Oscilliscope Problem - can I fix it?

P

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.


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On Sun, 16 May 2004 17:21:32 GMT, "Patrick"
<transactoid@rogers.com> wrote:

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...
Click to expand...
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
 
Patrick wrote:
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.
Click to expand...

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.
 
Patrick wrote:
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.
------------------
Click to expand...
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
--
-Steve Walz rstevew@armory.com ftp://ftp.armory.com/pub/user/rstevew
Electronics Site!! 1000's of Files and Dirs!! With Schematics Galore!!
http://www.armory.com/~rstevew or http://www.armory.com/~rstevew/Public
 
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.

"R. Steve Walz" <rstevew@armory.com> wrote in message
news:40A816CE.3941@armory.com...
Patrick wrote:

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.
------------------
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
--
-Steve Walz rstevew@armory.com ftp://ftp.armory.com/pub/user/rstevew
Electronics Site!! 1000's of Files and Dirs!! With Schematics Galore!!
http://www.armory.com/~rstevew or http://www.armory.com/~rstevew/Public
Click to expand...

---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.679 / Virus Database: 441 - Release Date: 07/05/2004
 
Patrick wrote:
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.
Click to expand...
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.

--
John Miller
Email address: domain, n4vu.com; username, jsm

Take me drunk, I'm home again!
 
John is probably correct. The power supply voltage for the vertical circuit
is likely unstable.
 
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