Electric lamp and discharge devices – Cathode ray tube – Shadow mask – support or shield
Reexamination Certificate
2000-01-27
2001-08-21
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Shadow mask, support or shield
C313S402000, C313S404000, C313S406000, C313S408000, C313S461000, C313S47700R
Reexamination Certificate
active
06278232
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a color cathode ray tube, and particularly to a color cathode ray tube in which the effective region of a shadow mask is enlarged and the magnetic shielding characteristic and the howling prevention characteristic for assembling the shadow mask are improved.
A color cathode ray tube generally comprises a vacuum envelope (glass bulb) provided with a panel which is arranged to close the front side and on which a fluorescent screen is formed, a slender neck portion which is arranged at the rear side and in which an electron gun is mounted, and a funnel connecting the panel and the neck portion. A shadow mask assembly is fixed in front of the panel inside the envelope; a magnetic shield is set in the tube in the vicinity of the joint between the panel and the funnel; and a deflection yoke is set to surround the tube at the joint between the funnel and the neck portion. Moreover, the shadow mask assembly includes a shadow mask having a surface curved toward the panel, and in which a plurality of electron beam passing holes are formed, and a skirt extending away from the margin of the curved surface, and a mask frame having a flange connected to a substantially rectangular side wall and to the magnetic shield, which is constituted by fitting and securing the skirt to the side wall. Moreover, the shadow mask assembly is secured in the panel so that the curved surface of the shadow mask faces the fluorescent screen formed on the front panel.
In the case of a color cathode ray tube having the above structure, three electron beams are emitted from three electron guns included in the neck portion of the envelope and the three beams are properly deflected by a magnetic field generated by the deflection yoke. Then, the three electron beams pass through the electron beam passing holes formed on the curved surface of the shadow mask and, thereafter, each of them is projected on a phosphor region having a corresponding color on the fluorescent screen. Thus, a multi-color image is displayed on the front panel of the color cathode ray tube.
FIG. 15
is a top view showing the structure of a shadow mask assembly of the type used for an already-known color cathode ray tube.
In
FIGS. 15 and 16A
and
16
B, symbol
40
generally denotes a shadow mask assembly,
41
generally denotes a shadow mask,
42
denotes a curved surface,
43
denotes a skirt,
44
generally denotes a mask frame,
45
denotes a side wall of the mask frame,
46
denotes a spring for supporting the shadow mask assembly in the panel of the tube envelope,
47
denotes a boss formed on the side wall of the mask frame and directed toward the inside of the frame, and
48
denotes a flange of the mask frame.
Moreover, the shadow mask
41
includes the curved surface
42
having a plurality of electron beam passing holes (not illustrated) and the skirt
43
extending from the margin of the curved surface
42
. The mask frame
44
has the side wall
45
and the flange
48
which are connected to each other or press-molded into a rectangular shape. The spring
46
for supporting the mask frame
44
in the panel is attached to the outside of the side wall
45
. Moreover, the mask frame has a plurality of bosses
47
for reinforcing the mechanical strength of the side wall
45
and for compensating for the clearance of the joint face between the side wall
45
and the skirt
43
. The skirt
43
of the shadow mask
41
is fitted to the side wall
45
of the mask frame
44
, and the skirt
43
and the side wall
45
are spot-welded to each other at several locations.
FIG. 16A
is a sectional view of a major side of the shadow mask shown in
FIG. 15
, and
FIG. 16B
is a sectional view of a corner of the shadow mask assembly shown in FIG.
15
. When the shadow mask
41
is press molded, a warp occurs due to springback because of the press-molding. The warp value SB of the skirt
43
usually tends to decrease at the corners with a relatively large degree of contraction due to press-molding and to increase at the central portion with a relatively small degree of contraction. The bottom margin of the skirt of the shadow mask
41
after being press-molded has a shape in which the middle of the side is slightly curved outward compared to the portion near a corner due to the warping. When fitting the skirt
43
of the shadow mask
41
into the side wall
45
of the mask frame
44
, there is a problem that the skirt
43
contacts the side wall
45
of the frame
44
at the middle of a side thereof, while a clearance
49
is produced between the skirt
43
of the shadow mask
41
and the mask frame
44
at a corner.
Moreover, the margin of the curved surface
42
is molded so that the middle of a side is slightly curved outward in accordance with the shape of the margin of the glass panel. That is, to fit the skirt
43
to the side wall
45
of the mask frame, the dimensions of various portions of the shadow mask
41
are set so that the middle of a side of the skirt
43
just meets the side wall
45
.
Moreover, when fitting the skirt
43
of the shadow mask
41
, which has been press-molded at the above dimensions, into the side wall
45
of the mask frame
44
, the clearance between the skirt
43
and the side wall
45
decreases at the middle of a side of the skirt
43
and increases at the corners of the skirt
43
. Therefore, as shown in
FIG. 15
, a boss
47
is formed at each corner of the side wall
45
so as to compensate for the increasing clearance. A boss
47
is also properly formed at the middle of a side of the side wall
45
in order to reinforce the mechanical strength of the side wall
45
.
Thus, the well-known shadow mask assembly
40
is molded so that the middle of a side of the margin of the curved surface
42
is slightly curved toward the outside when press-molding the shadow mask
41
. Therefore, the clearance between the skirt
43
and the side wall
45
increases at the corners of the skirt
43
. Moreover, problems occur in that it is necessary to form a boss
47
at each corner of the side wall
45
, so that the effective region of the curved surface
42
relative to the size of the mask frame
44
is narrowed by a value equivalent to the increase in the clearance.
Furthermore, in the case of the well-known shadow mask assembly
40
, the contact area between the skirt
43
and the side wall
45
is restricted to the arranged portion of the boss
47
at each corner. Therefore, the contact area between the skirt
43
and the side wall
45
is substantially small. In addition, because the contact area is small at the corners, there is a problem that an effective magnetic shielding characteristic cannot be obtained.
The present invention is made to solve the above problems and its object is to provide a color cathode ray tube comprising a shadow mask assembly which is capable of enlarging the effective region of the curved surface thereof to the size of the mask frame of the shadow mask assembly, thereby obtaining a superior magnetic shielding characteristic.
Though U.S. Pat. No. 4,308,485 discloses an example of the prior art, it does not suggest the present invention. Moreover, Japanese Utility Model Application No. 40942/1985 discloses a prior art arrangement, in which springback when forming a shadow mask is considered. However, neither the shape of the mask frame for holding the shadow mask nor the positional relation between the shadow mask and the mask frame are disclosed at all.
SUMMARY OF THE INVENTION
To achieve the above object, the present invention mainly comprises the following features.
(1) The side of the margin of the curved surface of a substantially rectangular shadow mask is set so that it is curved inward. That is, the side is set to a curvature which is convex to (toward) the tube axis of the cathode ray tube. Moreover, the substantially rectangular side wall of a mask frame is set so as to be linear or so as to have a curvature which is concave toward the tube axis of the cathode ray tube. Thereby, it is possible to obtain a sufficient cleara
Iwata Tsuyoshi
Maehara Mutsumi
Nose Hisashi
Antonelli Terry Stout & Kraus LLP
Haynes Mack
Hitachi , Ltd.
Patel Ashok
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