| Distorted / scewed traces on oscilloscope, I think it's magnetized - Click HERE for Original Thread |
| bigwill |
| 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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| Iain McNeill |
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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| Conrad Hoffman |
| 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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| Charles Hansen |
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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| bigwill |
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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| bigwill |
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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| bigwill |
| Is there any chance that this would've damaged the internal electrode structure of the crt? |
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| poynton |
| 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.
Andy |
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| Charles Hansen |
| quote: | Originally posted by poynton
I use a Weller soldering gun to degause old CRT type TVs.
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Be careful!
Please read post #5. |
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| poynton |
| quote: | Originally posted by Charles Hansen
Be careful!
Please read post #5. |
Most of the "unwanted" magnetism on TVs is not in the coil region but the shadow mask or the fixings round the screen.
I never had any problems in over 12 years
Andy |
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| Charles Hansen |
| quote: | Originally posted by poynton
I never had any problems in over 12 years
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Good for you. In the meantime:
http://www.laserfaq.org/samnew/tvfaq/tvdegacrt.htm
"WARNING: Don't attempt to degauss inside or in the back of the set (near the CRT neck. This can demagnetize the relatively weak purity and convergence magnets which may turn a simple repair into a feature length extravaganza!" |
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| poynton |
| quote: | Originally posted by Charles Hansen
purity and convergence magnets ................ |
They are an absolute horror when not aligned.
I usually scrapped the monitor !!
Andy |
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