Electric lamp and discharge devices – Cathode ray tube – Plural beam generating or control
Reexamination Certificate
1998-09-09
2001-11-06
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Plural beam generating or control
C313S412000, C313S413000
Reexamination Certificate
active
06313575
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to a color picture tube, and more particularly to a color picture tube in which an electron gun having a large-diameter main lens is mounted.
BACKGROUND ART
In general, a color picture tube is constructed to display a color image by scanning a phosphor screen in horizontal and vertical directions by a plurality of electron beams emitted from an electron gun.
An example of the electron gun applied to the color picture tube is an inline electron gun, which emits three electron beams in line: a center beam and a pair of side beams on both sides of the center beam, all traveling on the same horizontal plane. A main lens portion of the electron gun is constituted by grids. The center axes of side beam passage holes, through which side beams pass in a grid on the higher voltage side of all the grids, are decentered to outer sides than those of a grid on the lower voltage side. In other words, the center axes of side beam passage holes in a grid on the higher voltage side are located on outer portions, which apart from center beam, than those of a grid on the lower voltage side. As a result, the three electron beams are converged in a central portion of the screen. The three electron beams aligned in line can be self-converged in all the region of the screen, if a deflection field is pincushion-shaped in the horizontal direction, i.e., the inline direction in which the electron beams are aligned, and barrel-shaped in the vertical direction, i.e., the direction perpendicular to the inline direction.
Of the electron guns used in the color picture tube, a type of electron gun for improving the focus characteristic in all the region of the screen is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 64-38947. The electron gun is called an extended field type electron gun, which comprises a number of focusing grids, and in which part of an anode voltage is resistance-divided by a resistor arranged within a neck of the color picture tube, so that the divided voltages can be supplied to the grids, thereby forming a large-diameter main lens having a long focus by moderate potential distribution.
FIGS. 1A and 1B
show an example of the extended field type electron gun.
As shown in
FIG. 1A
, the electron gun comprises three cathodes KB, KG and KR aligned in line, each incorporating a heater (not shown) for emitting electron beams. The electron gun further comprises a first grid
10
, a second grid
20
, a third grid
30
, a fourth grid
40
, a fifth grid
50
, a plurality of intermediate electrodes
70
and
80
, a sixth grid
60
and a convergence cup
90
. These components are arranged in this order in the direction of travel of the electron beams and supported and fixed to each other by an insulating support (not shown).
A resistor
100
is provided near the electron gun, as shown in FIG.
1
B. One end
110
of the resistor
100
is connected to the sixth grid
60
, while the other end
120
is grounded. Intermediate points
130
and
140
are respectively connected to the intermediate electrodes
70
and
80
. The end
110
of the resistor
100
is also connected to an operation voltage supplying device
131
.
As shown in
FIGS. 1A and 1B
, the first grid
10
is a thin plate electrode having three beam passage holes of a small diameter to allow passage of electron beams. The second grid
20
is also a thin plate electrode having three beam passage holes of a small diameter to allow passage of electron beams.
The third grid
30
is formed of two cup electrodes
31
and
32
, open ends of which are joined together. The cup electrode
31
, on the side of the second grid, has three beam passage holes having a diameter slightly larger than that of the beam passage holes formed in the second grid
20
. The cup electrode
32
, on the side of the fourth grid, has three beam passage holes having a large diameter.
The fourth grid
40
is formed of two cup electrodes
41
and
42
, open ends of which are joined together. Each of the cup electrodes
41
and
42
has three beam passage holes, having a large diameter. The fifth grid
50
is formed of a plurality of cup electrodes
51
,
52
,
53
and
54
, each having three beam passage holes having a large diameter.
The intermediate electrodes
70
and
80
are thick plate electrodes, each having three beam passage holes of a large diameter. The sixth grid
60
is formed of two cup electrodes
61
and
62
, open ends of which are joined together. Each electrode has three beam passage holes having a large diameter. The convergence cup
90
is fixed to the bottom of the cup electrode
62
.
A DC voltage of about, for example, 100 to 150V and a modulation signal corresponding to an image superimposed thereon are applied to the three cathodes KB, KG and KR. The first grid
10
is grounded. The second and fourth grids
20
and
40
are connected to each other in the tube, and a DC voltage of about 600 to 800V is applied thereto.
The cathodes KB, KG and KR, the first grid
10
and the second grid
20
constitute a triode. The triode emits electron beams and forms a crossover.
The third and fifth grids
30
and
50
are connected to each other in the tube, and a focusing voltage of about 6 to 9 kv is applied thereto. An anode voltage of about 25 to 30 kV is applied to the sixth grid
60
.
The second and third grids
20
and
30
form a prefocus lens, which preliminarily focuses the electron beams emitted from the triode.
The third, fourth and fifth grids
30
,
40
and
50
form an auxiliary lens, which further preliminarily focuses the electron beams output from the prefocus lens.
A voltage corresponding to about 40% of the anode voltage is applied to the intermediate electrode
70
by the resistor
100
provided near the electron gun. A voltage corresponding to about 65% of the anode voltage is applied to the intermediate electrode
80
. Thus, the voltages of the values substantially intermediate between the voltages, applied to the fifth and sixth grids
50
and
60
, are applied to the intermediate electrodes
70
and
80
.
In the above structure, the fifth grid
50
, the intermediate electrodes
70
and
80
and the sixth grid
60
form a main lens, which finally focuses the electron beams on the screen. The main lens, having a main lens region extended by the intermediate electrodes
70
and
80
, is called an extended field lens.
The side beam passage holes formed in the intermediate electrode
80
and the cup electrode
61
are decentered to outer sides from the center axes of the holes. Therefore, the side beams are deflected toward the center beam, with the result that the three electron beams are converged substantially at the center of the screen.
In the conventional electron gun of the extended field type, the main lens formed of the fifth grid
50
, the intermediate electrodes
70
and
80
and the sixth grid
60
has a large diameter, so that the focusing performance in the all region of the screen is greatly improved. However, while the color picture tube is operating, the current tends to leak from the resistor
100
arranged within the neck. Since no measures to cope with the current leak are taken in the conventional electron gun, if the current leaks, the voltages applied to the intermediate electrodes
70
and
80
are unstable, resulting in change in the focusing characteristic of the main lens. If the focusing characteristic changes, a so-called convergence characteristic, for converging the three electron beams onto one point, also varies on the phosphor screen side.
DISCLOSURE OF INVENTION
The present invention has been made to solve the above problems, and its object is to provide a color picture tube which prevents a change in convergence characteristic due to a current leaked from the resistor arranged in the neck during an operation of the color picture tube, so that a stable and satisfactory convergence characteristic can be obtained in the overall screen.
According to an aspect of the present invention, there is provided a color picture tube comprising: an
Hoshino Fumitaka
Kimiya Junichi
Sugawara Shigeru
Day Michael H.
Gerike Matthew J.
Kabushiki Kaisha Toshiba
Pillsbury & Winthrop LLP
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