TVs and unshielded speakers

Television CRTs have internal elements (shadow mask and electrodes) that can be misaligned, bent, or damaged by a strong magnet.

Speakers usually have strong magnets, BUT those are for the voice coil and the field doesn't extend beyond an inch or two. A few compact speakers may have enough field outside the box to distort a color picture (but that doesn't mean that there's any permanent damage), but only if you disassemble the speaker will you get any magnetic parts capable of doing real, permanent damage. All speaker magnet designs are shielded by design, because it's expensive to waste the field on empty space...

I've tested magnetic linear motors (big 'uns, over 20 pounds of magnet material) and the field is harmless at a few inches. For a boombox speaker, maybe two inches magnet-to-screen is too close. Four inches should be OK.

Little or none.

Several years ago, as an experiment, I shut off a powerful bulk tape eraser right to a 36" Trinitron to see what happened. It produced what was apparently a permanent color splotch that would not go away, no matter what I did.

When the set was moved to my bedroom in March, the splotch magically vanished. I was told that my original "abuse" has caused the aperture grill wires to "tangle", and that moving (and therefore vibrating) the set had made them come loose. In fact, Sony had a guy on the production line who whacked CRTs with a rubber mallet to prevent or reverse this.

A TV set's automatic degaussing coil can remove all but the worst magnetization from the shadow mask. It simply isn't something to worry about.

Another poster said "All speaker magnet designs are shielded by design, because it's expensive to waste the field on empty space." This is misleading. Simply blocking the external magnetic field does not make it available to strengthen the field around the voice coil. Rather, many, if not most, speaker magnets are "shielded" by using two magnets, arranged so that their fields add at the voice coil, but largely cancel outside.

Last night I realized there was a green spot at the top of my TV screen. That's when I realized some jerk had put their boombox right on top the TV earlier in the day. There's two inches between the spot and the top of the TV. So the speaker in the boombox was probably three to four inches away from the spot. It seems like that boombox is what caused the green spot. It would be too much of a coincidence for the spot to appear the same day the boombox had been placed on the TV.

So, will this go away by itself? It sounds like you're saying the degaussing thing should fix it. I would imagine something like this would void the warranty and the free repair program thing I purchased.

There's no reason you would need an external degaussing coil. * The next time you turn it on, its built-in degaussing system should clear the magnetization. And if you did use an external demagnetizer, the manufacturer would have no way of knowing.

• As flat-panel displays displace CRTs, degaussing coils will become the proverbial hen's teeth.

They are only strong enough to counter the earth's magnetic field. If you put a magnet close enough to magnetize the CRT, the degauser will probably never be able to undo it.

Use a bulk tape eraser.

Have you ever tried it? Or are you just theorizing?

Why don't you actually do it, then report back what you find?

I have on several TV's and monitors. I used to play with magnets and have never seen any degauser remove the slightest spot. Bulk tape erasers work although you have to back off slowly before killing the power or they'll leave new spots.

Why the don't you try it yourself before making more of an ass of yourself?

I've done it. And it works.

A boom box is sitting right under my crt tv as we speak. Has been there for the last 10 years. No problem.

"William Sommerwerck" wrote in news:gv70pd$aia$ snipped-for-privacy@news.eternal-september.org:

I used to work on TEK 650 series video monitors(TEK service centers),and we used an external degaussing coil,called for in the calibration procedure. The internal 'DG' coils are not as strong as the external ones,and you have to move them around the face of the tube,you cant do that with the internal coil.

TV repair shops all used them,made good money degaussing TVs people had screwed up with magnets. It should not affect your warranty.

"William Sommerwerck" wrote in news:gv772q$rvd$ snipped-for-privacy@news.eternal-september.org:

that's the way you do it with an external TV degaussing coil.You work the coil around the CRT face several times,gradually back away until you're a few feet away and -then- click off the coil.

And now that the trade secret is out... A blowdrier might be a fair substitute, or even a wall-wort on a long extension cord. Another possibility is purity magnets getting thrown off by being remangnitized. Might need a degauss of the entire chassis followed up by purity and convergence alignment. Speakers and TVs don't mix.

Would you humor me with an experiment? Move the boombox away, and see whether the picture changes, then put it back.

I once put a 30" length of railroad rail on top of my TV. You know, a bachelor's idea of home decorating. It sat there for a few years, and never caused any problem with the picture.

When I finally moved it, the color blotches were horrible. But the external degaussing coil cleared it right up.

Done that a number of times, moving furniture around. I have never seen localized color errors, except when I was fooling around with some magnets, mainly from old diskdrives. The de-gausing build into my tv took care of the results of those experiments :)

In fairness, I was claiming that the internal degaussing coil would be sufficient. (It's generally more powerful than would be needed simply to clear the magnetization induced by moving the set through the Earth's magnetic field.) I stand by that, but it's certainly true that the shadow mask could be so badly magnetized that a more-powerful external coil would be needed.

My 36" Sony WEGA sat between two Apogee Diva speakers. There was no visible impurity from the speakers. On one occasion, when I had to move the right-front speaker to re-glue the midrange ribbon, all sorts of colorations appeared on the screen. They pretty much disappeared when the speaker was returned to its usual position. If you bring a big magnet near the CRT, then move it away in the same direction it arrived, much of the induced magnetization will be removed.

To put it another way -- you have to go out of your way to badly magnetize a shadow mask. If you remember the example I gave, despite having shut off a very powerful bulk eraser right next to the CRT, I hadn't permanently magnetized the aperture grille -- simply knocked it out of alignment.

Do any of you remember Zenith's spinning wand? It was a small bar magnet that could be spun by hand. The idea was to allow demagnetizing small areas without having to plug in a coil.

Too strong of a magnetic field from that bulk tape eraser can warp the shadow mask It can even pull it loose inside the bell. I've seen the damage on some CRTs, but its yours to destroy. I've degaussed thousands of CRTs in the last 40+ years.

Did it have a loco or two on the track? with permanent magnets in the motors?

Ron

Some of the worst offenders are the small speakers used for car hands-free telephone installations. They are powerful little fellows, and can affect CRT pictures from a distance of more than 18". They are not exactly hi-fi, but are sometimes used as external speakers for CB and amateur radio equipment, and must be kept well away from any TV set in the vicinity.

Just to be clear: a magnet outside the TV can magnetize a bit of steel inside the TV, so it takes BOTH moving the source AND cycling the TV's internal degaussing coil to rid oneself of the color.

Unplug the TV for at least 5 minutes to be sure it's cold, then plug it in and turn it on, to trigger the degausser cycle.

This should be done if magnetized items are moved to or from the TV's vicinity, or even (for careful work) when the compass orientation of the TV is changed. Any color splotches that remain, require purity magnet adjustment (if you have to ask what that means, you should let someone else do it).