EP0269498A1 - Earth-magnetic field protection device for cathode ray tubes with a shadow mask - Google Patents

Abstract

To improve the shielding effect of three-color cathode ray tube conferred by the frame-mask assembly and by a shielding screen of material with low magnetic permeability, the outer strands (16, 17) of a demagnetization coil are passed through. two loops in front of the attachment ears (7) and not behind the fried plane (5) as was the case previously, and the position of the inner strands (12, 13) is adjusted to obtain the best compromise.

Description

The present invention relates to a device for protecting cathode ray tubes with a mask with respect to the earth's magnetic field.

We know that the Earth's magnetic field acts on the path of the electron beams emitted by the barrel of a cathode ray tube and can cause what is called in a trichrome tube a "loss of register", that is to say -to say an offset of the impact on the screen of the beams of the three fundamental colors compared to the corresponding phosphors. The amplitude of the parasitic deviation depends on the amplitude of the terrestrial magnetic field, but also on the geometry of the tube (distance gun-mask, parameter called "p", and distance mask-slab, parameter called "q").

, la composante horizontale latérale Δ

, et la composante horizontale axiale Δ

. La composante verticale provoque une déviation parasite vers la gauche dans l'hémisphère Nord, alors que la composante latérale entraîne une déviation parasite du point d'impact vers le bas ou vers le haut selon que le tube cathodique est face à l'est ou face à l'ouest, et la composante axiale une déviation de type rotationnel.The Earth's magnetic field acts through its three components, namely the vertical component Δ

, the horizontal lateral component Δ

, and the axial horizontal component Δ

. The vertical component causes a parasitic deviation to the left in the Northern hemisphere, while the lateral component causes a parasitic deviation of the point of impact down or up depending on whether the cathode ray tube is facing east or facing to the west, and the axial component a deviation of the rotational type.

In current cathode ray tubes, to avoid the action of the earth's magnetic field, the tubes are shielded over most of the path of the beams. The shielding is obtained by the frame-mask assembly and by a stamped metal screen disposed in the tube and fixed to the frame substantially matching the shape of the cone.

The shielding operates in the following way: the external magnetic field realigns in the material constituting the screen the limits of the WEISS domain, creating an induced magnetic field tending to oppose the action of the external field which gave it birth. If the material constituting the screen has a high permeability magnetic, for example the metal, the induced field is totally opposed to the external field. There is therefore no longer any disturbing field inside the shielding. If a material with low magnetic permeability (for example mild steel) is used, chosen essentially for its low cost, the compensation is no longer total. To obtain more effective compensation, it is necessary to demagnetize the screen material using an initially intense and gradually decreasing magnetic field. This demagnetization makes it possible to locate the magnetization curve on the so-called anhyteresis curve, and thus to obtain a magnetic field of opposition to the more intense terrestrial field, hence better shielding. This demagnetization is conventionally carried out using a coil fixed to the rear of the tube, on the cone.

The Applicant has noticed that the frame-mask assembly has a shielding action with respect to the lateral and axial horizontal components and to the vertical component, the effects of which are reduced respectively by at least 90% at least, 50% and 60%. The introduction of a magnetic screen made of a material with low permeability allows, after demagnetization, better compensation for the effects of the axial horizontal component and the vertical component (reduction of the effects of approximately 87% and 97% respectively), but at to the detriment of the compensation for the lateral horizontal component, the effects of which are only reduced by approximately 78%. The variations in the effects of these components are simply determined by measuring the corresponding offsets of the registers.

The subject of the present invention is a device making it possible to improve the shielding effect of a screen made of a material with low magnetic permeability cooperating with a cathode ray tube of the frame and mask type, this improvement relating to the protection vis-à-vis of the three components of the Earth's magnetic field at once.

The device according to the invention comprises a demagnetization coil of the type with two oblong loops, the long sides of which are arranged substantially horizontally, these two loops being fixed largely on the cone of the cathode ray tube, symmetrically with respect to a plane passing through the axis of the neck of the tube, and connected to a device for generating demagnetization current, and according to a characteristic of the invention both long sides of these loops furthest from each other are arranged on the anterior half of the anti-implosion belt of the tube, advantageously near the longitudinal axis of this belt, just in front of the attachment ears of the tube of this belt.

• - la figure 1 est une vue arrière d'un tube cathodique muni du dispositif de l'invention,
• - la figure 2 est une vue de profil du tube de la figure 1,
• - les figures 3 à 6 sont des vues de profil du tube de la figure 1 pour différentes positions des brins extérieurs des boucles de déma­gnétisation, les brins intérieurs étant fixes,
• - les figures 7 à 9 sont des vues de profil du tube de la figure 1 pour différentes positions des brins intérieurs des boucles de déma­gnétisation, les brins extérieurs étant fixes, et
• - les figures 10 et 11 sont des diagrammes montrant l'effi­cacité des corrections obtenues dans les cas des figures 3 à 6 et 7 à 9 respectivement.

The present invention will be better understood on reading the detailed description of an embodiment taken as a nonlimiting example and illustrated by the appended drawing, in which:

• FIG. 1 is a rear view of a cathode ray tube fitted with the device of the invention,
• FIG. 2 is a side view of the tube of FIG. 1,
• FIGS. 3 to 6 are side views of the tube of FIG. 1 for different positions of the outer strands of the demagnetization loops, the inner strands being fixed,
• FIGS. 7 to 9 are side views of the tube of FIG. 1 for different positions of the inner strands of the demagnetization loops, the outer strands being fixed, and
• - Figures 10 and 11 are diagrams showing the effectiveness of the corrections obtained in the cases of Figures 3 to 6 and 7 to 9 respectively.

There is shown in the drawing a cathode ray tube 1 essentially consisting, for its glass part, of a slab 2, a cone 3 and a neck 4. We see in Figure 2 the trace 5 of the plane of "fried", that is to say the joint plane between the slab and the cone.

An anti-implosion belt 6 surrounds the skirt of the slab 2. This belt 6 is provided, near its corners, with four fixing ears 7. These ears 7 are fixed approximately in the middle of the width of the belt 6. A deflection block 8 is fixed to the neck 4, flush with the cone 3. The other details of the tube 1 have not been shown, since they are not necessary for understanding the invention.

The tube 1 has a shielding screen (not shown) at inside the cone, this screen being made of a material with low magnetic permeability, such as mild steel. This screen is demagnetized, whenever necessary (in particular when the orientation of the television or monitor incorporating the cathode ray tube is changed) using a demagnetization coil 9.

The coil 9 has two identical oblong loops 10, 11 fixed by any suitable means, for example by fixing clips, to the external surface of the tube 9, and supplied in parallel by a conventional demagnetization current source (not shown) so their respective fields are directed towards the front of the tube 9. The corners of these loops relative to their long sides 12, 13 closest to the neck 4 (inner strands of the loops) are connected two by two by retaining wires 14 , 15 which extend substantially vertically. The long sides 12, 13 as well as the other two 16, 17 of the loops 10, 11 extend substantially horizontally. The length of the long sides 16, 17 (outer strands of the loops) is substantially equal to the length of the long sides of the screen of the tube 9. These long sides 16, 17 are fixed to the belt 6, practically at its axis longitudinal, and pass at the level of the ears 7, and in this case in front (for an observer looking at the screen of the tube 9) the ears 7, which are fixed approximately in the middle of the width of the belt 6. The large sides 12, 13 of loops 10, 11, slightly shorter (about 1 to 2 cm) than the long sides 12, 13, are fixed, as was the case for the coils of the prior art, on the cone 3 at a short distance (generally a few centimeters) from the deflector 8. However, the positions of these long sides 12, 13 as well as the sides 16, 17 can be adjusted as a function of the dimensions of the tubes on which the coils are fixed, to obtain optimal results, as will be seen below with reference to FIGS. 3 to 9.

The paths 18, 19 of the loops of the demagnetization coils of the prior art are shown in broken lines. The large outer sides of these loops pass just behind the fried plane 5.

In the embodiment shown, the long sides 16, 17 of the loops 10, 11 are passed along the longitudinal axis of the belt 6, just in front of the ears 7. However, the long sides 16, 17 could be fixed anywhere located in front (for an observer looking at the screen) of the fried plane, the results varying as shown in Figure 10 and as explained below.

= 42%, ΔZ= 9% dans les coins et 22 % à 6 heures et 12 heures (c'est-à-dire dans la zone des milieux des grands côtés de l'écran), et Δ

= 29 %, et ce , pour un tube asphérique de 68 cm de diagonale.The results obtained with the coils of the invention, by sending them a demagnetization current of usual shape and duration are as follows, by placing the strands 16, 17 at the ears 7, and the strands 12, 13 at mid path between the front face of the deflector 8 (its face closest to the screen of the tube) and the anode button 23 (or its symmetrical 23A), by expressing the gain in terms of the field effect as a percentage, that is to say the residual displacement of the register: Δ

= 42%, ΔZ = 9% in the corners and 22% at 6 a.m. and 12 p.m. (i.e. in the middle area of the long sides of the screen), and Δ

= 29%, for an aspherical tube 68 cm diagonal.

To determine, in each case, the best positions of the strands 16, 17 and 12, 13 relative to the tube, two series of tests were carried out: first, keeping the strands 12, 13 at the level of the anterior face of the diverter 8 and in positioning the strands 16, 17 at about 5 cm behind the fried plane (Figure 3), at the fried plane (Figure 4), at the ears 7 (Figure 5), and at about 6 cm in front of the fried dish (Figure 6). The results obtained have been plotted on a diagram (FIG. 10) giving the efficiency E of the movement of the register as a function of the position P of the strands 16, 17. The efficiency obtained at 100% was arbitrarily taken as that obtained at the plane fried, with abscissa P zero. The different results are referenced respectively A, B, C, D, for the EAST / WEST correction (curve interpolated in solid line), and for the NORTH / SOUTH correction (curve interpolated in broken line). It can be seen that in front of the plane of the fixing ears the efficiency varies only very little for the NORTH / SOUTH correction and more for the EAST / WEST correction depending on the position of the strands 16, 17.

In a second series of tests, the strands 16, 17 were fixed at the level of the fried plane, and the register displacements were measured for a position of the strands 12, 13 at the level of the front face of the deflector 8 (FIG. 7), halfway between this position and the anode button 23 (for the strand 12, and the symmetrical 23A of the button 23 relative to the axis of the tube 1 for the strand 13) (Figure 8), and at 23, 23A (Figure 9). The respective results E, F, G have been plotted on the diagram of FIG. 11 in solid lines for the EST / QUEST correction and in broken lines for the NORTH / SOUTH correction, the curves being plotted by interpolating between the results. This figure 11 gives the efficiency E of the correction as a function of the position P of the strands 12, 13, the efficiency of 100% being taken arbitrarily for the average position, the abscissa 0 being taken at the level of the anterior face of the diverter 8.

Of course, to carry out a correct adjustment of the demagnetization loop, it is necessary to adjust both the position of the strands 16, 17 and that of the strands 12, 13, noting however that it is preferable not to bring the strands 12 too close together. , 13 of the diverter to avoid a disturbing action of the loop on the operation of the diverter, unless provision is made for means opening the loop when it is not in service.

Claims (4)

1. Device for protecting cathode ray tubes, with mask and shielding screen made of material with low magnetic permeability, with respect to the earth's magnetic field, comprising a demagnetization coil of the type with two loops (10, 11) oblong, the long sides (12, 13, 16, 17) of which are arranged substantially horizontally, these two loops being fixed largely on the cone (3) of the cathode ray tube, symmetrically with respect to a plane passing through the axis of the neck (4) of the tube, and connected to a demagnetization current generating device, characterized in that the two long sides of these loops furthest from each other (16, 17) are arranged in front of the fried plane (5) of the tube. 2. Device according to claim 1, characterized in that the two aforementioned large sides (16, 17) are arranged near the longitudinal axis of the anti-implosion belt of the tube (6). 3. Device according to claim 2, for cathode ray tubes with fixing lugs (7) fixed on the anti-implosion belt, half-width of this belt, characterized in that the two aforementioned large sides (16, 17) pass between the plane of attachment of the ears and the plane of connection of the skirt with the screen panel. 4. Device according to one of the preceding claims, characterized in that the two long sides closest to each other of these loops (12, 13) are arranged substantially halfway between the inner face of the diverter (8) and the anode button (23) or its symmetrical (23A).

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