analog oscilloscope slanted waveform help.

[diydidi]

Hi all.
I have had this scope for a while now. Works fine, except the waveform is slanted to the right as in the picture below.
I have tried adjusting some of the trimpots inside.....still nothing.
Trace rotation adjustment also doesn't help. I have also noticed that bringing my screwdriver close to the front and sides of the screen, also affects the trace position. Any ideas??
It is a 20Mhz GW Gos-623 scope. Can't find service manual either.

[doctor]

Hi all.
I have had this scope for a while now. Works fine, except the waveform is slanted to the right as in the picture below.
I have tried adjusting some of the trimpots inside.....still nothing.
Trace rotation adjustment also doesn't help. I have also noticed that bringing my screwdriver close to the front and sides of the screen, also affects the trace position. Any ideas??
It is a 20Mhz GW Gos-623 scope. Can't find service manual either.

It looks as generator’s problem, not the scope.
Take another scope and check.

[bkahuna]

How does the built-in calibration waveform on the scope look?

[olddawg]

How does the built-in calibration waveform on the scope look?

Don't mess with the internal calibration pots! If the calibration waveform still is slanted and there is no external adjustment and you are using a fresh probe, it may be a mechanical adjustment needed from the scope being bang around. There should be a yoke adjustment on the CRT. You loosen the clamp on the yoke and carefully rotate the yoke until the trace is straight. Do not move it forward or backward. If in a week or so it is slanted again you have an old cap changing value somewhere.

[guitardad]

Don't mess with the internal calibration pots! If the calibration waveform still is slanted and there is no external adjustment and you are using a fresh probe, it may be a mechanical adjustment needed from the scope being bang around. There should be a yoke adjustment on the CRT. You loosen the clamp on the yoke and carefully rotate the yoke until the trace is straight. Do not move it forward or backward. If in a week or so it is slanted again you have an old cap changing value somewhere.

Might be a bad cap. I had a Kikusui scope with a similar issue. Try a sq wave and see if it looks pincushioned.

[bkahuna]

Don't mess with the internal calibration pots!

Sorry, but where exactly did I suggest he twist the internal pots? All I asked was for him to hook his probe to the built-in generator most scopes have for calibrating probes. This is usually a square wave and might give some additional clues as to what is happening.

[Enzo]

You didn't, but the original poster said he had done so already. Thus if the internal calibration waveform looks funny as you sugested checking, the OP should not adjust the calibration controls as he had already tried.

[Steve Conner]

Also, an oscilloscope tube doesn't have a yoke. The deflection is electrostatic by tiny plates inside the tube.

Looks to me like either the sig gen is faulty, or the scope's vertical amplifier.

[J M Fahey]

I think that scope is just telling the truth.
By the way, the tangent to top and bottom peaks is perfectly horizontal, so don't turn the scope tube or anything.
And the screwdrivers and other tools, are obviously magnetized.

[olddawg]

I think that scope is just telling the truth.
By the way, the tangent to top and bottom peaks is perfectly horizontal, so don't turn the scope tube or anything.
And the screwdrivers and other tools, are obviously magnetized.

That's a good point. Anything with a magnetic field close to the scope could cause this problem. A magnetized screwdriver setting on one side of it could certainly do it. Clear the area around it and see if it makes a difference.

[diydidi]

OK. I have cleared the area around the scope of all steel etc. still no change. I did however notice that if a put the scope on its side, the scope trace comes right and looks symmetrical again. So surely it must be a mechanical adjustment within the CRT? Thing is I see no screws or adjustments on the base.

[J M Fahey]

No scope problem.
This test just proved it.
THERE IS A MAGNETIC FIELD OR MAGNETIC ANOMALY IN YOUR HOUSE.
Simple as that.
Maybe there is an ironmine (more precisely iron ore) under where you live, and I'm not joking.
There are devices made to discover them, which work on a similar principle.
By the way, one experiment we made at the University Physics Lab when I started studying Engineering, was precisely that.
We measured Earth's magnetic field intensity and orientation (it's a Vector magnitude) by pointing an unshielded Scope tube, no sweep (so the dot was static) in different directions, and graphing it.
So first we got the orientation.
Then, knowing Electron Mass and Speed (which could be calculated knowing high voltage used to accelerate it) , plus dot deviation (yes, the electron beam hit the screen in different points according to orientation, same as yours) we calculated strength.

[bkahuna]

OK. I have cleared the area around the scope of all steel etc. still no change. I did however notice that if a put the scope on its side, the scope trace comes right and looks symmetrical again. So surely it must be a mechanical adjustment within the CRT? Thing is I see no screws or adjustments on the base.

I agree with Juan. It sounds like a local magnetic field causing it. When you rotated the scope, that is akin to rotating your electric guitar guitar to minimize hum pickup.

Has this scope ever worked properly in this location or is this a recent development? You might try using it somewhere else to isolate the problem between scope and location.

[Enzo]

A hopefully related story. ANyone who works with color CRTs will relate.

I had a field tech call for help once, on an arcade video game. The kind you sit at, like in a restaurant or bar. (We call them cocktail table game cabinets) He was trying to adjust the picture. The cabinet swings open leaving the monitor facing the side. He'd get the picture right, but when he closed it up, a dark band crossed the screen and distorted the colors. He did this a few times. He then pulled the table away from the wall into the aisle way, and now the picture was perfect. But the game was in the way of the aisle. Push it back into place, back to the distortion. Cutting to the chase, he thought I was joking when I told him the problem was his CRT screen was magnetized and the thing was reacting to the lines of force in the earth's magnetic field. Didn't believe me. I told him to move the game around and watch the screen. Over the phone I could hear the sound of furniture being dragged and in the background an "I'll be damned" was heard. Sure enough, as he slid the game, and thus the CRT, across the room, he could see the lines of force in the earth's field as the screen crossed them. A quick degauss (demagnetizing) of the CRT and all was well.

[Sneaky Pete]

Having noticed that diydidi's location is in the Southern Hemisphere, could it be that the scope was designed for use in the northern hemisphere and is now being used where the earth's magnetic field is different? That might explain the slightly skewed trace.
I found this article on CRT monitors which offered a plausible explanation
Northern vs. Southern Hemisphere Monitors - Webopedia.com
Might explain why turning the thing on its side helped!