Background Art A color picture tube for embodying a color image can be classified as an aperture type alternately exciting R (Red), G (Green), and B (Blue) phospors by a single beam, a trio type where three electron guns are arranged independently, and an in-line type where three electron guns are arranged horizontally.

Among these, the trio and aperture types are generally used in color picture tubes for industry, and the in-line type for the public sector.

As an electron gun assembly comprises three electron guns arranged horizontally in parallel, in the in-line type color picture tube, as shown in FIG 1, electron beams 20 fired from three electron guns 10 should converge to a point (essentially, a position on the corresponding phospor in a pixel, i. e., three phospor points in a pixel) through a shadow mask 30 in a raster pattern.

Here, the electron beams 20 scan each position on a phospor secreen 40 by vertical and horizontal deflection means 50, and at this time the three electron beams 20 at each position should also converge to one point.

Accordingly, a color picture tube is equipped with static and dynamic converging means, and the static converging means consists of three pairs of magnetic rings (p2, p4, p6; generally denoted by p), each pair having 2,4, and 6 poles, respectively, which constitute a convergence device called a Convergence Purity Magnet (CPM) in general.

Here, each pair (p2, p4, and p6) of the convergence device

includes two magnetic rings each of which is partially magnetized and constitutes two kinds of N and S poles First, a 2 pole pair p2 shown in FIG. 2 (A) is called a purity magnet and moves the three beams for emitting light from the three phospors R, G and B in one direction by a magnetic flux between the N and S poles, as shown.

Meanwhile, a 4 pole pair p4 shown in FIG. 2 (B) moves the two beams R and B in opposite directions, a 6 pole pair p6 shown in FIG.

2 (C) moves the two beams R and B in the same direction, and the 4 pole and 6 pole pairs of magnetic rings are together called a convergence magnet. The magnet configurations shown in FIGS. 2 (B) and 2 (C) are set forth in a previous application by the present inventor where they form a non-symmetric magnetic system of the 4 pole and 6 pole pairs of magnetic rings, thus making it easy to control the electron beams.

Here, the convergence device is able to control the three electron beams R, G, and B for emitting light from the phospors, which are not arranged precisely due to errors in the manufacturing process, such that the three electron beams land precisely at a point. The reason that pairs p2, p4 and p6 constitute a set of magnetic rings is to control beam movements by adjusting relative angles between the two rings of each pair in order to compensate for manufacturing errors which differ for each picture tube.

In an electron gun, an electron beam is formed by collecting and accelerating thermoelectrons produced by heating with a heater, and thus high heat is generated during the operation of the electron gun.

Therefore thermal deformation of the electrodes of the electron gun cannot be avoided. This can be seen in FIG. 11. When electrodes are not deformed by heat, an electron beam B1 is collected and accelerated by the electrodes and lands on the screen S as an adjusted beam Bc by a convergence device. If electrodes are heated, the electrodes expand and an electron beam Bf diffuses and lands outside of the proper area

on the screen. The difference in this landing position is called the heat drift displacement f. In particular, since this heat drift displacement varies from the initial operation of the parts such as the electrodes, etc., until the time when the electrodes are heated sufficiently, it is an obstacle to obtaining a stable and clear image.

On this point, it has been widely selected in the conventional art to use an electrode material whose heat deformation is small, but the cost of developing new materials is high and the effect is not sufficient. Also, it causes, on the contrary, the problem that its processability and its yield become low, thus increasing the manufacturing cost.

Disclosure of the Invention In consideration of the above problems in the conventional art, it is an objective of the present invention to provide a convergence device that can compensate for heat drift without lowering the convergence characteristics.

To achieve the above objective, a convergence device of the present invention comprises; a first thermally stimulated magnetic body consisting of a magnetic material with a relatively low thermal expansion coefficient; and a second thermally stimulated magnetic body with a relatively high thermal expansion coefficient, wherein the first and second thermally stimulated magnetic bodies are arranged in a consecutive manner on the path of electron beams.

Preferably, the first and second thermally stimulated magnetic bodies diverge and converge electron beams in an mutually opposite way, and the first thermally stimulated magnetic body is made of a Al-Ni- Co material and the second thermally stimulated magnetic body is made of a ferrite material.

According to these compositions, when magnetic bodies are heated, their magnetic forces decrease at the same time. Thus the divergence caused by one magnetic body decreases and at the same

time the different amount of convergence caused by the other magnetic body decreases, thus mutually compensating for change in each other's adjustment of the geometrical path. Therefore, the electron beam path is rendered substantially invariant to a change in temperature.

As a result, the present invention provides a convergence device whose adjustment is easy.

Brief Description of the Drawings FIG. 1 is a schematic diagram illustrating the construction of an in- line type color picture tube; FIGS. 2 (A)-2 (C) are schematic diagrams illustrating the function of each pair of magnetic rings of a convergence device; FIGS. 3 and 4 are schematic diagrams illustrating the structure of a convergence device of the present invention; FIGS. 5 and 6 are a top view and a side view, respectively, of an assembled state of the present invention; FIGS. 7 and 8 are a top view and a side view, respectively, illustrating the construction of holders in FIGS. 3-6; FIGS. 9 (A)-9 (B) are top views illustrating a preferred construction of a first thermally stimulated magnetic body; FIG. 10 is a top view illustrating a preferred construction of a second thermally stimulated magnetic body; FIG. 11 is an electron beam path view illustrating the principle of the generation of heat drift; FIG. 12 is an example of an electron beam path view explaining the operation principle of a means of compensating for heat drift; FIGS. 13-15 are electron beam paths views explaining the operation principle of a means of compensating for heat drift according to the present invention; FIG. 16 is an electron beam path view briefly explaining the operation principle of a means of compensating for a heat drift for

comparison with FIG. 17; FIG. 17 is an electron beam path view explaining the operation principle of another construction of a means of compensating for heat drift in the present invention; and FIGS. 18-20 are top views explaining the convergence adjustment principle of a magnetic ring.

Best mode for carrying out the Invention Preferred embodiments of the present invention are explained in detail below with reference to the attached figures.

A method proposed as an effective alternative to counteract heat drift is a method of using a so-called thermally stimulated magnetic body, and this is seen in FIG. 12.

Looking at the left half of FIG. 12, a thermally stimulated magnetic body P4f with a magnetic force of, for example, 10 gauss is attached to the perimeter of a electron beam path, i. e., a convergence device, and expands the horizontal distance OCV (Outer Convergence Valence) between the three electron beams (R, G, B). Looking at the right half of FIG. 12, if electron guns are heated, the electrodes indicated by a dotted line are deformed by heat and, at the same time, a thermally stimulated magnetic body is also heated. At this time, the magnetic force of the thermally stimulated magnetic body decreases from 10 gauss to, for example, 8 gauss and the amount of expansion of OCV decreases relatively so that the heat drift displacement thus decreases.

Here, a thermally stimulated magnetic body P4f can be made of a ferrite series material whose variation in magnetic force according to temperature change is large, but this may have a variety of problems including the following.

First, since the compensation for heat drift depends on the physical properties of a thermally stimulated magnetic body P4f, the amount of compensation is not large enough. For example, the heat drift

displacement f generated in a general in-line type picture tube is about 0.3 mm, but the amount of drift that can be compensated for by a ferrite series thermally stimulated magnetic body is only 0.1 mm or so.

Also, since this heat drift compensation structure expands OCV in advance and thereafter the expansion amount decreases depending on the temperature change, the OCV expansion is a precondition and this may make the design and organization of a convergence device difficult in reality. In general, it is easy to organize and control a convergence device when the distance across all the three electron beams (R, G, B), OCV, is in the range of 1.5-2 mm. However, according to the above heat drift compensation structure, OCV becomes larger by 1-2 mm or so by using a thermally stimulated magnetic body P4f, and the magnetic force of the magnets constituting a convergence device should then be large, which may be causing a variety of problems in control.

For example, when the pure OCV of an electron gun is 2 mm and the OCV of a beam rotation magnet in a deflection unit is 2 mm, the total OCV is 6 mm if the OCV of a thermally stimulated magnetic body P4f is 2 mm. To adjust this amount of 6 mm, the required OCV adjustment of the magnetic rings that will constitute a convergence device is larger than 6 mm. Considering a deviation of 1 mm in the manufacturing process, this means that the required adjustment is as much as more than 8 mm.

As such, if the required adjustment is more than 8 mm, far larger than the ideal adjustment 1.5-2 mm, a magnetic ring with high magnetic force should be used, so that the magnetic force affects not only the beam (s) to be adjusted but also other beams. This may make it difficult to obtain a satisfactory adjustment even with increasing the number of adjustments.

At this point, the magnetic rings (p2, p4, p6) each of which consists of a pair of electrodes as explained in FIG. 2 are seen in detail in FIGS. 18-20. In FIG. 18 (A), for instance, a 4 pole pair consists of a pair of magnetic rings R1 and R2 each having a handle H, and they are

initially aligned on the same axis as shown in FIG. 18 (B), and are assembled in a state having no magnetism in a convergence device.

The relative angle between the poles (N and S) on the magnetic rings R1 and R2 is adjusted by rotating the handle H, thus adjusting the convergence. FIG. 19 shows an example where the OCV is adjusted with a magnetic force corresponding to a relative angle of 30 degrees between the poles, and FIG. 20 shows a relative angle of 90 degrees between the poles, which generates the maximum magnetic force.

In the figures above, the poles (N and S) are shown ideally to affect the outside beams (R or B) near the poles, but when the magnetic force is larger, the magnet field may be applied not only to the outside beam (R or B) but to the central beam (G) and the other outside beam (B or R), thus changing the path and making it very difficult to adjust convergence. Also, the large magnetic force allows the convergence to be changed largely with a small adjustment of the handle H. This may make a fine adjustment of less than 0.5 mm difficult.

Therefore, the art that uses a ferrite thermally stimulated magnetic body P4f not only may have a limitation in compensating for heat drift, but also may make the convergence adjustment difficult and degrades the convergence precision.

FIGS. 3 and 4 show preferred embodiments of a convergence device of the present invention. In the neighborhood of a neck N of a picture tube T is installed a holder D where pairs of each magnetic ring (p2 through p4 shown in FIG. 2, which will not be indicated for convenience) for convergence adjustment are loaded. A first thermally stimulated magnetic body P4a and a second thermally stimulated magnetic body P4f are arranged in a consecutive manner in front and in back of the holder, i. e, toward the front and toward the back of the path of electron beams. This convergence device can be attached to a deflection unit according to its construction, and even if the convergence device is attached to the neck, a construction according to an

embodiment of the present invention described below can be attached to a deflection unit independently if needed, as in the case of parts of other convergence devices.

Here, the first and second thermally stimulated magnetic bodies P4a and P4f are preferably made of materials with different thermal expansion coefficients. For example, the first thermally stimulated magnetic body P4a positioned in front of the path of the electron beams is made of, for example, a Al-Ni-Co material with a thermal expansion coefficient of 0.012%/°C, and the second thermally stimulated magnetic body P4f is made of a conventional ferrite material with a thermal expansion coefficient of 0.2%/°C. More preferably, magnetic poles of the first and second thermally stimulated magnetic bodies P4a and P4f are arranged such that the convergence and divergence of the electron beams are in opposite directions. As described later, it is preferable that the first thermally stimulated magnetic body with a smaller thermal expansion coefficient converges the electron beams and the second thermally stimulated magnetic body with a larger thermal expansion coefficient diverges the electron beams.

The principle of such construction and operation is seen in FIG.

13. Looking at the left half of FIG. 13, the first and second thermally stimulated magnetic bodies P4a and P4f prior to heating have the same magnetic force of 10 gauss, and perform the opposite operations of converging and diverging. According to this, the OCV of the composite electron beam landing on the screen is almost the same as the original OCV and in reality is slightly larger.

Next looking at the right half of FIG. 13, when the electrode is heated, the electron beam diverges, following the path B1 due to the thermal deformation, and after this divergence, the beam is deflected inward again, following the path B2, by the first thermally stimulated magnetic body P4a. Next, the electron beam following the path B2 is caused to diverge again by the second thermally stimulated magnetic

body P4f and landing on the screen S following the path Bc, thereby compensating for heat drift.

Here, the first and second thermally stimulated magnetic bodies P4a and P4f originally have the same magnetic force, and thus, if the magnetic force did not decrease due to heating, the beam would follow the path Bf and a drift displacement f will be generated. But since the magnetic force of the second thermally stimulated magnetic body P4f with a relatively larger thermal expansion coefficient decreases to, for example, 8 gauss, the divergence decreases and the beam lands following the path Bc that is approximately a converged state.

Meanwhile, the magnetic force of the first thermally stimulated magnetic body P4a is indicated to be 10 gauss even after heating, as it was before heating. This means that the magnetic force is smaller than that of the second thermally stimulated magnetic body P4f. Here, the magnetic force of the first thermally stimulated magnetic body P4a has to change according to the temperature, though the change is relatively small. This is because there is a need to adjust the convergence by controlling the inward deflection of the path such that when heating, the OCV of the electron beam does not excessively decrease due to a large decrease in the divergence caused by the second thermally stimulated magnetic body P4a.

In the figures after FIG. 5 are shown various views of embodiments of the present invention in assembled states. On a holder D shown in FIGS. 5 and 6 are loaded pairs (P2, P4, P6) of magnetic rings and the first and second thermally stimulated magnetic bodies P4a and P4f. Preferably, the second thermally stimulated magnetic body P4f is constructed to be of a C-ring type with 4 poles as shown in FIG. 10, and is secured to the holder D by a coupling jaw Hb as shown in FIGS. 7 and 8.

Meanwhile, the first thermally stimulated magnetic body P4a is preferably constructed to be of an 0-ring type with 4 poles as shown in

FIGS. 9 (A) and 9 (B), and can be constructed to have a plurality of poles such as 6 or 8 poles when required. In the case of 4 pole construction as shown in FIG. 9 (A), it is possible to adjust the OCV, but since the perpendicular distance between electron beams, CCV (Center Convergence Valence), becomes large, it is preferable to use a construction such as that shown in FIG. 9 (B) where the relative magnetic force between N and S poles is not symmetric.

Such operation principle of the present invention is seen in FIGS.

13 through 15. Here, FIG. 13 illustrates the case where the magnetic forces of the first and second thermally stimulated magnetic bodies P4a and P4f are the same, FIG. 14 illustrates the case where the magnetic force of the first thermally stimulated magnetic body P4a is larger than that of the second thermally stimulated magnetic body P4f, and FIG. 15 illustrates the case where the magnetic force of the first thermally stimulated magnetic body P4a is smaller than that of the second thermally stimulated magnetic body P4f. Meanwhile, it is assumed that the magnetic force of the second thermally stimulated magnetic body P4f changes relatively largely before and after heating, whereas that of the first thermally stimulated magnetic body P4a does not change very much before and after heating, for convenience.

First looking at the left half of FIG. 13, since the second thermally stimulated magnetic body before heating has the same magnetic force as the first thermally stimulated magnetic body, an electron beam follows the original path unchanged by the convergence and divergence and the OCV does not change very much. However, if the electrode expands by heating, the electron beam diverge to follow the path B1 and is then deflected inward along the path B2 by the converging magnetic field of the first thermally stimulated magnetic body. If the magnetic force of the second thermally stimulated magnetic body remained unchanged, the beam would land following the path Bf indicated by a dotted line and a displacement f would be generated, but since the magnetic force of the

second thermally stimulated magnetic body decreases to about 8 gauss by heating, the beam follows the approximately converged Bc path and lands on the screen S, thus compensating for heat drift.

This process works according to the same principle in the cases of FIG. 14, where the magnetic force of the first thermally stimulated magnetic body P4a is larger than that of the second thermally stimulated magnetic body P4f, and of FIG. 15, where the magnetic force of the first thermally stimulated magnetic body P4a is less than that of the second thermally stimulated magnetic body P4f. The physical properties of the first and second thermally stimulated magnetic bodies P4a and P4f are determined when the materials are selected. However, the compensation is determined by the relative magnetic forces of the two magnetic bodies P4a and P4f, not by their physical properties thereof.

Meanwhile, FIGS. 16 and 17 show, in comparison, the case where the first thermally stimulated magnetic body P4a is used all by itself. First in FIG. 16, the construction uses the method of increasing the OCV using a thermally stimulated magnetic body P4f with 4 or 6 poles and thereafter decreasing the degree of divergence with reduction in the magnetic force by heating.

In comparison, in FIG. 17, the OCV is previously reduced by the first thermally stimulated magnetic body P4a and thereafter divergence of OCV caused by the expansion of the electrode during heating is compensated for. Here, the thermal expansion coefficient of the first thermally stimulated magnetic body P4a is relatively small, and thus the problem of deteriorating beam characteristics practically does not occur.

Also, this compensation does not depend on the physical properties of the first thermally stimulated magnetic body P4a, but is a geometrical compensation where the heat expansion of the electrode has been previously considered, and an appropriate compensation can thus be expected.

As described above, in the present invention, the compensation

for heat drift is made by the physical and geometrical interaction of two different thermally stimulated magnetic bodies, not just by the physical properties of the magnetic bodies, and thus, an excessive initial OCV is not required and heat drift can be compensated for sufficiently. While the conventional construction requires an OCV adjustment as wide as 8 mm and the compensation for heat drift is as small as 0.1 mm, the present invention can compensate for 0.3 mm of heat drift while the required OCV adjustment is within 5 mm. Therefore, the required magnetic force is not large, thus enabling fine adjustment within 0.05 mm, and it is possible to adjust an OCV in the range of 0.05-5 mm.

As a result, the present invention provides a convergence device which can completely compensate for heat drift so that the convergence characteristics are not deteriorated, and it is easy to adjust the convergence.