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HP 400 VTVM question

Discussion in 'Electronic Repair' started by R. D. Davis, Nov 21, 2003.

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  1. R. D. Davis

    R. D. Davis Guest

    Should the meter in an HP 400 VTVM deflect to the far right, pegging,
    then, go all the way to the far left, as far is it can physically go,
    and then return to zero, when switching from one scale to another?

    The meter is giving off a very faint burning smell, like the smell of
    heating elements in a toaster or electric heater, but looking inside,
    I don't see any smoke or anything that looks charred. Didn't pinpoint
    the source of this yet.
  2. Dbowey

    Dbowey Guest

    rdd asked:
    Every one I ever used did that. Irritating, but it didn't result in harm to
    any of them.

  3. Jim Adney

    Jim Adney Guest

    Hmmm, I don't recall the exact cause of that, but I think it may be a
    rather standard symptom wiht the HP 400. It's in the manual. What's
    the serial number on yours?
    Is it possible that you're just not used to tube operated gear? These
    can put out a bit of a smell from the hot tubes, especially if they
    have not been fired up for a long time.

    The most common problem on these old units is that there are 4 large
    axial lead molded paper capacitors in there and they are probably all
    leaky. 3 of these are AC coupling capacitors in the meter amplifier,
    while the 4th is in the back, part of the power supply.

    The amp ones are paired with some resistors and mica caps. If these
    are leaky they will bias the control grids of the next amp stage high
    and cause poor gain as well as tubes that run extra hot, as well as
    shortening their lifetimes.

    Let me know your serial number and I may be able to send you a
    schematic with voltage values to check. I generally find that LOTs of
    things start to work well on these once I get the leaky caps replaced.

    There's a very nice old HP 400H sitting right behind me. ;-)

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