The 20AX System

THE “20AX” SYSTEM

Thanks to Stan (valveman49) for the materials for this section.

Principles

It is fundamental to all commercially-available colour television tubes that the picture displayed on the screen is made up of three primary colours, red, green, and blue, generated when electrons originating from a three-gun assembly in the neck of the tube strike the phosphor-coated viewing screen.

In the past, the three guns have been arranged in triangular or “delta” formation and the beams have been directed on to the viewing screen by an electron lens system of high accuracy. The image seen by the viewer is the result of light emitted when the electrons strike coloured phosphors deposited on the back of the screen. Traditionally, the phosphor pattern consists of around a million accurately-located red, blue, and green dots each of which can be illuminated as required by the modulated beam from the appropriate gun.

Since the resultant colour image must be seen as a complete picture rather than a single spot of light, it is necessary to “scan” the area of the viewing screen, both horizontally and vertically, whilst the required colours are modulated in the necessary sequence. To achieve this, it is obvious that all three beams must scan together with great accuracy to avoid imperfections in the final registration of the beams with their respective phosphors.

In practice, it is impossible to achieve the required accuracy without some form of additional correction to obtain the necessary convergence. This is because the three beams cannot emanate from exactly the same source, and the bending, or “deflection” of the beams, during the scanning process means that one or other of the three is inevitably displaced on arrival at the screen.

Traditionally, correction is achieved by means of an electrical correction system which is designed to influence the three beams after they leave the guns by means of additional components mounted on the neck of the tube. These components receive a series of complex signals which aim to correct the inherent misalignment of the three beams and ensure accuracy of “convergence” at any point in the picture.

Special electrical circuits are needed to achieve this, together with a complex adjustment process of typically 14 controls which must be adjusted by the manufacturer of the receiver; it is also important that the accuracy of adjustment is maintained during the life of the receiver.

It has long been recognised that the process of achieving correct convergence without recourse to these cumbersome methods is a goal that must be achieved, and a number of attempts to realise an alternative tube system have been made from time to time in various parts of the world. Some simplified systems have already been commercially exploited but, in the main, these have been limited either to smaller screen sizes or to the more bulky 90° tube systems where the problems are less acute.The “20AX” system is the first in the world to achieve a self-converging system for large screen sizes — up to and including 26in — in the 110° format. (The 110° tube system achieves a more compact picture tube by deflecting the beam through a wider angle, allowing a usefully shorter neck and considerably reduced cone bulk.)

The realisation of such a system requires a number of important changes to the previously-accepted picture tube and its associated deflection coil, each of which is associated with more closely controlled manufacturing processes than has been possible in the past. For example, the delta gun format is now replaced by a three-in-line array to remove one source of misalignment of the beams.

AX20 Section
AX20 Section

Cutaway View of 20AX Picture, Tube Deflection

This change is matched by a conversion of the colour screen from a pattern of small phosphor dots to one where each coloured phosphor is applied as a strip running from top to bottom of the tube face, each strip being precisely located with respect to its neighbouring colour. A corresponding change in the special “shadowmask” is also necessary — this being an internal metal structure (now slotted) which assists in guiding each colour beam on to its respective colour phosphor.

Of equal importance are changes to a major component associated with the colour tubes, the deflection coil. in the “20AX” system, this unit now achieves not only its own primary function of deflecting the three beams over the entire tube face but also applies the necessary individual corrective influence to each beam to ensure that accurate location on the colour screen is achieved without the necessity for the normal con­vergence components, adjustments, or the complex associated circuitry. This is achieved by realising a complex electromagnetic field distribution within the coil known as a “parastigmatic” field, and is the result of an extended development programme which originated in 1954.The main characteristics of a deflection field that determines its convergence properties are curvature of the image field, astigmatism and coma. The curvature of the image field causes misconvergence of the three beams which now form an equi­lateral triangle. The additional effect of astigmatism causes these image spots to change to isosceles triangles. The principal effect of coma in a delta-gun tube is to displace the blue beam relative to the red and green beams towards the direction of deflection. In a system with horizontal-in-line guns, coma caused by the horizontal and vertical deflection displaces the centrally positioned green beam relative to the red and blue beams against the direction of deflection.

The G11 20AX Tube
The G11 20AX Tube

In “20AX”, errors resulting from coma have been cancelled by controlling the degree of barrel and pincushion distortions. The curvature of field is compensated for by increasing the astigmatism of the deflection field in such a way as to eliminate misconver­gence in the horizontal direction at the expense of increasing it in the vertical direction. However, the latter is of no consequence, since the horizontal-in-line gun array eliminates mis-convergence in a vertical direction, resulting in good convergence at all positions on the screen.

The tube has the standard 36.5mm diameter neck and a “quick vision” heater as used previously, but because of modifica­tions to the gun made possible by the elimination of the pole pieces required for dynamic convergence corrections, the neck length is reduced by 20mm (1″) compared with its 110° delta-gun equivalent.

The only dynamic corrections required in “20AX” are those to compensate for small residual manufacturing tolerances. These adjustment compensations are few (7), and relatively straight­forward and expend very little energy compared with a com­parable delta-gun system.

In comparison with a delta-gun system, the deflection fields required in the self-converging “20AX” in-line system give greater pincushion E-W raster distortion but less N-S distortion. The N-S raster distortion is inherently barrel shaped but is decreased by shaping the deflection coil ferrite ring. Shaped cut-outs at the gun-end of the ring affect only the vertical deflection field, giving a N-S raster shape which is acceptable without further correction. The non-linear distribution of the field requires the application of modulated “S” correction. Both this and the E-W pincushion distortion is corrected by means of the E-W diode modulator circuitry.

G11 20AX Deflection Coil Assy
G11 20AX Deflection Coil Assy

20AX Deflection assembly

“20AX” Picture Tube

Externally there is little to distinguish a “20AX” picture tube from a comparable delta-gun 110° type apart from the slightly shorter neck and the deflection yoke centring-ridge on the cone. Internally, however, there are fundamental differences.

As with some of the earlier 110′ tube designs the magnetic shield is fitted inside the glass cone. Advantage has been taken of the unlimited vertical landing reserve inherent in a vertical stripe picture tube system by rotating the degaussing coils through 90: compared to previous designs. By this means, the vertical component of residual fields that may cause horizontal landing errors is completely eliminated. Because the mask material is not interrupted by holes in the direction of the de­gaussing field, the required magnetomotive force can be smaller thus allowing an appreciable reduction in the ampere-turns of the degaussing coils.

Electron Guns
The electron guns are mounted side by side, the two outer guns (red and blue) being slightly inclined towards the centre gun (green). The green beam is positioned between the other two in order to minimise the visual effect of small residual convergence errors (the human eye is more sensitive to con­vergence errors between red and green, or blue and green, than between red and blue). The cathode of each gun is of the “Quick-vision” type with low thermal capacity and improved heater-to-cathode heat transfer. It is thus possible to obtain a picture within five seconds from switch-on.
G11 20AX static correction unit
G11 20AX static correction unit

Static correction unit

Static Correction, Purity and N-S Symmetry. The object of static correction (convergence) is to cause the landing positions of the two outside electron beams (red and blue) to coincide with the central (green) beam in the centre of the picture tube screen. This requires a displacement in the X and Y directions for each of the outside beams. The static correction unit is of the permanent magnet type and is combined with the purity adjuster and a N-S raster symmetry adjuster. The com­bination consists of four pairs of magnetised plasto-ferrite rings, each pair coupled by pinion gears, and is fitted on the tube neck behind the deflection coils.

Two pairs of rings are used for static correction adjustments. With each pair, the direction and the strength of the magnetic field can be adjusted. By turning each pair of rings in opposite directions the strength of the field is influenced, while turning the two rings together, results in a change of the direction of the field. One pair of magnets forms a four-pole field, which moves the outside beams (red and blue) equally in opposite directions. The other pair forms a six-pole field, which moves the outer beams equally in the same direction. When adjusting the four-pole or six-pole rings, the centre beam is unaffected, and it is thus possible to bring all three beam spots into coincidence.

Four Pole magnets (a), move the two outer beams equally in opposite directions. The six pole magnets (b), move them equally in the same direction

One pair of rings magnetised as a vertical two-pole magnet has only the strength and polarity adjustable, not the direction, and thus provides the purity adjustment which requires a horizon­tal displacement only. All three beams are similarly affected. The vertically slotted shadowmask makes colour purity independent of beam landing in a vertical direction.

The remaining pair of rings (N-S Symmetry), magnetised as a horizontal two-pole magnet, again adjustable in strength and polarity only, corrects any vertical misalignment between the beams and the axis of the tube-yoke system. Such misalignment would otherwise cause curvature of the horizontal axis of the raster.

Two pole field of colour purity magnets shift all three beams in a horizontal direction

Dynamic Correction. Convergence errors in the outer areas of the display are corrected by means of four additional electro­magnetic four-pole fields. Two of these are produced by means of four small toroidal windings on the deflection coils ferroxcube ring and located on the horizontal and vertical axes of deflection. The series-connected four-pole windings are driven by field and line frequency sawtooth or parabola currents derived from the deflection currents. The other two four-pole fields are produced by means of balancing sawtooth or parabola currents through each half of the field and line deflection coils.

To define terminology all corrections in the X-direction are called “symmetry” while those in the y-direction are called “balance”.

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