GB2046559A - Tilted unitary degaussing coil arrangement - Google Patents

Abstract

A single degaussing coil (20) for a kinescope (10) has a first portion (20a) located along a first side of the base of a generally pyrimidal kinescope envelope or bulb. Second and third portions of the coil extend along portions of second and third sides of the base and are joined by a fourth portion of the coil lying between the juncture of the neck and the bulb. The first portion 20a tends to degauss the shadow-mask (332) while the diametrically opposite portion, due to a tilt in the axis of the single coil in relation to the tube axis, lie adjacent the small end of an internal magnetic shield of frusto-conical shape and hence more efficiently magnetically interact therewith. The single tilted coil requires lower degaussing energies in operation and a simpler wiring in manufacture. <IMAGE>

Description

SPECIFICATION Degaussing coil arrangement This invention relates to an arrangement for degaussing color pictures tubes having magnetic shields and proposes use of a unitary degaussing coil in a tilted or skewed position.

Color television displays are often accomplished by means of shadow-mask picture tubes. In such tubes, a plurality of electron beams are directed from slightly different points or origin towards a screen coated with fluorescent phosphors. The phosphors are grouped in triads or groups of three, each member of which fluoresces in a different primary color when illuminated by an electron beam. Each member of each triad is made responsive to a single one of the plurality of electron beams by means of a shadow-mask. The shadow-mask is a thin conductive shield with a multitude of perforations precisely located with respect to each triad of color phosphors. The shadow-mask ideally allows the phosphor of a particular color to be illuminated only by an electron beam originating at a particular location.

Thus, with three electron beams originating in slightly different locations, each of the color phosphors of a trio is pure, i.e., is illuminated by only one electron beam, and gradations of color may be achieved by appropriate control of the source of electrons.

In the past, color television displays were sensitive to the influence of the earth's magnetic field. The magnetic field passing through the kinescope deflects the electron beams away from their intended paths and changes the apparent source of the electron beams arriving at the shadow-mask in a manner dependent upon the orientation of the picture tube relative to the earth's magnetic field, thereby impairing color purity. Early attempts to correct this problem icvolved the use of field neutralizing coils, as described in United States Patent No.

2,921,226 issued January 12, 1960 to Vasilevskis. In the field neutrilization arrangement, a direct current is passed through one or more coils arranged about the kinescope in such a manner as to balance to zero or neutralize the earth's field. This required adjustment by an expert, and required readjustment if the television was moved to a new location.

Another approach to correcting color purity problems resulting from the earth's magnetic field involves the use of magnetic shields. In such an arrangement, a hollow shield in the form of the frustum of a cone surrounds the region through which the electron beam travels before reaching the shadow-mask and screen. The high permeability of the shield diverts extraneous magnetic fields away from the electron beam. It was found, however, that the permeable magnetic shield was capable of becoming magnetized and, when so magnetized, itself perturbed the color purity. Degaussing arrangements were then devised to demagnetize the shield and the shadow-mask. The degaussing is accomplished by passing an alternating current of initially large magnitude through one or more coils arranged about the shadow-mask and magnetic shield as described in United States Patent No.

2,962,621 issued November29, 1960 to Fernald. The magnitude of the alternating current is then gradually reduced to zero and the magnetization of the shield and shadow-mask is reduced. A coil disposed along the periphery of the shadow-mask as illustrated by Fernald tends to produce a relatively small magnetic field near the center of the shadow-mask and screen. Thus, effective degaussing with such a coil requires currents which may be undesirably large.

In order to increase the magnetic flux through the magnetic shield and through the screen without excessive coil currents, arrangements such as those described by Norley in United States Patent No.

3,322,998 issued May30, 1967 and by Matsushima et al., in United States Patent No.3,872,347 issued March 1975 where devised. In the Norley and Matsushima arrangements, a pair of coils are arranged symmetrically on the picture tube, with each coil having a portion lying along the periphery of the shadow-mask and a portion extending towards the neck of the tube. The two-coil arrangements produce a magnetic field the lines of which are transverse to or across the longitudinal axis of the tube. This "cross axial" degaussing field approach using a pair of coils provides effective degaussing. However, the two coils require additional labor for mounting as compared with a single coil, and their interconnection may give rise to incorrect wiring.Furthermore, the two-coil arrangement tends to require a greater length of conductor than the single-coil type, and may respond to the deflection yoke field with currents which perturb color purity.

It is desirable to have a degaussing arrangement using a unitary coil requiring a relatively short conductor length which provides degaussing performance substantially equivalent to that of the two-coil system, combined with the simplicity and short conductor length of the single-coil type. - In a preferred embodiment, the present invention includes a degaussing arrangement for a colortelevision picture tube. The picture tube comprises an envelope having a flared portion with large and small ends, a faceplate portion adjacent the large end, and a neck portion coaxial with the flared portion and joined to the small end. The picture tube further comprises a shadow-mask enclosed within the envelope adjacent the faceplate portion and an electron gun assembly enclosed within the neck portion of the envelope.A flared magnetic shield is enclosed within the envelope and has large and small ends. The shield is adjacent a segment of the flared portion of the envelope. The degaussing arrangement further comprises a single degaussing coil encircling the flared portion of the envelope, with the axis of the degaussing coil tilted relative to the axis of the flared envelope portion in such manner as to dispose a first segment of the circumference of the coil in a position overlying a region of the envelope adjacent a portion of the large end of the shield.A second segment of the circumference of the coil, diametrically is disposed oppositely to the first segment, in a position which is contiguous with a region of the envelope adjacent a portion of the small end of the shield and which is located more remotely from the axis of the flared envelope portion than is the periphery of the opening at the small end of the shield. Means are also provided for supplying a variable alternating current solely to the single degaussing coil to effect degaussing of the shadow-mask and the magnetic shield.

FIGURES 1 a and 1 b illustrate in perspective views a picture tube with a degaussing arrangement embodying the invention; FIGURES 2a and 2b illustrate in respective rear and side views the arrangement of FIGURE 1 a and 1 b; FIGURE 3 is a cross-sectional view of the picture tube of the preceding FIGURES; FIGURE 4b illustrates magnetic field distributions associated with the invention and FIGURES 4a and 4c illustrate magnetic field distributions associated with the prior art; and FIGURE 5 is a semipictorial diagram containing block, schematic and pictorial portions depicting the arrangement of the invention as used with a television receiver.

In FIGURES la and 1 b, a picture tube designated generally as 10 includes an envelope consisting of three distinct portions. A frusto-conical or pyramidal portion 1 2a of the envelope is joined at the small end of the frusto-conical section with a neck portion 1 2b of the envelope. A flattened faceplate portion 1 2c closes the large end of frusto-conical portion 1 2a. A base plate 14 at the end of neck section 12b remote from the frusto-conical portion supports pins by which connections are made to the electron gun assembly mounted within neck portion 1 2b. A highvoltage, anode or ultor connection button 16 is located on the surface of frusto-conical section 1 2a.

A degaussing coil designated generally as 20 is located on the exterior surface of envelope 12. Coil 20 has a generally rounded triangular shape. One leg 20a of coil 20 is adjacent the periphery of the junction of faceplate 1 2c and frusto-conical section 1 2a of the envelope. Additional legs 20b and 20c of coil 20 form a vertex extending towards and around the junction of the neck and frustum portions of the envelope. A pair of leads 22 are taken from coil 20 for connection to a source, not shown, by means of which a current of variable amplitude is passed through coil 20 for degaussing.

FIGURES 2a and 2b illustrate in respective rear and side views the arrangement of FIGURES 1 a and 1 b.

The rearview of Figure 2a illustrates the somewhat rectangular outline of the large end of the frustoconical section which is common in the picture tubes currently in use. The neck portion, however, is circular as seen in this rear view for ease in mounting deflection yokes and other neck components.

The top cross-sectional view of kinescope 10 in FIGURE 3 illustrates a phosphor screen 330 adjacent the inside of faceplate 1 2c of the envelope. A shadow-mask 332 is mounted near phosphor layer 330 by means of mounting brackets 334 and 336.

Mounting brackets 334 and 336 also support a hol low magnetic shield 338 formed of a magnetically permeable material. Shield 338 preferably has the general form of frusto-conical portion 12a of the envelope of picture tube 10. Both the large and small ends of shield 338 are open to allow the passage of one or more electron beams from an electron gun assembly 340 mounted within neck portion 1 2b to phosphor layer 330.

The direction of magnetic field distribution generated by a degaussing coil mounted concentric with the picture tube is illustrated in cross-sectional view in FIGURE 4a. In a test of such an arrangement, the magnitude of the field available for degaussing at the center of screen 332 was 0.2 gauss with a coil producing a maximum of 20 gauss. The coil also produced between 0.3 and 0.6 gauss at the periphery of the opening at the small or entrance end of magnetic shield 338.

By contrast, the inventive coil arrangement has as its core more of magnetic shield 338, and produces a magnetic field distribution somewhat as shown in FIGURE 4b. The center of the screen is at a region of greater field strength. Using a degaussing coil producing the same maximum field strength as that of FIGURE 4a, the degaussing field at the center of the screen was 0.5 gauss, which is almost double that of the prior art arrangement. The corresponding field strengths at the entrance end of magnetic shield 338 were 7.5 gauss at the side nearest the coil, 1 gauss at the side farthest from the coil and 4 gauss at the other two sides. Thus, the fields available for degaussing the magnetic shield are much greater in the inventive arrangement than in the arrangement of FIGURE 4a when the coils have the same number of ampere-turns.

FIGURE 4c illustrates the magnetic field distribution associated with two-coil arrangements such as those of Norley and Matsushima. In the arrangement of FIGURE 4c, the combined effect of the pair of coils causes a substantial magnetic field flux transverse to the axis of the picture tube in the region of magnetic shield 338. At the faceplate and shadow-mask end of the tube, the magnetic field lines are parallel with the shadow-mask and tend to have a uniform strength through the center of the shadow-mask. The degaussing provided by the inventive arrangement is substantially equivalent to that provided by the two-coil arrangement of FIGURE 4c in that it provides degaussing for the internal magnetic shield as well as for the mask and frame assembly. The inventive degaussing arrangement is less costly by virtue of requiring less conductor length than either the single-coil arrangement of FIGURE 4a or the two-coil arrangements. The inventive arrangement is easier to assemble than the two-coil arrangements and cannot be wired incorrectly.

FIGURE 5 illustrates a circuit by which the degaussing coil may be energized with an alternating current having an initially large amplitude which decreases to a substantially zero amplitude. In FIG URE 5, a portion of a television receiver includes a plug 510 adapted to be connected to the alternating power mains. A switch 512 and fuse 514 couple the mains across primary winding 51 6a of a power transformer 516. A secondary winding 51 6b of transformer 516 produces alternating current which is utilized by a power supply illustrated as a block 518 for energizing the remainder of the television receiver, illustrated as a block 520. The alternatingcurrent mains are also coupled through a thermallyreponse resistor 530 to leads 222 of coil 220.

Thermally-responsive resistor 530 has a positive temperature characteristic, so that the resistance of the thermistor is low at room temperature and increases with increasing temperature.

At room temperature, the resistance of thermistor 530 is low. When switch 512 is closed in order to energize the television receiver, power is applied through transformer 516 to power supply 518 to energize the receiver. At the same time, an alternating voltage is applied across the series combination of thermistor 530 and coil 220. Because the resistance of thermistor 530 is initially iow, a relatively large alternating current flows through coil 220. The current flow through thermistor 530 causes the thermistor to heat, and its resistance to rise. The increasing resistance decreases the current flow resulting from the applied voltage and partially compensates for the reduced power dissipated in the thermistor at the decreased current.The temperatu re of the thermistor continues to rise, resulting in a rapid reduction in the current to a small limiting value as the resistance of the thermistor reaches a maximum. A circuit arrangement using temperature-responsive resistors is described in United States Patent 4,024,427 issued to Belhomme on May 17,1977.

While the arrangement as described uses an internal magnetic shield, those skilled in the art will recognize that the principles of the invention are equally applicable to a kinescope fitted with an external shield.

The described embodiment thus describes a colour television tube with degaussing facilities, comprising a magnetic shield adjacent to and extending some distance along a flared portion of the tube, a shadow mask generally at the wide end of the flared portion, and one sole degaussing coil encircling the tube and the shield, but having an axis tilted or skewed with respect to the axis of the tube so as to extend from a position adjacent the mask towards the small end of the flared portion, the sole coil being diminishingly energizable with ac so as to degauss both the shield and the mask.

Claims (5)

1. Adegaussing arrangementfora colortelevision picture tube, said picture tube comprising an envelope comprising a flared portion having large and small ends, a faceplate portion adjacent said large end, and a neck portion coaxial with said flared portion and joined to said small end, said picture tube further comprising a shadow-mask enclosed within said envelope adjacent said faceplate portion and an electron gun assembly enclosed within said neck portion of said envelope, said picture tube further comprising a flared magnetic shield (e.g.

enclosed within said envelope and) having large and small ends, said shield being adjacent a segment of said flared portion of said envelope, said degaussing arrangement being characterized by a single degaussing coil encircling said flared portion of said envelope, with the axis of said degaussing coil tilted relative to the axis of said flared envelope portion in such manner as to dispose a first segment of the circumference of said coil in a position overlying a region of said envelope adjacent a portion of said large end of said shield, and as to dispose a second segment of the circumference of said coil, diametrically opposed to said first segment, in a position which is contiguous with a region of said envelope adjacent a portion of said small end of said shield and which is located more remotely from said axis of said flared envelope portion than is the periphery of the opening at said small end of said shield; and means for supplying a variable alternating current solely to said single degaussing coil to effect degaussing of said shadow-mask and said magnetic shield.

2. A degaussing arrangement in accordance with claim 1 characterized in that said first coil segment is disposed in a position overlying a region of said envelope adjacent an upper portion of said large end of said shield, and wherein said second coil segment is disposed in a position contiguous with a region of said envelope adjacent a lower portion of said small end of said shield.

3. A degaussing arrangement in accordance with claim 1 or 2, characterized in that said picture tube envelope flared portion is in the general form of the frustum of a cone, and said magnetic shield is in the general form of the frustum of a cone.

4. A degaussing arrangement in accordance with claim 1 or 2 characterized in that said picture tube envelope flared portion and said magnetic shield each have the general shape of a truncated pyramid, and said shadow-mask is generally rectangular.

5. A degaussing arrangement substantially as hereinbefore described with reference to Figures la and 1b or Figures 2a and 2b or all Figures.