MXPA99005755A - A deflection yoke with geometry distortion correction - Google Patents

Abstract

A video display deflection apparatus includes a vertical deflection coil. A separator is used for mounting the vertical deflection coil thereon. The separator has a funnel shaped first part conforming to a shape of a neck of the cathode ray tube and a second part forming a front portion of the separator close to the screen. A degree of flare of the first and second parts is substantially different. A saddle shaped, horizontal deflection coil (3) is mounted on the separator (2) for producing a deflection field to scan the electron beam along a horizontal axis of the display screen (9). The horizontal deflection coil includes a plurality of winding turns forming a pair of side portions, a rear end turn portion (19), close to an electron gun of the tube and a front end turn portion (29), close to the screen (9). At least a portion of the front end turn portion, in a radial angular position ranging between 0 and 30 degrees, is supported on the second part of said separator away from the boundary. Consequently, an effective length of the horizontal deflection coil is extended in a direction of the screen to provide north-south raster distortion correction. A magnetically permeable core (5) cooperates with the deflection coils to form a deflection yoke free of permanent magnets upfront.

Description

A YOGNE OF DEVIATION WITH CORRECTION OF DISTORTION OF GEOMETRY The invention relates to a deflection yoke for a color cathode ray tube (CRT) of a video display apparatus. In particular, the invention relates to a deflection yoke having a pair of horizontal deflection coils in the form of a depression, for correcting the North-South geometry distortion of the image formed on the screen of the cathode ray tube.

BACKGROUND A cathode ray tube for generating color images usually contains an electron gun that emits three coplanar electron beams (electron beams R, G, and B), to excite a color or phosphorus material on a screen. primary given red, green, and blue, respectively. The deflection yoke is mounted on the neck of the tube to produce deflection fields created by the horizontal and vertical deflection coils. A ring or core of ferromagnetic material surrounds, in a conventional manner, the deflection coils. It is required that the three generated beams converge on the screen to avoid a landing error of the beam called convergence error that otherwise would produce an error when transmitting the colors. In order to provide convergence, it is known to use astigmatic deflection fields termed aconvergent. In a self-converging deflection coil, the non-uniformity of the field that is illustrated by the flow lines generated by the horizontal deflection coil, generally has a pincushion shape in a portion of the coil located in the front part, closer to the screen. A coma error occurs due to the beams R and B, which penetrate into the deviation zone at a small angle in relation to the longitudinal axis of the tube, undergo a complementary deviation with respect to that of the central beam G. With respect to the horizontal deviation field, the comma is usually corrected by producing a barrel-shaped horizontal deflection field in the region or area of entry of the deviation yoke beam, behind the aforementioned pincushion field that is used for the error correction of convergence. A combo parabola distortion is manifested in a vertical line on the image side, by a gradual horizontal directional change of the green image relative to the midpoint between the red and blue images, as the line is followed from the center to the corner of the screen. If the change is made to the exterior or to the side of the image, this coma parabola error is conventionally referred to as positive; if it is done towards the inside or towards the center of the image, the coma parabola error is referred to as negative. A distortion of geometry referred to as pincushion distortion occurs in part due to the non-spherical shape of the screen surface. The distortion of the image, referred to as North-South at the top and bottom, and East-West at the sides of the image, is stronger as the radius of curvature of the screen is greater. When the screen has a relatively large radius of curvature greater than IR, such as 1.5R or greater, for example, it becomes increasingly difficult to resolve beam landing errors, such as geometry distortion, without using magnetic auxiliaries. , such as shunts or permanent magnets. For example, in the deflection yoke of the prior art of Figure 2, permanent magnets are placed in front of the deflection yoke to reduce distortions of North-South geometry. It is a common practice to divide the field of deviation into three successive action zones along the longitudinal axis of the tube: the rear or rear zone, closest to the electron gun, the intermediate zone, and the frontal zone closest to the screen. The comma error is corrected by controlling the field in the back zone. The geometry error is corrected by controlling the field in the frontal zone.

The convergence error is corrected in the posterior and intermediate zones, and is less affected in the frontal zone. It may be desirable to reduce the distortion of North-South geometry by controlling the coil distributions of the deflection coils without using magnetic auxiliaries, such as shunts or permanent magnets. The elimination of shunts or permanent magnets is desirable because, in an inconvenient manner, these additional components can produce a heating problem in the yoke, related to the higher horizontal frequency, particularly when the horizontal frequency is 32 kHz or 64 kHz and higher. These additional components also, undesirably, may increase the variations between the yokes produced in a way that the error corrections such as geometry error, comma error, coma parabola, or convergence error are degraded. In the deflection yoke of the prior art of Figure 2, a separator is composed of a main part 161 that conforms to the shape of the tube on which the deflection yoke is mounted for a substantial length of the separator. However, a front end 160 of the separator deviates away in a plane perpendicular to the Z axis from the funnel-shaped contour of the tube. An internal surface of the front end 160 is used to support the front end turn of the horizontal deflection coil. The circular shape of the inner boundary 162 of the front end 160 forms a boundary between the part 160 that is perpendicular to the axis Z, and the part 161 having the enlarged shape conforming to the conical shape of the tube funnel. The wall surface of the enlarged front end 160 of the separator is flat and perpendicular to the main axis Z. During the reeling process of the coil, retractable bolts perpendicular to the XY plane are inserted to form corners in the coil. In the yoke of the prior art of Figure 2, the bolts are placed substantially in the limiting circle 162 of the front end 160 of the spacer, between the parts 161 and 161. It may be desirable to use the front end 160 to increase the effective length of the spacer. the coil The increase in the effective length of the coil facilitates the change of the center of deflection of the horizontal deflection coil with respect to that of the vertical deflection coil. In accordance with an inventive feature, the corners of the coil produced by the bolts are placed remote from the limiting circle. Thus, conveniently, a substantial portion of the coil extends at the front end 160. The result is that the effective length of the coil is increased in a manner that reduces the distortion of North-South geometry.

COMPENDIUM A video display deviation apparatus, incorporating a feature of the invention, includes first and second deviation coils. A spacer is used to mount the first and second deflection coils thereon. The separator has a first part formed in the form of a funnel that conforms to a shape of a neck of the cathode ray tube, and a second part that forms a front portion of the spacer near the screen. A degree of widening of the first and second parts is substantially different. The second deflection coil includes a plurality of coil turns that form a pair of side portions, a rear end turn portion, close to an electron gun of the tube, and a front end turn portion, close to the screen. At least a portion of the front end turn portion, at a radial angular position of between 0 and 30 degrees, is supported on the second part of the separator away from the boundary in a manner extending an effective length of the second coil of deviation, in one direction of the screen to provide grid distortion correction.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a deflection yoke, according to a configuration of the invention, mounted on a cathode ray tube. Figure 2 illustrates a front view separated into parts of a deflection yoke according to the prior art. Figure 3 shows a cross section of a depression coil according to a configuration of the invention, formed in the intermediate zone of the coil. Figures 4a, 4b, and 4c represent a side view, a top view, and a front view, respectively, of a coil according to a configuration of the invention. Figures 5a and 5b show the position of the winding bolts in front of the depression coil of Figures 4a, 4b, and 4c, in relation to the separator. Figure 6 shows the variation, along the main axis Z of the tube, of the distribution function coefficients of the horizontal deflection field generated by a coil according to a configuration of the invention, and the influence of the coil extension in the front end turn of the XY plane.

DESCRIPTION OF THE PREFERRED MODALITIES As illustrated in Figure 1, a self-converging visual display device includes a cathode ray tube (CRT) having an evacuated glass envelope 6, and a configuration of phosphor or luminescent elements. , which represent the three primary colors R, G, and B, configured in one of the ends of the envelope, forming a visual display screen 9. Electron guns 7 are configured in a second end of the envelope. The set of electron guns 7 is configured to produce three electron beams 12 aligned horizontally, in order to excite the corresponding luminescent color elements. The electron beams sweep the surface of the screen by operating the deflection yoke 1 mounted on a neck 8 of the tube. Deviation yoke 1 includes a pair of horizontal deflection coils 3, a pair of vertical deflection coils 4, isolated from each other by a separator 2, and a core of ferromagnetic material 5 provided to improve the field of beam paths . Figures 4a, 4b, and 4c illustrate, respectively, the side, top, and front views of one of the pair of horizontal coils 3, having a depression shape in accordance with an aspect of the invention. Each loop of the coil is formed by a loop of a conductive wire. Each of the pair of horizontal deflection coils 3 has a rear end turn portion 19 of Figures 4a and 4b, close to the electron gun 7, and extending along the longitudinal axis or Z. A back portion of front end 29, disposed near the visual display screen 9, is curved away from the axis Z in a direction generally transverse to the axis Z. Each of the core 5 and the separator 2, conveniently, can be manufactured in the form of a single piece, instead of assembling from two separate pieces. The lead wires of the front end turn portion 29 of the depression coil 3 of Figures 4a-4c, are connected with the rear end turn portion 19 by bundles of side wires 120, 120 ', which form a section of lateral winding, along the Z axis, on one side of the X axis, and by lateral wire bundles 121, 121 ', on the other side of the X axis. The portions of side wire bundles 120, 120' and 121 121 ', located near the beam exit region of the deviation magnetic field of the deflection coil 23, form the front spaces 21, 21', and 21"of Figure 4a. The front spaces 21, 21 'and 21"affect or modify the harmonic current distribution, to correct, for example, the geometric distortions of the image formed on the screen, such as the North-South distortion. In the same way, the portions of the lateral wire bundles 120, 120 ', and 121, 121' located in the entrance region of the deviation coil 3, produce rear spaces 22 and 22 '.

Spaces 22 and 22 'have coil distributions selected to correct horizontal coma errors. The end turn portions 19 and 29, as well as side wire bundles 120 and 121 ', define a main coil window 18. The vacuum coil of Figures 4a-4c can be wound with a small copper wire. dimensions, covered with electrical insulation and with thermosetting adhesive. The winding is performed in a winding machine that cools the vacuum coil essentially in accordance with its final shape, and introduces the spaces 21, 21 ', 21", 22, 22' of Figures 4a-4c during the winding process. The shapes and fittings of these spaces are determined by retractable bolts on the winding head, which limit the shapes that these spaces can assume.Each bolt produces a corresponding winding corner in the vicinity of the bolt to change the direction of the wire. Each winding is held in a mold, and a pressure is applied to obtain the required mechanical dimensions.A current passes through the wire in order to soften the thermosetting adhesive, which is then cooled again with in order to adhere the wires to each other, and to form a depression coil that is self-supporting.The region along the longitudinal axis Z of the end portion 29, defines the exit region or region of the bundle 23 of the reel 3. The region along the longitudinal axis Z of the window 18, defines an intermediate region or region 24, and extends, in an end, from the Z-axis coordinate of a corner portion 17 where the side wire bundles 120 'and 121' are joined. The other end of the window 18 is defined by the portion 29. The region of the coil located in the rear part behind the window 18, which includes the rear end turn 19, is referred to as the region or area of entry of the beam. 25. The comma error is corrected mainly in the back or inbound zone 25. Geometry errors, such as East-West and North-South distortions, are corrected mainly in or near the output zone 23. The error of convergence is the least affected in the exit zone 23, and is corrected mainly in the intermediate zone 24 and in the entrance zone 25. Figure 3 is a cross-sectional view of the depressed line coil 3 in a plane parallel to XY in the intermediate zone 24. For the consideration of symmetry, only the cross section of the half of the coil is represented. This half coil includes the bundles 120, 120 'of the conductors 50. The position of each conductor is identified by its radial angular position?. The conductor wires of group 120 are configured between 0 degrees and TL, while those of group 120 'are configured between? L and T2. Due to the consideration of coil symmetry, the Fourier series expansion of the amperes turns density N (?) Of a coil is written as follows: N (?) = Al * cos (?) + A3 * cos (3?) + A5 »cos (5?) + + AK» cos (KT) + (EQ1) with: N (?) «eos (K?)» d? (EQ2) The magnetic field assumes the expression: H = A1 / R + (A3 / R3) »(X2-Y2) + (A5 / R5) • (X4-6X2« Y2 + Y4) + ... (EQ3) where R is the radius of the magnetic circuit of the ferrite core surrounding the deflection coils. The term Al / R represents the zero order coefficient or the fundamental field component of the field distribution function, the term (A3 / R3) • (X2-Y2 represents the second order coefficient of the field distribution function at a point of the X and Y coordinates, and is related to the third harmonic of the coil distribution.The term (A5 / R5) (X4-6X2 »Y2 + Y4) represents the fourth order coefficient of this field or the fifth harmonica, etcetera.

The positive term A3 corresponds to a second-order coefficient of positive field on the axis that produces the pin-shaped field. In the case where the current circulates in the same direction on all conducting wires, N (?) Is conventionally positive, and the term A3 is positive if the wires are set to? = 0 degrees and? = 30 degrees. This is so because cos (3?) Is positive. By configuring the wires in the previously defined angular range, it is possible to locally introduce a significant positive second-order coefficient of the field, as well as a positive fourth-order coefficient of the field that is globally positive. In order to maintain the convergence of the electron beams coming from an in-line gun, it is known to make the second order coefficient of the line deviation field positive in the intermediate zone 24 of Figures 4a and 4b. For this purpose, the majority of the wires of the lateral wire bundles 120, at least a part of the intermediate zone 24, are maintained in a radial angular position of between 0 degrees and 30 degrees. However, because this method to control the convergence of the beams introduces a strong coma parabola error, the coma parabola error has to be corrected. The comma errors are corrected by introducing spaces 22, 22 'in the area 25, where the end return section 19 is located. An additional space 26, which opens into both zones 24 and 25, allows make the adjustment of the residual coma and parabola coma errors. Accordingly, the convergence and comma errors to acceptable values are reduced by a coil structure, as illustrated by Figures 4a, 4b, and 4c, where the comma errors are adjusted by the spaces 22, 22 ' , 26, and the convergence of the beams through the spaces 26 and 21. "The configuration of the wire bundles in the front portion of the intermediate region near the front end return section 29 contributes to the reduction of the distortion of North-South geometry of the image created on the screen Beams 150, 151, 152 of Figure 4a contain together most of the coil wires, and are configured at a radial angular position in the XY plane, which is between 0 and 30 degrees As illustrated by Figure 1, the separator is composed of a funnel-shaped main part 161 that conforms to the shape of the tube on which the deflection yoke is mounted for a substantial length of the separator. addition, an e The front end 160 of the separator forms a plane perpendicular to the axis Z which extends in the perpendicular plane XY away from the funnel-shaped contour of the tube. An internal surface of the front end 160 is used to support the front end turn 29 of the horizontal deflection coil. The circular shape of the inner periphery or boundary 162 of the front end 160 forms a boundary between the part 160, which is perpendicular to the axis Z, and the part 161, which has the enlarged shape conforming to the conical shape of the tube funnel . The internal boundary 162 has a half circle shape in each half of the separator. During the winding process of the coil, retractable bolts are inserted perpendicular to the XY plane in the area where the bundles of the lateral section are connected to the end turn 29. In the embodiment of an inventive feature, the corners of the coil produced by the bolts, they are located further away from the limiting half circle 162 of the front end 160 of Figure 4c and the main part 161 of the separator. The residual North-South geometry errors are reduced to acceptable values by placing the internal periphery 163 of the end turn 29 further away from the limiting half circle 162. As indicated above, the limiting circle means 162 is separated between the body 161 of the separator, and its front end 160. Conveniently, the change in the position of the corners of the coil produced by placing the bolts on the front end 160 and away from the limiting half circle 162, extends in the action zone by the effective length of the horizontal deflection field towards the front of the tube, and provides an additional correction of the North-South geometry of the image produced by this type of field. Moreover, the change in the position of the corners of the coil, produced by placing the bolts on the front end 160, and moving away from the limiting circle 162, increases the distance between the centers of horizontal and vertical deflection. As explained by N. Azzi in a paper presented at the Society of Information Display (SID) conference in 1995, entitled "Design of a North - South pin - coma free 108 degree self-converging yoke for CRTs with super flat face píate ", the increase of the distance between the centers of deviation allows to have a better control of the North-South geometry of the image. In a preferred embodiment of the invention, the deflection yoke is mounted on a tube of the type A68SF, which has a screen of the aspheric type whose horizontal edges have a radius of curvature of the order of 3.5R. The spacer has the front end 160 in the shape of a circular ring that forms a surface supporting the end turn 29. The front end 160 is flat and is parallel to the XY plane. The front end turn 29 extends in a direction perpendicular to the Z axis, which offers the advantage of keeping the size of the deflection yoke short along the Z axis direction. In addition, easier fabrication of the Z axis is facilitated. coil in the mold because, during winding, the retractable bolts are inserted perpendicularly to the surface of the mold; in this way, a better retention of the wires is obtained during winding. Figure 5a illustrates, in a front view, the positions of the front bolts at locations 165, 166, and 167 with respect to the spacer. Figure 5b illustrates, in a radial cross section, the radial positions of the front bolts at location 165. The bolts at locations 165, 166, and 167 of Figure 5a are inserted during the winding process, to create the beams of wires 150, 151, and 152, respectively. Each bolt produces a corresponding winding corner on the coil in the region that makes contact with the bolt. Beam 150 contains 57 percent of the total number of wires, and beams 151 and 152 contain 11 percent and 26 percent, respectively. The pins are configured in the radial annular positions in the XY plane, equal respectively to 10, 20, and 30 degrees, respectively. The bolts are displaced or changed in relation to the limiting circle means 162, on the ring 160. The limiting circle circle 162 is essentially circular, with a radius equal to 54.5 millimeters. The position of the bolt, and therefore that of the winding corner, is positioned away from the limiting half circle 162 by a distance from the center of the circular cross-section of the bolt which is the same for each of the bolts. This distance is equal to a delta value = 4 millimeters. Accordingly, each winding corner is disposed away from the limiting half circle 162. Various combinations have been considered which include changing only the bolt which is at 10 degrees, changing only the bolt which is at 20 degrees, changing only the bolt which is at 30 degrees, and then change the bolts two at a time. It has been shown that the change of the pin at location 167, located at approximately 30 degrees, was what offered the greatest sensitivity to the control of the error of external North-South geometry in relation to the horizontal edges of the image. In the case of the deflection yoke for tube A68SF, a change of 4 millimeters from the location of the bolt at location 167, by itself, conveniently causes a North-South external pinion deviation of -1.11 percent relative to a reference situation of 0 percent. The reference situation is obtained when the bolts in the locations 165 to 167 are not changed, and they are located on the edge or the limiting circle 162. In a convenient way, the improvement in the deviation of the North-South outer pincushion is obtained without degrading the convergence parameters. A deviation of -1 percent is desirable, because it provides a pin-shaped pattern on the screen. A viewer perceives a pin-shaped pattern of -1 percent at a distance from the screen equal to five times the height of the image as free of geometry distortion. The radial change in the positions chosen for the three bolts in the locations 165 to 167 of 4 millimeters, simplifies the manufacture of the coils without this structure being limiting. If necessary, a finer North-South geometry control can be selected, as a function of screen size and smoothness, by changing the front bolts for different amounts in relation to the limiting half circle of the edge 162. This configuration results in an external pincushion offset of -1.06 percent, and an inner pincushion offset, measured midway between the horizontal edges and the center of the screen, of -0.40 percent. These values are acceptable without having to use auxiliary field configurators, since the external North-South geometry deviations remain in the form of a pincushion. The ideal values for the external pincushion form are of the order of -1%, and for the internal pincushion form of the order of -0.4% to -0.8%. Figure 6 shows the variation of the zero order and highest order coefficients of the field distribution function of the horizontal deviation field. In particular, Figure 6 shows a slight change towards the front of the zone of action of the coefficients of zero and second order, HO and H2. The following values, calculated from the curves in Figure 6, show this change towards the front: Bolts on the pins Location 162 Changed Comprehensive HO 261,699 262,869 Center of gravity of the field Z = -22.568 mm Z = -22.304 mm Effective field length 101.021 mm 101.616 mm Although the value differences between the two structures appear to be small, they are sufficient to provide the desired geometry correction. The sensitivity of the device to the displacement of the center of deviation is more significant when the face plate of the tube is flatter. The previous example is not limiting. According to an implementation mode not shown, the enlarged front end has an internal wall of revolution whose widening is not perpendicular to the Z axis, but inclined towards the front of the tube, with a surface, for example, of a truncated conical shape. This configuration makes it possible to increase the effect generated by the change of the bolts to the outside, but in the same way increases the influence on the other parameters, such as the convergence and the comma, making the error control of residual geometry less dependent of these last parameters. In the same way, the number of bolts, and therefore the number of bundles of wires formed in the radial opening from 0 to 30 degrees, depends on the size of the screen and its planarity, and therefore, may be greater or less than three. Finally, this principle of controlling residual geometry errors can be used in the same way to control the East-West geometry, and therefore, can be used for the design of vertical deflection coils.

Claims (10)

1. CLAIMS 1. A video display deflection apparatus, comprising: a first deflection coil for producing a deflection field in order to sweep an electron beam along a first axis of a visual display screen of a cathode ray tube; a second deflection coil in the form of a depression, to produce a deflection field, in order to sweep the electron beam along a second axis of the visual display screen of the cathode ray tube; a separator for mounting the first and second deflection coils thereon, this separator having a first funnel-shaped part that conforms to a shape of a neck of the cathode ray tube, and a second part forming a front portion of the separator near the screen, the first and second portions having substantially different degrees of spreading, the second deflection coil including a plurality of coil turns forming a pair of side portions and a front end turn portion, close to the screen , such that a winding corner is formed between one of the side portions and the front end turn portion within a radial angular position of between 0 and 30 degrees, the winding corner being arranged on the second part of the separator away of a boundary between the first and second parts, in a manner extending by an effective length of the second deviation coil. ion in one direction of the screen to provide a grid distortion correction; and a magnetically permeable core for cooperating with the first and second deflection coils, to form a deflection yoke free of permanent magnets. A video display deflection apparatus according to claim 1, wherein the front end turn portion within the range of 0 and 30 degrees is supported on a surface of the second part of the separator, and wherein the second support part is perpendicular to a longitudinal axis of the tube. 3. A video display deflection apparatus according to claim 1, wherein most of the wires that are in a front portion of the side portions are in the angular position of between 0 and 30 degrees. 4. A video display deflection apparatus according to claim 1, wherein the second deflection coil is a horizontal deflection coil. A video display deflection apparatus according to claim 1, wherein the second deflection coil is formed as a unit outside the separator, and assembled after being formed with the separator, to form the deflection yoke. 6. A video display diverting apparatus according to claim 1, wherein the second deflection coil is formed in a winding machine using retractable bolts that determine a distance from the boundary of the winding corner. 7. A video display deflection apparatus according to claim 1, wherein the effective length of the second deflection coil extends in said direction of the screen, to provide a North-South grid distortion correction. 8. A video display diverting apparatus according to claim 1, wherein the deflection yoke is free of permanent magnets in a beam exit region. 9. A video display deflection apparatus, which comprises: a first deflection coil for producing a deflection field, in order to sweep an electron beam along a first axis of a visual display screen of a cathode ray tube; a second deflection coil in the form of a depression, to produce a deflection field, in order to sweep the electron beam along a second axis of the visual display screen of the cathode ray tube, which includes a plurality of winding turns forming a pair of side portions, a front end turning portion, near the screen, and a winding corner formed between at least one of the side portions and the return portion of the winding. front end within a radial angular position of between 0 and 30 degrees, the at least one side portion having a first part conforming to a neck shape of the cathode ray tube, and a second part interposed between the first part and the second part. winding corner, having a different degree of broadening different from the first part, such that the winding corner is disposed away from a boundary between the first and second parts, in a manner extending by an effective length of the second deflection coil, in one direction of the screen that provides a grid distortion correction; and a magnetically permeable core for cooperating with the first and second deflection coils, in order to form a deflection yoke. A video display diverting apparatus according to claim 9, further comprising a spacer wherein the front end turn portion within the range of 0 and 30 degrees is supported on a surface of a part of the spacer which is perpendicular to a longitudinal axis of the tube. SUMMARY A video display deflection apparatus includes a vertical deflection coil. A separator is used to mount the vertical deflection coil on it. The separator has a first funnel-shaped part that conforms to a neck shape of the cathode ray tube, and a second part that forms a front portion of the spacer near the screen. A degree of widening of the first and second parts is substantially different. A horizontal deflection coil in the form of depression (3) is mounted on the separator (2), to produce a deflection field, in order to sweep the electron beam along a horizontal axis of the visual display screen (9) The horizontal deflection coil includes a plurality of winding turns that form a pair of side portions, a rear end turn portion (19), near an electron gun of the tube, and a front end turn portion (29). ), near the screen (9). At least a portion of the front end turn portion, at a radial angular position of between 0 and 30 degrees, is supported on the second part of the separator, away from the limit. Accordingly, an effective length of the horizontal deflection coil is extended in one direction of the screen to provide a North-South grid distortion correction. A magnetically permeable core (5) cooperates with the deflection coils to form a deflection yoke free of permanent magnets in the front.