I'm guessing Tellies will be the one with an answer to this!

My son loves playing with magnets and today was mucking about and put them near the V screen and the colours all went a bit crazy ... it's just a regular older type TV with a tube, I think .. it was quite a good effect and we played around with it for a while.

But then I got to thinking .. what is happening when we do it? And can it damage the TV???

There's a perforated steel plate in the CRT just behind the glass called a 'shadow mask'. It covers the entire area of the screen. It's function is to make sure that the red, green and blue electron beams (coming from the electron guns in the neck of the CRT) only hit the correct phosphors. For example a beam from the red gun will only hit red phosphors. The mask is very precisely placed in relation to the guns and screen in order to achieve this. A good illustration here.

It's important that the mask is not magnetised, even in a minor way. If it is, an electron beam intended for red phosphors (from the red gun) might be diverted hit the wrong coloured phosphors. A strong external magnet held near the CRT will certainly magnetise the shadow mask and make this happen. Your son and his magnet will do it for instance .

The danger is that this could lead to mis-registration of colours even after the magnet has been taken away. So getting magnets near the screen should be discouraged really. Sorry, I know it's good fun but that's life .

There are some coils external to the CRT that demagnetise the shadow mask every time it is switched on from cold at the on/off switch (stand-by won't do). It has to be from cold because the coils are driven by a part called a 'thermistor'. This starts off cold and then warms up quickly when the power is switched on. It's during this short cycle that demagnetising (or degaussing) takes place. It won't happen again until the thermistor has cooled right down. This takes about 10 minutes.

About the colours staying wrong after the magnet has been taken away: Colours should return to normal after a degaussing cycle has happened. Sometimes though, the degaussing components may not be able to entirely demagnetise the shadow mask. Then the colours will not be correct (electron beams are diverted and hit the wrong colour phosphors). For instance, an overall white picture may have purple or green tinges in some areas of it. Two or three degaussing cycles will often clear this problem. This can be a lengthy business because of the time it takes for the thermistor to cool down.

Sometimes the thermistor goes wrong and won't do its job (not because of the magnet though). You have to call the repairer when this happens. A simple repair but most people don't get any fault mended these days.

I was wondering, is this the same for computer screens, because some people I know put magnets on the computer screen because of the pretty colours, then degauss it after. Is it still damaging even if the degaussing gets rid of the pretty colours?

It applies to anything with a colour CRT in it. It's not damaging in a way that it can physically wreck a CRT. The danger is that degaussing might not restore things to normal. If the degaussing circuit is working well, then it will probably be OK. If the magnet was a really strong one, the shadow mask my remain magnetised even after degaussing.

Whether degaussing cures the problem or not, we bunch of merry TV menders (such as I used to be part of) considered it bad practice to get magnets near a colour CRT. Hi-fi speakers and even the Earth's magnetic field can have an effect on them.

My spouse built a console color television some years back, tubes and all. Well, he didn't build the picture tube, but put it in the machine and all that stuff. I can remember his every so often "degaussing" the thing.

I dont even know how to degauss a TV. The only way I know how to degauss a computer is that there is a little menu at school when I press a little button. It seems to work when my friends start playing with old hard drive magnets though.

Some monitors have a button I believe. My CRT monitor didn't though. It degaussed when switched on and that was all.

We used to have a 'degaussing stick' that could be used externally in people's houses. It wasn't as good as the degaussing coil we had for workshop use though. This had the diameter of a car wheel.

I always used my soldering gun. This worked pretty well and saved carrying the extra item of a degaussing stick.

How do you use a soldering gun to degauss a TV?

A soldering gun has transformer in it that produces a fairly strong alternating magnetic field. Just what's needed for degaussing.

Ahhhh. So they are useful for more than just sticking stuff together. DO you just hold it up to the spot that has the pretty colours?

No. You have to move it around all over the place just in front of the glass. Then slowly back away so that the field is gradually removed from the shadow mask. Add a toss of the head and the whole action can look quite graceful really ..unless you burn yourself on the soldering tip. That produces a whole different kind of movement.