Distorted / scewed traces on oscilloscope, I think it's magnetized - diyAudio
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Old 26th June 2008, 01:12 PM   #1
bigwill is online now bigwill  United Kingdom
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Default Distorted / scewed traces on oscilloscope, I think it's magnetized

My CRO is acting up. When you move a trace to the top of the screen, the trace angles upward, and when it's at the bottom it's angled down. This happened since I temporarily placed a small laser printer on top of it and made a couple of prints, in hindsight probably not the best idea. I supposed something has gotten magnetised. Is it possible to fix this?
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Old 26th June 2008, 11:59 PM   #2
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Old 27th June 2008, 12:13 AM   #3
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you've played with astigmatism, rotation, intensity etc to no avail?

CRT's are electrostatic if I remember correctly although still affected by magnetism. Wouldn't expect a degauss function.

What model? how old?

If it's a single channel why worry. It's probably a dual right?
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Old 27th June 2008, 03:35 AM   #4
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It seems plausible that the shielding around the CRT was magnetized by some current pulse in the printer, but I stack all sorts of test equipment on my scope, and have never had the slightest problem. So have a lot of other people over the decades. Can you describe the scope- is the enclosure metal? If something got magnetized, it should be possible to demag it using an old TV degaussing coil, but I'm betting something else happened.
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Old 27th June 2008, 04:21 AM   #5
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All 'scopes use CRT's with electrostatic deflection. There is usually a magnetic shield around the CRT to minimize interference from external B fields. It sounds like this got magnetized. Get a bulk tape eraser and try demagnetizing the shield around the CRT.

WARNING! Do not try this trick with a video display. A video display uses magnetic deflection coils. They usually have permanent magnets attached near the CRT to compensate for (cancel) the earth's magnetic field. If you try to de-gauss the shielding around a video display, you will probably demagnetize the permanent magnets and screw it up.
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Old 27th June 2008, 01:21 PM   #6
bigwill is online now bigwill  United Kingdom
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It's an LG-9020G 20Mhz scope.

I havn't tried the rotation control yet but it was perfect before this happened.


I have some 6v mains toroids. If I make a coil of wire around an old file or screw driver, will this make an adequate degaussing wand? (Powered by the toroid)
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Old 27th June 2008, 01:39 PM   #7
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Hah! I had a reel of speaker cable, the complete reel measuring about 5ohms DCR, so I put 6vac across it, waved it by the scope a bit and slowly pulled it away and it worked! Woo, nice straight traces again!

I think the steel case must've been magnetized
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Old 27th June 2008, 01:59 PM   #8
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Is there any chance that this would've damaged the internal electrode structure of the crt?
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Old 27th June 2008, 03:20 PM   #9
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Quote:
Originally posted by bigwill
Is there any chance that this would've damaged the internal electrode structure of the crt?

I doubt it.

I use a Weller soldering gun to degause old CRT type TVs.


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Old 27th June 2008, 03:41 PM   #10
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Quote:
Originally posted by poynton
I use a Weller soldering gun to degause old CRT type TVs.
Be careful!

Please read post #5.
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