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Author Topic: the effect of magnets on a tv screen.  (Read 18249 times)

paul.fr

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the effect of magnets on a tv screen.
« on: 01/06/2007 22:51:45 »
I ran a magnet over a CRT TV screen, this had the effect of changing the colour of the image and the effect lasted for a few days! Why?


 

Offline DoctorBeaver

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the effect of magnets on a tv screen.
« Reply #1 on: 01/06/2007 23:07:15 »
I ran a magnet over a CRT TV screen, this had the effect of changing the colour of the image and the effect lasted for a few days! Why?

Are you sure it wasn't the drugs you were taking?  :D

Seriously though, folks; blame electrons. Yup, those little buggers are the source of all ills. In this particular case, the magnetic field makes them go silly. The colour change is probably due to the magnet changing their velocity, or some such. Why the effect should persist for a few days, how the hell would I know that? I'm just a dumb rodent!

What I do know, though, is that a few years ago I was living in a flat & the guy next door was a radio ham. His rig was the other side of the dividing wall where my TV was. I could always tell when he was using his radio because my TV picture went mental.
 

paul.fr

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the effect of magnets on a tv screen.
« Reply #2 on: 01/06/2007 23:15:52 »
The part of the screen where the magnet was placed went green, if that helps?

As for radio hams, you should have complained. part of his licence is that he must have filters to prevent the interference (well, it used to be. They were pretty simple to make with coax.
 

Offline syhprum

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the effect of magnets on a tv screen.
« Reply #3 on: 01/06/2007 23:17:45 »
I was a TV service technician in 1965 when colour TV first came to the UK and phenomena was given great publicity, many Technicians had 50 Hz powered electromagnets which were regarded as a sort of magic wand to get rid of this screen magnetism and made good money waving them about.
Modern CRT's are less sensitive to stray magnetic fields although speakers sometimes cause problems.
With care the effect can be got rid of by waving the other pole of the magnet near the screen.
 

Offline Soul Surfer

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the effect of magnets on a tv screen.
« Reply #4 on: 02/06/2007 09:59:44 »
This effect only applies to Cathode ray tube (CRT) types of colour televisions and modern LCD and LED screens are not affected by magnetic fields and I don't think that plasma screens will be affected either.

The reason for this is that the CRT generates light by shooting a finely focused beam of electrons to create a spot of light and deflecting it using magnetic fields to create the picture so magnetic fields will affect the picture.  For black and white televisions this will distort the picture but for colour televisions there are three separate beams for red green and blue These beams shine through a set of fine holes in a metal plate(the shadow mask) to strike the red green and blue phosphor dots.  A magnetic field will cause the wrong beam to strike the dot and affect the colour.  You should be very careful with magnets near colour CRT tubes because the affect can be permanent because you can magnetise the shadow mask to permanently distort the colour.  MOST TVs have demagnetising coils that get rid of stray fields like this but they can take quite a long time.  You will notice that there is a brief buzz when you switch the TV on after it has been switched off for some time this is the demagnetising coils operating.
 

Offline syhprum

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the effect of magnets on a tv screen.
« Reply #5 on: 02/06/2007 10:35:42 »
The first generation of CRT type colour TV's used the colour mask system developed by RCA as described by Soul Surfer, later TV's use the vertical line shields as developed by SONY which is much less sensitive to magnetization.
Of course LCD screens are quite immune but there might some effect with plasma but I don't think it would be permanent
 

lyner

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the effect of magnets on a tv screen.
« Reply #6 on: 03/06/2007 00:15:34 »
Each hole in the shadow mask (and the idea is the same for modern colour CRTs, which use slots) lets electrons from each of the three guns through but only the electrons from the green gun get to the green phosphors (etc.) To achieve this, 'electron optics' are used and this involves a system of magnets and electromagnets. For this to work, the three guns should really be separated by less than a millimetre.  The guns are quite bulky and magnetic fields are used to bend the paths of the three beams so that they 'appear to' come from almost the same place in the neck of the tube. Once you have the colour purity correct, you have to make sure that the three (red,green and blue) beams are deflected by the same amounts  so that the three pictures are registered, or 'converged' over the whole area of the screen.
Modern tubes are much shorter than old tubes and the angles that the electron beams are deflected through are large. This makes the problem even greater and it is an amazing feat to make everything work right and to be reliable for years, in spite of changes in temperature , voltages and local magnetic fields. Lining up the old Shadow mask CRTs was an absolute nightmare and if you even  knocked the set a bit hard, you'd have to do it all over again.
That is almost a thing of the past, now. I wonder if anyone is still producing CRTs any more.
 

Offline syhprum

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the effect of magnets on a tv screen.
« Reply #7 on: 03/06/2007 12:28:27 »
Setting up the convergence on the first generation of colour TV's in the sixties was quite easy as the convergence controls were good quality components and the broadcasting stations carried test cards much of the time.
The seventies it became a nightmare , as TV,s became cheaper poorer quality components were fitted which were very unstable and the test cards disapered so that one had to try and setup convergence on a moving picture!!!!!.
I was fortunate to get out of the TV business in 1975 before the simpler SONY type CRT's came in into the much more lucrative and less demanding printing business other wise I think I would have been driven mad.
Now I am retired I now have a testcard generator and all my TV's are either LCD or SONY and life in this area is very easy.
 

lyner

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the effect of magnets on a tv screen.
« Reply #8 on: 06/06/2007 16:00:38 »
I agree about the convenience of the broadcast test card but a grille pattern was more useful. After a year or two of exposure to the steaming hot conditions inside some of the big valve colour tellys, the convergence controls used to get very sticky and iffy. They were just starting to be problematical when you had the good sense to leave the business Syphrum!  I remember a huge Phillips monster with a convergence box with a 4ft cable on it so you could sit in front of the tv and sort it out. The beast was too big to move so that you couldn't do the trick with a mirror.
Broadcast monitors had to be lined up every damned morning for critical viewing - I remember an earnest man on a 'colour course' who had us all doing it from scratch. Nightmare.
I was thinking the other day- my career coincides almost exactly with that of broadcast PAL TV; we're both on the way out now. I plan to be more useful than analogue tv after 2012, though!
 

Offline daveshorts

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the effect of magnets on a tv screen.
« Reply #9 on: 25/10/2007 13:46:46 »
If you are interested I have written this up as a kitchen science at:
http://www.thenakedscientists.com/HTML/kitchen-science/exp/magnets-and-tvs/
 

lyner

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the effect of magnets on a tv screen.
« Reply #10 on: 25/10/2007 14:07:42 »
Quote
With care the effect can be got rid of by waving the other pole of the magnet near the screen.
You really need to be careful doing this - you will make it worse before you know it.
You need to use AC through a coil and gradually remove the coil - that re-shuffles the magnetic domains and leaves no permanent magnetism. It might even work with a beefy transformer - but don't put it through the screen.
 

paul.fr

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the effect of magnets on a tv screen.
« Reply #11 on: 25/10/2007 21:38:19 »
If you are interested I have written this up as a kitchen science at:
http://www.thenakedscientists.com/HTML/kitchen-science/exp/magnets-and-tvs/

I thought of this topic when i heard the radio show, well podcast, thanks Dave.
 

Offline Karen W.

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the effect of magnets on a tv screen.
« Reply #12 on: 25/10/2007 22:28:51 »
« Last Edit: 29/10/2007 07:23:15 by Karen W. »
 

Offline syhprum

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the effect of magnets on a tv screen.
« Reply #13 on: 26/10/2007 07:08:09 »
Altough all TV's are fitted with auto-degaussing the thermistor type devices that control the current surge to the degaussing coils often fail with age.
these devices only cost a pound or so or can easily be salvaged from old computer monitors from you local dump.
They are only three terminal devices and need the minimum of soldering skills to replace.
 
 

lyner

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the effect of magnets on a tv screen.
« Reply #14 on: 27/10/2007 23:12:19 »
I don't think the built-in de-gaussing coils carry enough current or have enough turns to cope with the sort of damage that a strong magnet can cause. They are really to deal with permanent magnetisation caused by the Earth's field and minor fields from nearby speakers ets.  A big, hand - held one may be necessary for magnetisation levels caused by  messing about with strong  magnets.
 

Offline syhprum

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the effect of magnets on a tv screen.
« Reply #15 on: 28/10/2007 11:46:54 »
An awful number of TV's are scrapped just because the thermistor device that controls the degaussing has failed.
the result is a badly discoloured display that leads people to assume that the expensive CRT has failed.
This is another good reason for not leaving receivers on standby (apart from saving the planet), although modern receivers have small transformers to power the remote control circuity that only take about 2 Watts when the power is on the degaussing device is being heated with consequent reduction in its life. 
 

lyner

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the effect of magnets on a tv screen.
« Reply #16 on: 28/10/2007 18:44:12 »
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when the power is on the degaussing device is being heated with consequent reduction in its life. 
Are you saying that the degauss coil has current through it all the time?
Why?
 

Offline syhprum

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the effect of magnets on a tv screen.
« Reply #17 on: 29/10/2007 06:12:09 »
No     although they are permantly connected to the the power via thermister device when the power switch is on.
When the power is first applied it is cold and allows a surge of current into the coils degaussing the surroundings when it is hot (which it is whenever the power is on the current drops to zero).
If the set is permanantly on stanby relying on the remote control to shut the main circuraty down the thermister stays hot all the time and you get no degaussing current surge when you bring the set to life vier the remote control.
This has two bad effects, if the magnetic enviroment is changed the set does not get degaussed next time you bring it to life, also the life of the thermister device is shortened.
 

lyner

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the effect of magnets on a tv screen.
« Reply #18 on: 30/10/2007 16:19:18 »
That's a naff system. Why can't the degaussing coil be controlled by the remote? There is no real limit to the current that a thyristor can control - take your washing machine as an example - the whole lot goes through the thyristor on the input.
 

Offline syhprum

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the effect of magnets on a tv screen.
« Reply #19 on: 30/10/2007 17:51:21 »
The terminal on which I am reading this does in fact have all its power switched by the computer to which it is connected but not your usual TV.
This is my own modification using an 25 amp opto triac from one of the old washing machine size 256 Mb disk drives
« Last Edit: 30/10/2007 18:50:21 by syhprum »
 

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the effect of magnets on a tv screen.
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