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Tek 475 Scope Problem

Discussion in 'Electronic Repair' started by Michael, Sep 21, 2004.

  1. Michael

    Michael Guest

    I picked up a Tektronix 475 (not 475A) scope. It works fine, except that
    there is no sweep at any speed faster than 1us. Trigger stays locked,
    and I can see the vertical working off the screen at the edge, but there
    just isn't any sweep. At 1us and below, sweep appears normal.

    This problem is there whether I use the A or B (delayed) mode. Seems
    like the oscillator just stops. Anybody have any ideas? I'm thinking
    maybe it's a switch contact on the sweep time knob, but it's a mess to
    try to get to it, so I don't want to take that much stuff apart unless
    I'm pretty sure that's where the problem is.

    I have the schematic in PDF form, but it's pretty hard to read and is
    separated into many sections, so I can't figure out what's going on.
    Thanks for any help you can give me.

    Michael
     
  2. Asimov

    Asimov Guest

    "Michael" bravely wrote to "All" (21 Sep 04 00:42:12)
    --- on the heady topic of "Tek 475 Scope Problem"

    Mi> From: Michael <>

    Mi> I picked up a Tektronix 475 (not 475A) scope. It works fine, except
    Mi> that there is no sweep at any speed faster than 1us. Trigger stays
    Mi> locked, and I can see the vertical working off the screen at the edge,
    Mi> but there just isn't any sweep. At 1us and below, sweep appears
    Mi> normal.
    Mi> This problem is there whether I use the A or B (delayed) mode. Seems
    Mi> like the oscillator just stops. Anybody have any ideas? I'm thinking
    Mi> maybe it's a switch contact on the sweep time knob, but it's a mess to
    Mi> try to get to it, so I don't want to take that much stuff apart unless
    Mi> I'm pretty sure that's where the problem is.

    Mi> I have the schematic in PDF form, but it's pretty hard to read and is
    Mi> separated into many sections, so I can't figure out what's going on.
    Mi> Thanks for any help you can give me.


    This is not specific advice for this problem but from past experience
    when an oscillator dies at a higher frequency this means a loss of
    gain in the oscillating device. Either a transistor has marginal gain
    or a dry decoupling electro in the supply line to the stage, could
    cause a loss of loop gain.

    A*s*i*m*o*v

    .... Electrical Engineers do it with more frequency and less resistance.
     
  3. Jim Yanik

    Jim Yanik Guest

    you need a service manual.

    The 475 has THREE different sweep generators,depending on which sweep mode
    is used.While working at Tek,I always had to refer back to the manual to
    troubleshoot sweep problems.
    The circuit description is priceless,VERY useful.
     
  4. With my 485 with somewhat similar symptoms, it was just one of those
    internal coax cables that was loose.

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    Important: Anything sent to the email address in the message header is ignored.
    To contact me, please use the feedback form on the S.E.R FAQ Web sites.
     
  5. TekMan

    TekMan Guest


    Most likely a problem with the capacitors in the range < 1 mikrosec.
    Or switch contacts: come lame or dirty with time.

    As Jim proposed: Check the manual section, THERE IS DETAILED section
    how to troubleshoot the timebase.

    YES, we all know it's hard to "dig" into the manuals. Because we have
    to do this every single day :))

    Be assured that it is worth your efforts - the Tek manual for the 475
    (and many other) are brilliant for troubleshoting and understanding
    the concept of the circuit.

    hth,
    Andreas
     
  6. Jim Yanik

    Jim Yanik Guest

    (TekMan) wrote in

    Personally,I mostly ignore the troubleshooting algorithms and "advice" that
    TEK put in their manuals;there's no substitute for reading the CIRCUIT
    DESCRIPTION and knowing how the circuit works in detail.
     
  7. TekMan

    TekMan Guest

    You are right Jim, that's how all we pro's and ex-pro's of this kind
    of work field do.

    But, one should not forget: The original poster seems to be not too
    experinced in this kind of scope repair (Sorry, was not meant
    negative). If you are sitting in front of a faulty scope first time,
    then for this kind of work, the troubleshoot alogo's in the tek 475
    manual are a big help. Like every good documentation is when you have
    a faulty system: It's written for easing the work, and to help repir
    technician that don't do this every day to get better, quicker and
    more reliably to find the fault.

    Of course, like in every electronics circuit: It's fundamental to
    "dig" into the schematics and try to understand HOW the circuit does
    what it does.

    Never forget Jim that we do so for more than 30 years, so that's why
    our perspective of "starting the work" is slightly difference.

    To the OP: Good luck for the repair.

    hth,
    Andreas
     
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