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tektronix 475a problem

Discussion in 'Troubleshooting and Repair' started by zermello, Dec 9, 2011.

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  1. zermello


    Dec 9, 2011
    Greetings, I thought some knowledgeable person would be able to suggest the location of the fault in my 475a, if I describe the symptoms. Intermittently, the vertical section goes wacky. The displacement is only a fraction of what it should be, and the two vertical position controls are no longer independent, in other words, they move both channels, by different amounts. Can only get a line from channel one barely up to the center of the screen and channel two only about a quarter of that. And then, for short periods of time, everything seems to work fine. Also, the (bad) behavior slowly changes as it warms-up.

    I know very little about electronics (started this project to learn). I'm guessing that there is a diode somewhere that is supposed to separate the channels. Any thoughts?

    Thanks, kw
  2. OLIVE2222


    Oct 2, 2011
    Hi zermello,

    You can dowload the 475 service manual here under:

    However the best think to do is to join the tekscope Yahoo group (the acceptance take few days) they are highly skilled people there to help you.

  3. electro_pa


    Aug 14, 2011
    Hi zermello,
    I am ex Tektronix field service tech.
    Sounds like you have trouble in 'Vertical Switching' circuit, around U370.
    Possibly the U370 itself.
    Obviously, your symptoms show the channels are now connected together, so the switch function is a fail.
  4. zermello


    Dec 9, 2011
    Great info. Thanks! That problem is on hold for the moment, as I slowly try to unravel all of the issues. One big hurdle for me is that I don't know what to expect with various settings. Btw, I have the scope in a somewhat stable state at the moment, with both channels working independently. I suspect it is after I push the Trig view button that this problem reoccurs, but I am reluctant to test that at the moment. Do you know what I should see when I press this button?

    Thanks, Keith
  5. electro_pa


    Aug 14, 2011
    475 Trigger View test.

    Hi zermello again,
    BTW have you down-loaded the 475 manual, which Olive2222 pointed to in the first reply?
    It's the real deal!
    Expect the Trigger View function only to be active, while the button is held down.

    The attachment is text from the 'test of Trigger View', from the manual page 5.11.
    If you have recovered from fault condition,then your problem is 'intermittent'. That's a real bummer!
    Have you checked the basic power supply voltages?
    That is always the first thing to do, when looking for faults. See manual page 5.14 for description. Make sure you are using the line supply voltage, the scope is set for, on the back-panel switch.

    Attached Files:

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