
    Scope useful for live testing but once again one needs to know how to use the tester and what results to expect. To add to this is the added danger of any live circuit testing, especially mains connected devices.
  3. needsleep


    Aug 2, 2017
    thank you. To your last point about live circuit testing, I was not going to do that. I was trying to communicate, obviously not very well, that I didn't want to have to remove the transistor from the circuit board. So you have the circuit board on the table with printer far from it. Now can I test the transistor in place on the board, or do I have to remove (de-solder) the transistor connections and separate it from the board?

    I actually have an analog multimeter to my name. So I'm thinking I could figure out a digital one. The oscilloscope is a whole different thing for me though. I haven't touched an oscilloscope since high school electronics. I will have to decide if I can train myself in a reasonable time frame to use it. Maybe I could spend a month learning. Do you have any recommendations for that? Thanks.
  4. Bluejets


    Oct 5, 2014
    I answered that above where I said testing in and out of circuit etc. etc.
  5. kellys_eye


    Jun 25, 2010
    You can make static test of transistors in-circuit but need to account for the other components that may be connected to it - this takes experience.

    You can make active tests (powered-up) of transistors in-circuit but this assumes a knowledge of the circuit operation and/or availability of the schematic.

    You can also diagnose 'probable' faults from the symptoms if you can described those symptoms accurately.

    PSU's - in particular Switched Mode PSU's (SMPS) - operate in a closed-loop manner meaning a fault in one area will 'display' results elsewhere and, as a result, make such devices difficult to diagnose without both experience and a schematic.

    For someone who is asking such basic questions (no insult intended - we all have to start somewhere) it is probable that this task, especially without the aforementioned schematic, may be a frustrating exercise for you!

    We can help were we can. Post clear pictures of the device (circuit board, make, model etc) and list the fault symptoms you have seen, measured or deducted.

    Note that it s often simpler, cheaper and quicker to purchase replacement PSU's.
  6. 73's de Edd

    73's de Edd

    Aug 21, 2015
    What brand and model of printer . . . .WHAT BRAND AND MODEL . . . . OF PRINTER . . . . . .WHAT BRAND AND MODEL . . . . OF PRINTER . . . . . . ?
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