Friday, January 6, 2012

Degaussing a TV With Everyday Items

Recently I had to fix an old CRT (cathode ray tube) television that showed significant discolouration on the screen and I thought that I might share some of the techniques I used.  Admittedly degaussing a TV is something that you will almost never have to do (thanks to flat screen displays), but it's still fun to know what causes the problem and how to fix it.

First, a few basic facts about how a CRT works.  The diagram below show the basic components of an RGB CRT.  The following components are identified.

1. Electron guns
2. Electron beams
3. Focussing coils
4. Deflection coils
5. Anode connection
6. Shadow mask for separating the red, blue, and green beams
7. Phosphor layer with red, blue, and green zones
8. Close-up of the phosphor coated inner side of the screen

Cathode Ray Tube

It looks more complicated that it really is.  Electrons are emitted from the gun and accelerated toward the screen by accelerator anodes (not shown) to form a beam.  The beam is then focussed with the focussing coils.  To get the beam to hit the part of the screen you want, it needs to be steered by the x and y deflection coils.  Now comes the part that we are interested in, the shadow mask.  The shadow mask is used to make sure the red, green, and blue beams hit the right phosphors.  As seen in the diagram, the RGB beams come from slightly different locations and converge at a point due to the focussing coils. This is seen at label 8.  After converging, the beams will then start to diverge again.  By placing a mask with holes at the focus point of the beam, and placing the phosphor dots on the screen at the right location, it ensures that the beams will hit their corresponding phosphors.  If the beam is not in the right position it will hit the screen and not the phosphor, but if it does pass though, the RGB beams will diverge to hit the right phosphor dots.  This is the part of the screen that causes colour problems.

The Problem

If the shadow mask is misaligned or deformed by an external shock, the beams hit the wrong dots and cause discolouration. It is similar to swapping the RGB components of an image around.  This isn't that common and it's something that we can't fix.

There is another kind of distortion that can occur at the shadow mask, and this one we can do something about.  If for whatever reason the shadow mask becomes magnetised the same distortion can occur.  Electrons moving through a magnetic field are deflected, and if this deflection happens after the electron has passed though the hole in the shadow mask it will deviate from its intended path and hit the wrong colour phosphor.  There are many ways for the mask to become magnetised, but nearly always it's caused by placing a large speaker near the screen.  The magnet in the speaker permanently magnetises the screen, so even if the speaker is removed the distortion remains.

Fixing It

There are three ways to control this problem.  Later model sets have and automatic degaussing circuit built in. Basically a coil placed around the edge of the tube has a current passed through it. This is a decaying alternating current that produces a corresponding magnetic field that demagnetises the mask, leaving the magnetic domains randomised and cancelling each other out.

Screens without built-in circuits can be degaussed with an external wand that does the same job.  If you are going to use one of these, buy one.  I have seen plans for these on the net that are basically death traps, and will either electrocute somebody or burn their house down.

The third method of degaussing a screen is the one I am going to describe.  It's tedious and can be frustrating.  That's right, doing by hand.  You can use permanent magnets from items around the house to get the same effect.  I used and old sub-woofer, the magnetic roller from an old fax machine, and a fridge magnet.  By moving them over the screen with the TV on, you can cancel out the field and see how you are effecting the distortion.  It's a bit of trial and error, if it get worse, move the magnet the other way or turn it over.  You can use a large speaker magnet that works from about three feet away to do large areas and the other smaller magnets to fix tiny spots.  It may take some time and you may look like an idiot waving magnets around a TV like a shaman performing some ritual, but you can do it.  Just remember that if the magnets you use are too powerful you can damage the mask permanently, so ease into it with the smaller magnets.  Another tip is to only do this from the front of the screen, if you go near the back with the magnets you may damage other parts of the system.




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