Degaussing and Demagnetising

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Picture tubes

Perfection comes at a price

Degaussing & demagnetising

Cathode ray tubes (CRTs) - the glass tube that you look at when watching TV - have magnetic properties which must be properly aligned with Earth's magnetic fields. If a TV is moved while turned on, colour spots may appear and adversely affect the colour and picture (as shown on the picture).

The larger the CRT, the more sensitive it is. Some 77cm and larger television tubes have a N-S/E-W alignment switch which has to be used depending on where the TV set is used - either in the earth's northern hemisphere or the southern hemisphere... A common mistake is place speakers next to the TV, or worse still, on top of it. Speakers have magnets which will affect the purity of the TV picture. However, speakers such as BeoLab 8000 and BeoLab 6000 are 'magnetically shielded' which means that they are designed in such a way as not to affect the CRT. The speakers - when used in a home theatre set-up, for example - can perfectly happily sit at the side of a TV without adversely affecting the picture.

Colour TVs have an automatic degaussing (demagnetising) circuit built-in. But it produces a weak field and may take a couple of days for it to go away. A simple method of ridding the screen from these unwanted blemishes is to isolate the TV from the mains for 30 minutes or so and then plugging the set back in and switching it on. This demagnetising function often causes a brief buzzing sound. Alternatively, place a compass on top of the cabinet, its needle will twitch violently when the degauss circuit is doing its stuff. Never turn the set round while it's running; the Earth's magnetic field will upset beam-landing and purity.

If this does not work, it may be necessary to contact a service company to manually degauss your TV. In some cases a purity alignment may be needed, especially if the TV has been subjected to a fall or has been handled roughly. This is when all three colour guns of a picture tube are aligned to the 3 primary colours: red, green and blue. Using a special video generator, a service technician can select and display a picture with just one gun on.

Better brightness comes from a shadow mask

If the degauss action takes place but there's still a colour-stain, it's possible that some local magnetic field is present. Move any speakers at least 2m from the set and degauss once more. If you live in a block of flats or other steel-framed building it may be that the structure itself is magnetic! Test for this by putting the set in the middle of the room, degaussing and trying once more. It may be possible (talk to the dealer) to get the steel joists demagnetised.

In rare cases, an internal component of the CRT called the 'shadow mask' can detach and cause this problem. When this occurs, on most occasions the TV set has been subjected to a fall or heavy blow to the cabinet.

Tube strikes

As screens get bigger and flatter, especially those with 16:9 faceplates, the maintenance of colour purity and good scanning geometry makes tremendous demands on tube design technology and manufacturing tolerances. These are now stretched to the limit in both triad (conventional, three gun tubes) and Trinitron types. Most people reasonably expect perfection in a TV for which they have paid over 1,400 euros. And as a general rule, the more you pay the closer you get to perfection.

Showing your true colours

Purity, in picture-tube terms, is the state where the electron beam for each individual colour strikes only its 'own' colour phosphors on the faceplate. Poor purity stains one colour with the phosphor light of another, and can be caused by poor manufacturing tolerances at the factory; incorrect setup and alignment of the tube's deflection yoke or neck-mounted magnetic 'beam-steering' rings; external magnetic fields, even those generated by the planet itself; and distortion of the metal shadow mask or aperture grille just behind the viewing screen.

The latter cause, 'mask-doming', usually arises from local overheating and expansion, and manifests as a soft-edged colour-stained patch anywhere on the screen where a picture highlight has been stationary for a while. Turning down the brightness and/or contrast settings generally cures it. If colour staining is always present in the same screen area, it's down to one of the other causes listed above.

If impurity remains after all of the above has been tried, a technician will be required. He will check the settings of the purity rings and the positioning of the deflection yoke, and possibly fix correction magnets on the bowl of the picture tube. If none of these measures work, he may reject the tube as being faulty or out of tolerance. Sometimes external degaussing cures the problem, and the set's auto-degauss circuit has failed due to a duff positor, relay or connection/contact.

Geometrically correct

Many TV viewers get hot under the collar about picture geometry, primarily the straightness - or rather lack of it - of lines at the edges of the image. This can be checked on a Teletext display. Geometry is governed by the accuracy of the magnetic scanning field generated within the tube neck by the scanning yoke and can be adjusted, to a greater or lesser degree - depending on what the manufacturer provides - by an engineer. The settings are stored in software in an EEPROM chip, and are initially set to optimum by an optical/computer system at the factory. Thereafter the data modulates the scan-coil currents at line and field rate to pull the picture into reasonable shape.

The scan geometry can rarely be made perfect. There are always 'tolerances' in the manufacture of the scan yoke, and almost everything in life is a compromise. So it comes down to what's acceptable, and here things get a bit woolly! It's very hard to get a figure from any manufacturer, though engineers work on the assumption that a 6mm deviation from a dead straight in a line near the border of a 32in widescreen acceptable.

Beyond the fringe

Another aspect of a colour TV tube's performance is colour registration, sometimes called 'convergence'. Bad registration shows up as colour fringing on the outlines of picture features near the corners and edges of the display, best judged in the absence of a pattern generator by looking at a Teletext display. Again it's difficult to quote a figure, but an error of 2-3mm is acceptable on a big screen, depending again on price, quality and personal taste. There should be virtually no visible colour fringing near screen centre. Good colour registration in outer screen areas depends, once again, on the deflection yoke.

Poor registration may be improved by a good engineer by means of panning and tilting the front end of the deflection yoke and/or by resetting software in the memory chip, the latter particularly on Trinitron models. As with purity, it's also sometimes possible to make corrections by fixing weak disc or strip magnets to the tube bowl. With all settings and adjustments at optimum, what you get is down to the design and build quality of the scan yoke.

Focussing on colour

Image focus (assuming that the internal focus control is correctly adjusted) is a function of electron-gun design and the sharper the edges and corners of the picture the better the manufacturers have done. Incorrect colour rendering is probably due to the need for colour-drive adjustment rather than any shortcoming of the picture tube, unless it's old and becoming worn. Even then modern auto-greyscale-tracking technology keeps the colour correct until the electron guns are well down. A good way to check colour-tracking performance is to turn the colour right down and look for tinting in the resulting black-and-white image, in both dark and bright areas.

Leaking colour on a black-and-white image indicates poor colour focus.

Some TV designs offer a black- or dark-stretch feature, in which the gain of the RGB (Red-Green-Blue) amplifiers is made dependent on signal level. This varies the picture's 'gamma' law, and allegedly brings out detail in dark areas of the image. This is no less than deliberate distortion, and is best left switched off!

Plasma and LCD displays

The newer LCD screens are immune to purity and geometry errors and the same applies to plasma panels, because in both these cases the RGB-triad pixels and column/row formations are printed on the rear surface of the faceplate: there is no 'scanning' function here. LCD projection sets can only develop colour-registration errors from (rare) optical or mechanical faults, though colour blotching can arise from dust and dirt deposits on the liquid-crystal panels and the optical surfaces; a thorough internal clean by a technician solves this.

Tube-type TV projectors (rear-projection sets) don't suffer from purity errors and screen-centre registration is easily adjusted automatically or by the user. If this is correct but colour-fringing is present near the picture borders, you require professional help. Numerous internal software presets help to trim beam deflection in the individual tubes and thus precisely overlay the three colours.

Created: 11th January 2007
Modified: 3rd April 2007

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