Cathode ray tube having reduced convergence drift

Information

  • Patent Grant
  • 6515411
  • Patent Number
    6,515,411
  • Date Filed
    Monday, October 16, 2000
    24 years ago
  • Date Issued
    Tuesday, February 4, 2003
    21 years ago
Abstract
A cathode ray tube having a reduced variation in convergence drift. An inner graphite layer coats the inner surface of a funnel, and a metal coating layer is electrically connected with the inner graphite layer on the inner surface of the neck portion. The metal coating layer does not extend to a position directly beside the focusing electrode most remote from the cathode of the electron gun of the cathode ray tube, and has a surface resistivity of 107Ω/□ or less.
Description




BACKGROUND OF THE INVENTION




1. Field of the Invention




The present invention relates to a cathode ray tube (CRT), and more particularly, to a CRT having reduced convergence drift, including a metal coating layer electrically connected to a built-in graphite layer on the inner surface of a neck portion.




2. Description of the Related Art




In general, if power is applied to a CRT, an electron gun emits electron beams from a cathode. The emitted electron beams pass through electron beam apertures of a plurality of electrodes are focused and accelerated. The accelerated electron beams are selectively deflected by a deflection yoke installed in the cone portion of a bulb and excite a phosphor layer coated on the inner surface of a panel which forms a screen, thereby producing a picture.




As shown in

FIG. 1

, a conventional CRT


10


includes a panel having a phosphor layer on its inner surface, a funnel


12


sealed in the panel


11


, and a shadow mask


13


inwardly spaced from the panel


11


.




The shadow mask


13


is coupled to a shadow mask frame


14


. The shadow mask frame


14


is fixedly positioned to a stud pin


15


on the inner surface of the panel


11


and a hook spring


16


connected to the stud pin


15


. Accordingly, the position of the shadow mask


13


in the panel


11


is determined.




An electron gun


17


for generating electron beams producing red (R), green (G) and blue (B) light, is inserted into a neck portion


12




a


of the funnel


12


. A deflection yoke


18


for deflecting the electron beams, is installed in a cone portion


12




b


of the funnel


12


.




An inner graphite layer


19


and an outer graphite layer


100


coated on inner and outer surfaces of the funnel


12


, respectively, and thus a high voltage applied to an anode can be stabilized by forming a condenser using the glass funnel


12


as an insulator.




As known very well, the electron gun


17


includes a triode consisting of a cathode, a control electrode and a screen electrode, a plurality of focusing electrodes opposed to the screen electrode, for forming a pre-focusing lens unit, and a final accelerating electrode opposed to the focusing electrodes, for forming a main focusing lens unit.




A shield cup


110


is installed in front of the electron gun


17


. A plurality of bulb spacers


120


are fixed on the outer circumference of the shield cup


110


. The bulb spacers


120


elastically contact the inner graphite layer


19


to supply a positive voltage to the final accelerating electrode.




The CRT


10


must optimize the convergence characteristic by which R, G and B electron beams emitted from the electron gun


17


converge onto a point throughout a screen, inclusive of the center and corners of the screen. In the CRT


10


, when the electron beams are deflected, they may be shifted from their proper positions, a phenomenon which is called convergence drift.




The convergence drift is divided into thermal drift and charge drift. Specifically, the charge drift is caused by a change in the potential of the neck portion


12




a


due to the condition of the outer surface of the neck portion


12




a


when a high voltage is applied to the CRT


10


. The initial potential of the neck portion


12




a


is attributed to accumulation of charge due to electron beam current, causing an increase in the convergence error.




To overcome the problem, U.S. Pat. No. 4,868,454 discloses a method of stabilizing the potential of the surface of the neck portion with a metallic mirror coating on the inner surface of the neck portion. However, according to this method, convergence drift is 0.2 mm or greater, that is, the effect of removing charge is weak and occurrence of arcing is highly probable.




U.S. Pat. No. 5,536,997 discloses that an enamel layer electrically contacting a conductive layer coating the inner surface of a neck portion. The formation of the enamel layer relatively reduces convergence drift. However, this method has the following problems. First, the process of manufacturing a CRT is relatively complex. In other words, a conductive layer made of graphite is applied to the inner surface of the neck portion and dried. Then, an enamel glass solution is placed in contact with the conductive layer. During this procedure, the conductive layer and the enamel layer are electrically connected. Thus, forming the enamel layer is further necessary. Second, the arcing characteristic is poor. In the course of sealing the electron gun into the neck portion, it contacts the inner surface of the neck portion. Here, since an enamel layer having a predetermined thickness is present on the inner surface of the neck portion, when the bulb spacer is mounted at a proper position it scratches the enamel layer, which provides for a path for discharge. Also, the particles of the scratched enamel layer float between the focus electrode and the final accelerating electrode, resulting in a discharge between electrodes.




SUMMARY OF THE INVENTION




To solve the above problems, it is an object of the present invention to provide a CRT which can definitely reduce convergence drift with a metal coating layer on the inner surface of a neck portion.




Accordingly, to achieve the above object, there is provided a cathode ray tube having reduced convergence drift including a panel having a phosphor layer on its inner surface, a funnel sealed in the panel and having an inner graphite layer and an outer graphite layer on inner and outer surfaces, respectively, an electron gun in a neck portion of the funnel and consisting of a cathode, a control electrode, a screen electrode, a plurality of focusing electrodes, a final accelerating electrode and a shield cup, and a metal coating layer electrically connected with the inner graphite layer on the inner surface of a neck portion and having the surface resistivity of 10


7


Ω/□ or less.




Here, the metal coating layer is preferably formed on the inner surface of the neck portion higher than the top surface of the focusing electrodes.




Also, the metal coating layer is preferably electrically connected with the final accelerating electrode via the shield cup.




Also, the metal coating layer may be selectively formed on the inner surface of the neck portion adjacent to side electron beam apertures for red and blue electron beams.




Further, the metal coating layer is preferably a metal thin film having either iron or chrome as a main component. Also, the metal coating layer may be a metal thin film having both iron and chrome as main components.











BRIEF DESCRIPTION OF THE DRAWINGS




The above object and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which:





FIG. 1

is a cross section of a conventional CRT;





FIG. 2

is a cross-sectional view illustrating a neck portion of a CRT according to the present invention; and





FIG. 3

is a graph illustrating the convergence drift according to variation in the resistivity of the inner surface of the neck portion of a CRT according to the present invention.











DESCRIPTION OF THE PREFERRED EMBODIMENT




In

FIG. 2

showing the structure of a neck portion


20


of a CRT according to the present invention, an electron gun


21


is installed inside the neck portion


20


.




The electron gun


21


includes a triode consisting of a cathode


22


, a control electrode


23


and a screen electrode


24


. A plurality of first and second focusing electrodes


25


and


26


opposed to the screen electrode


24


for forming a pre-focusing lens unit, are disposed in front of the screen electrode


24


. A final accelerating electrode


27


opposed to the second focusing electrode


26


for forming a main focusing lens unit, is disposed in front of the second focusing electrode


26


. A shield cup


28


is installed in front of the final accelerating electrode


27


. A plurality of bulb spacers


29


are fixed on the outer circumference of the shield cup


28


.




The respective electrodes are supported by a bead glass


200


parallel to both internal sides of the neck portion


20


. Leads


210


for applying voltages to the respective electrodes are installed in the lower portion of the cathode


22


. The leads


210


are drawn outside the neck portion


20


and supported by a stem


220


. The stem


220


is sealed when the electron gun


21


is in the neck portion


20


.




A deflection yoke


230


is installed in the cone portion of the neck portion


20


so that electron beams from the electron gun


21


are deflected to the phosphor layer on the inner surface of a panel.




An electrically conductive inner graphite layer


240


coats on the inner surface of the neck portion


20


. The inner graphite layer


240


reaches a position adjacent to the shield cup


28


. An outer graphite layer


250


coats the outer surface of the neck portion


20


. The inner and outer graphite layers


240


and


250


form a condenser using the funnel made of glass as an insulator, thereby stabilizing the high voltage applied to the anode.




The bulb spacers


29


elastically contact the inner graphite layer


240


. The bulb spacers


29


receives the high voltage applied to the anode and transmit the received high voltage to the final accelerating electrode


27


through the shield cup


28


. The shield cup


28


serves to correct minute convergence of the corner of a screen by adjusting the movement of an electron beam when the electron beam emitted from the cathode


22


passes through the electron beam apertures of the respective electrodes and lands on the phosphor layer of the panel.




Here, a metal coating layer


260


coats the inner surface of the neck portion


20


for purpose of preventing the convergence drift of R and B electron beams to the corner of the screen due to accumulation of positive charge at a main focusing lens unit between the second focusing electrode


26


and the final accelerating electrode


27


.




The metal coating layer


260


overlaps with and is electrically connected to the inner graphite layer


240


, and extends lengthwise by a predetermined length with respect to the neck portion


20


. The predetermined length preferably does not extend to be directly beside of the second focusing electrode


26


in order to reduce a discharge possibility.




Also, the metal coating layer


260


may be present only on the inner surface of the neck portion


20


adjacent to side apertures for R and B electron beams, among R, G and B electron beam apertures formed in-line on the same plane as that of the final accelerating electrode


27


.




The metal coating layer


260


is made of metal such as nickel or chrome a thickness of several micrometers or less. Here, the metal coating layer


260


preferably has surface resistivity of 10


7


Ω/□ or less for the purpose of attaining stable charge drift.




Although the metal coating layer


260


may be formed by inserting a separate metal material into the neck portion


20


, the electron gun


21


is preferably used for facilitating assembling work. In other words, the metal coating layer


260


may be formed by evaporating some of components contained in the material of the electron gun


21


on the inner surface of the neck portion


20


through local inductive heating.




Alternatively, the metal coating layer


260


may be electrically connected with the final accelerating electrode


27


via the shield cup


28


, or may be electrically connected with the final accelerating electrode


27


using a connection means, e.g., a conductive wire


270


.




The process of forming the metal coating layer


260


will now be described in more detail.




In evacuating a CRT, a bombardment step is necessarily further provided for removing foreign matter adsorbed into the electrodes and for preventing adsorption of the gas produced during decomposition of the cathode


22


from being adsorbed into the electrodes. In other words, an induction coil is placed outside the neck portion


20


at a position corresponding to the electron gun


21


. Then, if high-frequency inductive heating is applied, the surfaces of the electrodes of the electron gun


21


are heated to 700 to 1000° C. for several seconds. Accordingly, the foreign matter is burnt and the gas adsorbed into the electrodes is evacuated.




The metal coating layer


260


, can be formed during the bombardment step. In other words, the induction coil is placed outside the neck portion


20


corresponding to the final accelerating electrode


27


and the shield cup


28


.




Subsequently, if local inductive heating is applied, metal components contained in the electron gun


21


coat the inner surface of the neck portion


20


due to high-frequency inductive heating. Here, in the electron gun


21


made of stainless steel containing 14 wt % of chrome and 16 wt % of nickel, the chrome and nickel coat the inner surface of the neck portion


20


in the form of a thin film having a thickness of several micrometers. A metal coating layer made of either nickel or chrome, or a multiple metal coating layer made of nickel and chrome may be formed on the inner surface of the neck portion


20


according to the heating temperature. Also, an iron alloy containing 42 wt % of nickel can be used as the material for electrodes. In this case, the nickel is inductively heated to a temperature in the range in which it can coat the inner surface of the neck portion


20


. Otherwise, the metal coating layer


260


may be formed by installing a separate metal element within the neck portion


20


.




Also, the metal coating layer


260


must be restricted in position on the inner surface of the neck portion


20


. In other words, the metal coating layer


260


must not be directly beside the second focusing electrode


26


, which is the final focusing electrode, in order to reinforce non-arcing characteristics.




In order to prevent discharging, a sufficient distance between the metal coating layer


260


and the main focusing lens unit must be provided. To this end, the distance between the inner surface of the neck portion


20


and the outer surface of the second focusing electrode


26


is preferably larger than the distance between the second focusing electrode


26


and the final accelerating electrode


27


, constituting the main focusing lens unit. This is for solving the problem caused by arcing by forming a discharge space first between the second focusing electrode


26


and the final accelerating electrode


27


, even if the resistance of the metal coating layer


260


is low.




In the aforementioned CRT according to the present invention, the R and B electron beams drift to the corner due to accumulation of positive charges in the neck portion


20


, caused by electron beam current during an initial stage of applying a high voltage, resulting in a change in the convergence for several hours.




Here, the metal coating layer


260


is electrically connected with the inner graphite layer


240


to which a positive electrode voltage is applied, thereby reducing accumulation of positive charges to reduce a convergence drift.




FIG.


3


and Table 1 show convergence drifts depending on a variation in the resistivity of the inner surface of the neck portion


20


according to the present invention.




According to the experiment carried out by the inventor, the metal coating layer


260


formed on the inner surface of the neck portion


20


not extending to a location directly beside the second focusing electrode


26


, by a high-frequency inductive heating method using the electron gun


21


as a target. Also, the convergence drift of the metal coating layer


260


was measured by evaporating the metal coating layer


260


so as to have different levels of resistance. The distance between R and B electron beams was measured for 24 hours after 30 minutes from the time when the power is applied, i.e., after the thermal convergence drift is stabilized.















TABLE 1












Convergence drift







Resistivity α/□ (mm)



























10


0






0.07







10


1






0.07







10


2






0.07







10


3






0.071







10


4






0.074







10


5






0.08







10


6






0.09







10


7






0.10







10


8






0.118







10


9






0.138







10


10






0.16







10


11






0.185







10


12






0.215







10


13






0.25







10


14






0.285







10


15






0.32















In the graph shown in

FIG. 3

, the x-axis indicates the surface resistivity of the inner surface of the neck portion


20


, and the y-axis indicates the convergence drift.




Referring to FIG.


3


and Table 1, when the metal coating layer


260


is not formed, the surface resistivity of the inner surface of the neck portion


20


is approximately 10


13


to 10


17


Ω/□. If the metal coating layer


260


has a surface resistivity of approximately 10


12


Ω/□, the convergence drift exceeds 0.2 mm. In general, a high-resolution CRT requires the convergence drift of 0.1 mm or less.




When the metal coating layer


260


has the surface resistivity of 10


7


Ω/□, the cvonvergence drift is 0.1 mm, that is, the potential stability increases. In other words, as the metal coating layer


260


has the surface resistivity of 10


7


Ω/□ or less, the convergence drift correcting effect is stabilized. Thus, in order to achieve stability of a charge drift, the surface resistivity of the metal coating layer


260


is preferably maintained at the level of 10


7


Ω/□ or less. On the other hand, when the metal coating layer


260


has a surface resistivity of 10


8


Ω/□ or greater, the potential stability decreases. Also, when the metal coating layer


260


has a surface resistivity of 10


12


Ω/□ or greater, the charge drift correcting effect is noticeably lowered.




As described above, in the CRT having a reduced convergence drift according to the present invention, the variation in the convergence drift can be minimized by maintaining a stable neck potential, with a metal coating layer on the inner surface of a neck portion, electrically connected to an inner graphite layer, and electrically connecting the metal coating layer to the final accelerating electrode.




Although the present invention has been described with reference to illustrative embodiment, the invention is not limited thereto and various changes and modifications may be effected by one skilled in the art. It is therefore contemplated that the true spirit and scope of the present invention be set forth in the appended claims.



Claims
  • 1. A cathode ray tube having reduced convergence drift comprising:a panel having a phosphor layer on an inner surface of the panel; a funnel sealed to the panel and having an inner graphite layer and an outer graphite layer on inner and outer surfaces, respectively of the funnel; an electron gun in a neck portion of the funnel and including of a cathode, a control electrode, a screen electrode, a plurality of focusing electrodes, a final accelerating electrode, and a shield cup; and a metal coating layer electrically connected with the inner graphite layer on an inner surface of the neck portion and having a surface resistivity of no more than 107Ω□.
  • 2. The cathode ray tube according to claim 1, wherein the metal coating layer on the inner surface of the neck portion does not extend to a position directly beside a surface of the focusing electrodes.
  • 3. The cathode ray tube according to claim 1, wherein the metal coating layer is only present on the inner surface of the neck portion adjacent to side electron beam apertures for electron beams producing red and blue light on the panel.
  • 4. The cathode ray tube according to claim 1, wherein the metal coating layer has nickel as a main component.
  • 5. The cathode ray tube according to claim 1, wherein the metal coating layer has chrome as a main component.
  • 6. The cathode ray tube according to claim 1, wherein the metal coating layer has nickel and chrome as main components.
  • 7. The cathode ray tube according to claim 1, wherein the metal coating layer is formed on the inner surface of the neck portion using the electron gun as a target.
  • 8. The cathode ray tube according to claim 7, wherein the metal coating layer is formed by high-frequency inductive heating.
  • 9. The cathode ray tube according to claim 1, wherein the metal coating layer is electrically connected to the final accelerating electrode via the shield cup.
  • 10. The cathode ray tube according to claim 1, wherein the metal coating layer is electrically connected to the final accelerating electrode via a conductive wire.
Priority Claims (1)
Number Date Country Kind
99-45311 Oct 1999 KR
US Referenced Citations (6)
Number Name Date Kind
4052641 Dominick et al. Oct 1977 A
4101803 Retsky et al. Jul 1978 A
4153857 Delsing et al. May 1979 A
4403170 Misono et al. Sep 1983 A
4868454 Paridaens Sep 1989 A
5536997 Van Hout Jul 1996 A
Foreign Referenced Citations (1)
Number Date Country
6-260112 Sep 1994 JP