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Vintage Dumont 292 Oscillograph

Discussion in 'Electronic Equipment' started by Rob, Mar 2, 2007.

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  1. Rob

    Rob Guest

    This looks to be '40s or so vintage. Plug in and it appears to work
    (I can get a trace), have not done much more than that. The power
    caps appear to be 2.5uF, 1500V and there are about a dozen of the
    paper/wax types used in the unit. Should I replace these right off,
    rather than continuing to fool with it? If so, which ones are the
    most critical for replacement? I do not have a schematic and limited
    knowledge, I could not tell you their function by looking at them. As
    far as calibration, I see a single trim pot, is there a generic
    approach to calibrating these that someone can explain?
  2. Rob

    Rob Guest

    Just want to add that this would be for learning purposes at the
    beginner's level, "fooling around" if you will. And yes, I am aware
    of the dangerous voltages involved even if unpowered (though not much
    more than that).
  3. Rob

    Rob Guest

    I just measured the power caps rated .5uf/1500V. One reads .67, the
    other .62. Not bad for 60 years. Spot checking other caps (Sprague
    brand), the worst I find are 50% out of spec, .33 or so when cap is
    rated at .2. None leak or show shorts, amazing build quality. Made
    in the USA.
  4. Alan Douglas

    Alan Douglas Guest

    The Du Mont 292 came out in March 1950. I have the schematic if you
    need it. The two electrolytic filter caps ought to be replaced.
    Normally paper capacitors of this vintage are leaky by now, but Du
    Mont sometimes used Aerovox oil-filled caps which hold up pretty well.
    I wouldn't run it for extended periods however. Capacitors are
    cheaper than power transformers.

  5. Jamie

    Jamie Guest

    I have a Dumont dual trace Solid State scope, M version, i think it
    covers up to 60 Mhz.. Don't ack me

    Yup, it still works how ever, i did have to change the Trippler in it.
    I stuck a TV trippler in there wrapped in Tin foil just out side the
    the cage. it works perfectly! :)
  6. Rob

    Rob Guest

    There is one Aerovox, beneath the CRT. I fired it up for awhile, all
    tubes work and I get some kind of response, but it is not working
    properly. I can center the spot, but can't move it to the right past
    the centerline. I connected up the probe and plugged into a socket
    with no response on the trace. Would it be worth replacing the power
  7. Alan Douglas

    Alan Douglas Guest

    My guess is the 0.1 600V caps from the 12AX7 plates to the CRT
    deflection plates are leaky. If the spot still won't go past center,
    check the X or Y position pots.

    This wasn't a high-end scope, so restoration would be more for its
    own sake than to get a working test instrument.

  8. Rob

    Rob Guest

    I've tallied up the capacitors:

    1x .02uF 600VDC paper type (sprague)
    8x .1uF 600VDC paper type (sprague)
    2x .25uF 600VDC paper type (sprague)
    1x .25uF 600VDC can type (sprague)
    1x .5uF 600VDC paper type (aerovox, not oil filled)
    2x 20uF 450VDC can type (sprague), this is the dual type can, one unit
    2 square type w/ the colored dots, unknown values, these go on the
    5500 and 30K setting for the Sweep Range

    Thinking of using Mallory 150 Series axial metallized polyester for
    these, about $1.25 apiece.

    Filter Caps:
    2x .5uF 1500VDC cans (sprague)

    Thinking of using 940C Cornell Dubilier axial PP film type for these,
    $8 or so. Mounting might be a problem.

    I also see a few trimmer capacitors.

    So, looking at about $30 in caps. Worth it? Tube appears to be fine
    and I am getting some response when probing household power, but
    things seem a little out of whack.

  9. Once again, you are completely off topic. This time, you are telling
    the man who had a book published about old test equipment. Alan is a
    wealth of knowledge on old test equipment. You should read his book,
    and learn.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  10. Alan Douglas

    Alan Douglas Guest

    You could change a few, in the time base and the coupling to the
    CRT deflection plates, and see if they make an improvement, before
    buying the others. The filter caps ought to be changed before you run
    the scope very much, since they could take out the power transformer
    if they short. But they'll probably work long enough to evaluate the
    rest of the circuitry.

    In general, scopes of this vintage are only collectors items or
    suitable for fooling around, which is to say, once you've used a more
    modern scope with triggered sweep and calibrated vertical and
    timebase, there's no going back. This particular model was not
    high-end even in its day. With its gas-tube sweep generator, it's a
    pre-war design with "modern" (late-40s) 12AX7s in the vertical amp.

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