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Oscilliscope Problem - can I fix it?

Discussion in 'Misc Electronics' started by Patrick, May 16, 2004.

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  1. Patrick

    Patrick Guest

    I have an HP Model 120B oscilliscope. It's quite old (from the 50s, I
    believe) but its been working perfectly until now. I recently moved across
    town and after setting the thing back up, I noticed it had developped a
    problem. The waveform trace does not "stabalize" vertically. That is, the
    trace drifts up and down the screen. As soon as I centre it, it'll start
    floating up and down again. In fact, the actual act of turning the vertical
    pot causes the trace to jump erratically around the screen - in fact any
    control adjustments cause this to happen. Note that the waveform itself is
    displayed perfectly, its just the vertical *position* that is problematic
    (horizontal is also fine).

    I realize this is an old machine and that my question is really just a shot
    in the dark...

    Thanks in advance.
  2. Gerard Bok

    Gerard Bok Guest

    First thing I would check is grounding.

    What type of mains are you connecting to ?
    Is there a safety earth connection (and is it working!) ?
  3. Michael

    Michael Guest

    For a second there I thought that was the scope that was one component
    of an equip. set I repaired while in the A.F. eons ago, and I remember
    the scope as a simple, reliable, no-nonsense unit. But on reflection
    (i.e. a lot of brain bending) I concluded that the one I had worked with
    was the HP-103. Actually the AP-103AR (rack-mount version).

    But thanks for the fond memory.
  4. ------------------
    Replace that pot!! They do this all the time, moving it dislodged
    some pollutant, or dinged the resistive material where the rotor
    was sitting.

  5. Patrick

    Patrick Guest

    Not sure it is the pot though. True, it jumps around when I move the pot,
    but the waveform also drifts, on its own accord, regardless of my fiddling
    with the pot.
  6. John Miller

    John Miller Guest

    My first guess would be that it's not the pot, *however* the first thing I
    would do (absent a schematic) would be to measure the voltage drop across
    the two outer terminals of the pot to see if it was changing. Knowing that
    can give you a good start.
  7. John is probably correct. The power supply voltage for the vertical circuit
    is likely unstable.
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