What Happened When I Shook My Flashlight Near My TV? Shocking Results Revealed!

[Werg22]

What the h*ll happened??!

I have this flashlight that when shaken generates light. I approached to a CRT TV by accident and the screen permanently changed color. Now my TV is gone, but I'm more stunned than angry. What's in that flashlight?

 

[turbo]

There is a strong permanent magnet in that flashlight that oscillates back and forth in a coil that to current to charge the capacitor(s). You can restore the color of the CRT by degaussing it.

 

[Werg22]

You mean a e.m.f. inducing motor?

Anyway... Where can I read about how the magnet comes to interfere with the cathode rays?

 

[cesiumfrog]

Read about how cathode ray tubes work, and about how magnetic fields affect cathode rays (see JJ Thompson's apparatus for an example).

 

[berkeman]

The permanent magnet in the hand-pumped flashlight has magnetized the metal in the shadow mask layer in the faceplate of your color TV. You can fix it by buying a "deGaussing coil" at a general electronics supply store or off the Internet. That is a coil about 15-20cm in diameter that contains multiple turns of wire. You plug the coil into the AC Mains wall plug, and hold it up to the TV sceen. The screen goes nuts from the AC magnetic field from the coil, but you wave it around gently as you back away from the screen slowly, and this presents a slowly decaying AC magnetic field to the screen (and its shadowmask), and when you get far enough away, this leaves the screen with a negligible DC magnetization, so the electron beam is not deflected at all by the shadowmask, and your colors are true again.

I used to work a lot with TVs (fixing them for friends, etc.), and I won't tell you why I had to buy a deGaussing coil...


EDIT, Quiz Question -- Why doesn't it hurt a black and white TV to hold a permanent magnet up to the faceplate?

EDIT2, Quiz Question -- Hey, why the heck don't they just make the silly shadowmask layer out of non-ferrous conductive material? That seems just plain dumb. There has to be a reason, though...

 

[turbo]

I used to work a lot with TVs (fixing them for friends, etc.), and I won't tell you why I had to buy a deGaussing coil...

Fun with rare-earth magnets?

 

[berkeman]

Fun with rare-earth magnets?

Sort of. Party at my apartment, showing off the pretty colors that a magnet can make on a TV screen, uh, ooops. Let me go snag my deGauss coil for a sec...

 

[turbo]

OK, typical boy meets girl/magnet meets CRT story.

 

[Astronuc]

My son used to do that. Took us a while to figure it out though.

We had bought my son and daughter magnetic wands with a set of metal balls (each covered in colored plastic). He would spend time playing with them.

One day we noticed the color TV was green and purple. We figured it was dying. Then it colors changed again. My wife called a repair person who basically figured out that the screen had been magnetized, be we didn't figure on my son at that point. I had some speakers nearby, so I thought it might have been the speakers, which got moved into another room. But then the TV changed color again. That's when we found my son with the magnet in the room with the TV. Problem solved.

 

[cesiumfrog]

You can fix it by buying [..]

I've never seen a TV (nor CRT computer monitor) that did not have a built-in degaussing function already included. I'd look for this first, in the in-screen menus, or at least try switching it on freshly from the power point, and maybe even read the manual.

 

[berkeman]

I've never seen a TV (nor CRT computer monitor) that did not have a built-in degaussing function already included. I'd look for this first, in the in-screen menus, or at least try switching it on freshly from the power point, and maybe even read the manual.

That's a very good point. Most of the CRTs I was referring to were older (like me). If your TV or CRT has a built-in deGauss coil, that will save you a bit of money. Good call cesiumfrog.

 

[Mk]

How do you build your own deGausser?

 

[cesiumfrog]

How do you build your own deGausser?

Wikipedia: "A degauss causes a magnetic field inside the tube to oscillate rapidly, with decreasing amplitude." Sounds plausible (first ensuring the field is uniform, then trying to move to the center of the hysteresis curve), you might want to already have some experience putting circuits together tho.

 

[antonantal]

EDIT, Quiz Question -- Why doesn't it hurt a black and white TV to hold a permanent magnet up to the faceplate?

Because it doesn't have a shadow mask.

EDIT2, Quiz Question -- Hey, why the heck don't they just make the silly shadowmask layer out of non-ferrous conductive material? That seems just plain dumb. There has to be a reason, though...

I think I read somewhere that they make the shadow mask out of ferrous material because of it's strength. I guess that if it was made from aluminium it would bend under it's own weight. And other conductive non-ferrous and strong materials are too expensive? I'm not sure. What is the answer?

 

[FredGarvin]

I did some reading on this when I had a problem. I found out that many of the built in degauss systems are not strong enough to handle anything out of the ordinary. In my case, I had a speaker that wasn't shielded very well and moved it too close to my TV. I had to get a pretty good sized coil to try to fix it. I still couldn't completely fix it though.

 

[antonantal]

I did some reading on this when I had a problem. I found out that many of the built in degauss systems are not strong enough to handle anything out of the ordinary. In my case, I had a speaker that wasn't shielded very well and moved it too close to my TV. I had to get a pretty good sized coil to try to fix it. I still couldn't completely fix it though.

If the magnet is very strong, it not only magnetizes the shadow mask, but it also bends it, and this can't be fixed.

 

[russ_watters]

You guys should see the computer monitors and tvs on a navy ship...