Hi all,

I have a question that's been bugging me for a while now. With cathode ray tube televisions, the earths magnetic field is a major impactor on the set, more specifically the tube. This is noticeable with some sets, when you change the direction the set faces, the colours change and sometimes distort. For this reason, it is recommended that once the facing direction of a set has been changed, that you wait 15 minuted before powering it on again.

My question is, how do they fix this issue for the televisions used in buses, trains and planes? These are all obviously on the move, so the magnetic fields, direction and intensity on the tubes would vary over the journey. How to they counteract this? Do they use a device/component in the set to create an artificial magnetic field, thus allowing for the use of the set whilst in a moving vehicle?

Also, with these sets, wouldn't the constant bumping and vibration (more for the sets in buses) gradually shift/distort the shadow mask?

Thanks,

Chris

The magnetism issue is well recognised, the Germans used it to advantage, as they had realised that ships gradually became magnetised and the "magnetic mine" was developed to exploit it.

Various devices were made to degauss both the ships & the TV's. The TV one was sold as a "degausing wand".Gradually the CRT's were fited with a permanent degaussing coil, that activated breifly when the set was turned on.

It is likely that the set that you have, that has experienced this known magnetic issue, has sat in the one position for a considerable time and has no, or a non function degausser coil. It will wrap around the front sides of the CRT.

The bus ones may be of a type that are well constructed? A these are constantly moving the earths magnetic field would likely not have time to magnetise them in a specific direction and degaussing coils would play their part.

You can tell if a CRT tube needs degaussing by blotchy patcheds on the screen.

Marc

I was watching a documentary on how the planet was believed to have formed about a year ago and it made mention of Earth's magnetic field. It was said that this changes polarity roughly every 300,000 years. Of course, none of us have been alive long enough to verify this though CRTs probably won't be in use by the time it's due to happen again.

It's already proven that the core of the magnetic field is drifting away from the planet's axis and not only directly away from the axis but it also turns to run in the same direction as the rim of the Arctic Circle. It may even be that the magnetic pole that we call "North" could well really be "South" but there's just no way of telling.

As for the field's effect on CRTs, I'd say that out of all the modes of transport listed, aeroplanes would provide the most severe environment as they are moving quicker and can bank left or right as well as going up and down. My bet is that the screens would have been heavily shielded as the radiation they give off could have been a problem for the avionics as mobile phones are believed to be these days. This shielding wouldn't stop the 'colours changing' when the front of the screen is zapped by magnetic radiation though, as Marc mentioned, the degaussing coils, if fitted, would correct this each time the monitor is turned on.

I'm aware of the degaussing coil (my Sony Trinitron has the loudest coil I've ever heard), but would the magnetic field provided upon power up be sufficient to keep the set working properly over the journey? I would think that the constant changes in magnetic field etc. would overpower the magnetic field first provided by the degaussing coil..?

Cheers,

Chris

If it was strong enough it'd overpower (or to be more accurate, take over from) it because the coils are only activated for a few seconds. Then it's a free-for-all. To be honest though I am not sure there'd be a problem. Even though the Earth's magnetic field is very large it is one of the weakest forces we know of in a practical sense. Even gravity is stronger and it is also considered a weak force.

From my experience with avionics (using them) I would expect the monitors in the passenger area to be "cable" types. Rather the same as your computer monitor.

There, in this situation would be no need for RF. just a decoder & optic fibre.

Marc

If you want to see what magnetic fields can do, work in an Aluminium Smelter with 1000v DC at 200,000amps going through the bus bars. Even in the offices 100m away we had to rotate the yokes on the old crt monitors to straighten up the screens. In the production line, you could stand a screw driver vertically in the palm of your hand. As you walked past each individual melting pot it had a north and south pole and the screw driver would sway left and then right, really weird.
As I walked under the bus bars, to do current monitoring calibrations, I would feel ill and I asked to be moved to another section of the plant away from the magnetic fields.

While CRT's are affected by the earth's magnetic field it is only a weak effect.

The effect is more noticeable the larger the screen size, so smaller CRT's are not going to suffer as much.

Colour CRT's also contain magnetic screening (external on very early types and internal on newer types) which helps to reduce the effect of weak magnetic fields.

The problem is not as big as you may be thinking as portable colour CRT TV's would not be be very practical if they could not retain reasonable colour purity when being lugged around from place to place!

I wouldn't imagine that you would have a portable television running while your on the move though. If you were to watch one on the move, I'd imagine it would be a smaller 12VDC 5" B&W set, which doesn't have the same issues as a colour set.

Chris

CRT's in adverse magnetic environments can be magnetically shielded, eg enclosed in a steel cabinet. They can also have a manual degaussing switch. Computer monitors have a manual trigger for their automatic degaussing.
On a bus YOU are also subject to the 'constant bumping and vibration', and I am sure picture quality is the last thing on your mind.
On some brands of sets manufactured in the Northern Hemisphere, the CRT was was rotated 180 degrees, or put in upside down, so the factory convergence did not have to be reset.
From memory the shadow mask is joined to a heavy frame. I don't think 'bumping and vibration' could distort it. Maybe if it it were dropped face first and came to a sudden stop? But then the electron guns would stretch forward wouldn't they? I think heating or localized heating would be the main cause of shadow mask distortion.

My 2c