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Tektronix 475 blanking problem

Discussion in 'Electronic Equipment' started by Andrej Nussdorfer, Nov 26, 2003.

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  1. Hi!

    I have recently aquired an old Tektronix 475 oscilloscope that works
    fine with the exception of the backtrace blanking, which seems to be
    faulty.

    Has anyone encountered such a problem? Or still better, does anybody
    know where the user and service manuals can be found?

    Thanks, folk!

    Andrej
     
  2. JW

    JW Guest

    http://www.aa4df.com/tek475sm.htm
     
  3. Jim Yanik

    Jim Yanik Guest

    (Andrej Nussdorfer) wrote in
    First,check/adjust CRT bias. That may be all that's needed.
    Check for proper Z-axis waveform.
    Then check sweep unblanking gate signal.(real fun,475 has 3 separate sweep
    generators.)*Read the circuit description in the manual!* (real TEK manual)
    Last,check DC restorer diodes with a curve tracer,or replace.They break
    down under HV,so using a DMM check usually will not find bad diodes.
    Lastly,it could be a G-K short,although that's unusual.
     
  4. TekMan

    TekMan Guest



    As Jim proposed, CRT bias and the -2950V should be checked.

    A manual can be downloaded for free from logsa.army.mil

    The manual contains detailed help for checking/debugging the sweep
    generators (BTW. Jim, I only found referenced in the manual two: fast
    and slow sweepgen, but these 2x for each A and B sweep - did I
    miscount?)

    If you follow the instructions (pp. 4-12 in the manual; thats section
    4, pp 12)
    the whole check of all sweep generators took me 25 minutes.

    In mine (ser# 700530) one decoup. resistor in the +15V was burned due
    to short in nearby tantalum cap (looks like this is often happening -
    it's the 28th repair case I've like this) and a bad Tran (Q 1044).

    The scope is now doing fine :)


    hth,
    Andreas
     
  5. TekMan

    TekMan Guest




    As Jim proposed, CRT bias and the -2950V should be checked.

    A manual can be downloaded for free from logsa.army.mil

    The manual contains detailed help for checking/debugging the sweep
    generators (BTW. Jim, I only found referenced in the manual two: fast
    and slow sweepgen, but these 2x for each A and B sweep - did I
    miscount?)

    If you follow the instructions (pp. 4-12 in the manual; thats section
    4, pp 12)
    the whole check of all sweep generators took me 25 minutes.

    In mine (ser# 700530) one decoup. resistor in the +15V was burned due
    to short in nearby tantalum cap (looks like this is often happening -
    it's the 28th repair case I've like this) and a bad Tran (Q 1044).

    The scope is now doing fine :)


    hth,
    Andreas

    PS: sorry for the term debugging..
     
  6. Jim Yanik

    Jim Yanik Guest

    (TekMan) wrote in
    The third sweep gen is for delayed sweep functions,if I recall correctly.
    I used yellow hi-lighter to hilight the sweep gens in the manual I used
    when I was with TEK.It and a chart in the circuit descriptions(IIRC) were
    valuable in figuring out which circuit was in use when a problem occurred.
     
  7. TekMan

    TekMan Guest

    Jeez! I bought on ebay a 475 serv. manual, section in the sweep gen
    schematics highlighted with yellow text marker.
    Maybe might be preowned by you.
    I wonder how it made the long way to Europe.

    Yes, the chart in the circuit descriptions is real helpful. Speeded up
    the repair a lot.

    Andreas
     
  8. Jim Yanik

    Jim Yanik Guest

    (TekMan) wrote in
    Mine will have voltages,notes and part numbers written on the
    schematics.Might even have my name on the inner cover.
    I'm pretty sure it went into the dumpster when we moved to a new
    location,though.

    You guys would puke at the number of TEK manuals I've seen thrown into the
    dumpsters after several moves.If I had a place to store them,I might have
    kept them.
    AACK,the stuff I could have saved!
     
  9. Hi everybody!

    Thanks for all the answers! they will be quite helpfull. In the
    meantime I discovered another suspicious thing. Three diode rectifiers
    (CR1442 +15V unreg, CR1452 - +5V unreg and CR1462 - -8V) are extremely
    hot. Actualy too hot to touch. Has anybody else noticed something like
    that?

    Thank you again!

    Andrej Nussdorfer
     
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