Oscilliscope Problem - can I fix it?

Question

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.

Gerard Bok · Accepted Answer

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!) ?-- Kind regards,Gerard Bok

Michael · Answer

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.

R. Steve Walz · Answer

------------------Replace that pot!! They do this all the time, moving it dislodgedsome pollutant, or dinged the resistive material where the rotorwas sitting.-Steve-- -Steve Walz     ftp://Electronics Site!! 1000's of Files and Dirs!!  With Schematics Galore!! or

Patrick · Answer

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 fiddlingwith the pot.acrossthestartverticalisproblematicshot---Outgoing mail is certified Virus Free.Checked by AVG anti-virus system ( 6.0.679 / Virus Database: 441 - Release Date: 07/05/2004

John Miller · Answer

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.

Charles Schuler · Answer

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

Oscilliscope Problem - can I fix it?

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