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Oscilliscope Problem - can I fix it?

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.

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Reply to
Patrick
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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!) ?

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Kind regards,
Gerard Bok
Reply to
Gerard Bok

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.

Reply to
Michael

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

-Steve

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Reply to
R. Steve Walz

across

the

start

vertical

is

problematic

shot

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Reply to
Patrick

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.

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John Miller
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Reply to
John Miller

John is probably correct. The power supply voltage for the vertical circuit is likely unstable.

Reply to
Charles Schuler

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