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~Tektronix~ oscilloscope repair 2246 ModA (but others may help too)

Discussion in 'Electronic Repair' started by [email protected], Nov 19, 2012.

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  1. Guest

    Hello everyone,

    I have a Tek 2246 ModA which has a slight but annoying problem:

    If I leave the scope off for some time, more than a couple hours, when I turn it back on, channel 1 is deflected off screen, below it.
    All other channels are there and appear fine, but not channel 1.
    Then after a couple minutes it comes back and everything works fine then.

    I know this is minor but is kind of annoying, so I have been trying to "chase this ghost" for a while now.

    Does anyone here know what it can be? Maybe had the same problem before?

    Thanks a lot everyone for even reading this far.

    Cheers,
    Marcelo.
     
  2. JW

    JW Guest

    Probably caps, but I've never worked on one.
    A place where you'll likely get help:
    http://tech.groups.yahoo.com/group/TekScopes/
     
  3. Marcelo

    Marcelo Guest

    Thanks for the reply ... yes it may be cap related ... I will check this other group, or maybe someone here that could maybe have the "rosetta stone" for this.

    Thanks again,
    Marcelo.
     
  4. Ray Carlsen

    Ray Carlsen Guest

    A similar problem I had with an HP scope deflected the beam to the
    right. It turned out to be an open collector resistor. Since yours
    appears to be a repeatable problem, it's likely to be a semiconductor
    and not a dry joint (not with a TEK scope, anyway) or PS problem since
    the other channel works. Deflection plates are fed with complementary
    balanced DC coupled stages, so an intermittently open transistor
    somewhere in the vertical deflection circuit is my best guess. I know
    I'm stating the obvious but you really do need a schematic to tackle
    that one. Failing that, you might try shocking each transistor with
    cold spray after the thing warms up to isolate the bad component.
    of course that assumes you can get to the area in question. I've
    heard there are extender boards to allow access. Note also that some
    parts are very carefully matched pairs and many are proprietary and
    hard to get. Good luck!

    Ray

    PS No flames please. I know I'm a novice at scope repair.
     
  5. Guest

    Hi Ray,

    Yeah ... novice here too :)

    I do have the schematics and I have been looking for the obvious issues first. I am getting to the point though where I will have to start looking ar less obvious stuff.
    Freezing some transistors seems like a good idea though. It has to be temperature related, so maybe I can make the problem reoccur under my control.

    Thanks for the post.

    Cheers,
    Marcelo.
     
  6. N_Cook

    N_Cook Guest

    turn it back on, channel 1 is deflected off screen, below it.
    "chase this ghost" for a while now.


    Have you seen it in the process of drifting? smooth or jerky?
    I would suspect any switch contacts like in the attenuator
    Put a DC monitor line in somewhere, while still using it (comparing with the
    same point in ch2) , plus trying
    freezer spray/ low setting hot air gun
     
  7. Guest

    First question :

    When you switch the deflection facor, say all the way down to 5 volt/div., does the trace come on screen and closer to center or does it remain off screen regardless ?

    J
     
  8. whit3rd

    whit3rd Guest

    The most likely reason for this, is a power supply sequencing problem.
    If it's only channel 1, there's possibly an RC filter in a channel one
    supply that has either a faulty resistor (high value) or faulty capacitor
    (leakage current too high). It could even be a shorted capacitor
    that heats up and behaves better (but that isn't as good as a working capacitor).

    It's also possible that offset adjustments of all your channels run off a
    highly regulated reference supply, and that secondary supply is
    slow coming up (but other channels have a smaller adjustment, so show
    less effect).
     
  9. N_Cook

    N_Cook Guest

    turn it back on, channel 1 is deflected off screen, below it.
    "chase this ghost" for a while now.

    I'm assuming as Tek that it has 2 separate CRT beams . Is it possible to
    swap ch1 and ch2 somewhere mid-process to localise to early or late circuit
    stages? ie
    problem stays on the same beam or swaps over
     
  10. Assuming this is a single-beam CRT, the problem can't be with the CRT.

    I'd look for an offset somewhere in the gain stages.
     
  11. Marcelo

    Marcelo Guest

    Hi J,

    Is remains offscreen regardless.
    I have tried messing around with the knobs and switches initially thinking it could be some bad contact.

    Thanks,
    Marcelo.
     
  12. Marcelo

    Marcelo Guest

    Hi William and N_Cook,

    Yes I will have to start looking further on the circuit now.
    The big problem is that I have only this 1 minute window to troublehsoot. Because when it comes back it works perfectly from then on. So I would have to leave it off for hous to see the problem again.

    Thanks all for the answers.
    And happy thanksgiving.

    Cheers,
    Marcelo.
     
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