Color picture tube having inline electron gun with coma correction members

The present invention provides an improvement in a color picture tube having an inline electron gun for generating and directing three inline electron beams, comprising a center beam and two outer beams, along initially coplanar paths toward a screen of the tube. The beams pass through a deflection zone adapted to have two orthogonal magnetic deflection fields established therein. A first of the fields causes deflection of the beams in a first direction perpendicular to the inline direction of the beams, and a second of the fields causes deflection in a second direction parallel to the inline direction of the beams. The gun includes two shunts for shunting portions of both deflection fields around the outer beam paths. Each shunt comprises one magnetically permeable member having an aperture therein. Each shunt completely surrounds one of the electron beam paths. The improvement comprises each shunt being longer in the first direction than in the second direction and being symmetric about a central axis of the shunt that parallels the first direction and symmetric about another central axis of the shunt that parallels the second direction.

BACKGROUND OF THE INVENTION 
The present invention relates to a color picture tube having an improved 
inline electron gun, and particularly to an improvement in the gun for 
obtaining equal raster sizes (also called coma correction) within the 
tube, without severely distorting the electron beams. 
An inline electron gun is one designed to generate or initiate preferably 
three electron beams in a common plane and direct those beams along 
convergent paths to a point or small area of convergence near the tube 
screen. 
A problem that exists in a color picture tube having an inline gun is coma 
distortion, wherein the sizes of the electron beam rasters scanned on the 
screen by an external magnetic deflection yoke are different because of 
the eccentricity of the positions of the two outer beams with respect to 
the center of the yoke. This coma problem has been solved in the prior art 
by including various shaped magnetically permeable members adjacent to or 
around the electron beam paths in a fringe portion of the yoke deflection 
field. For example, Hughes, U.S. Pat. No. 3,873,879, issued Mar. 25, 1975, 
teaches the use of small disc-shaped enhancement elements above and below 
the center beam and ring or washer-shaped shunts around the two outer 
beams. The enhancement elements concentrate the vertically extending 
horizontal deflection field lines at the center beam path. The shunts 
completely surround the outer beams and bypass fringe portions of both 
vertical and horizontal deflection fields around the outer beams. The 
shunts also concentrate the horizontally extending vertical deflection 
field at the center beam path, thereby enhancing the vertical deflection 
of the center beam. If further enhancement of the vertical deflection of 
the center beam is required, the outer diameter of the washer-shaped 
shunts can be enlarged to collect more of the vertical deflection field. 
However, there is a limit to the maximum size shunt diameter. If the 
shunts are made too large, they will begin to extend into the area of the 
center beam. Recently, a yoke has been developed that requires a very 
large vertical coma correction. It is not possible to use washer-shaped 
shunts to provide the required coma correction, because the shunts would 
overlap the center beam. Although coma correction could be provided by the 
use of other types of shunts, such as C-shaped shunts or D-shaped shunts, 
the lack of symmetry of such shunts can severely distort the electron 
beams. Therefore, there is a need for a shunt design that will provide the 
large coma correction required by the new yoke and will not severely 
distort the electron beams. 
SUMMARY OF THE INVENTION 
The present invention provides an improvement in a color picture tube 
having an inline electron gun for generating and directing three inline 
electron beams, comprising a center beam and two outer beams, along 
initially coplanar paths toward a screen of the tube. The beams pass 
through a deflection zone adapted to have two orthogonal magnetic 
deflection fields established therein. A first of the fields causes 
deflection of the beams in a first direction perpendicular to the inline 
direction of the beams, and a second of the fields causes deflection in a 
second direction parallel to the inline direction of the beams. The gun 
includes means for shunting portions of both deflection fields around at 
least one beam path. The shunting means comprises at least one 
magnetically permeable shunt having an aperture therein. The shunt 
completely surrounds one of the electron beam paths. The improvement 
comprises the shunt being longer in the first direction than in the second 
direction and being symmetric about a central axis of the shunt that 
parallels the first direction and symmetric about another central axis of 
the shunt that parallels the second direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a plan view of a rectangular color picture tube 10 having a glass 
envelope comprising a rectangular faceplate panel or cap 12 and a tubular 
neck 14 connected by a rectangular funnel 16. The panel comprises a 
viewing faceplate 18 and a peripheral flange or sidewall 20 which is 
sealed to the funnel 16. A three-color phosphor screen 22 is carried by 
the inner surface of the faceplate 18. The screen 22 is preferably a line 
screen with the phosphor lines extending substantially perpendicular to 
the high frequency raster line scan of the tube (normal to the plane of 
FIG. 1). A multi-apertured color-selection electrode or shadow mask 24 is 
removably mounted, by conventional means, in predetermined spaced relation 
to the screen 22. An improved inline electron gun 26, shown schematically 
by dotted lines in FIG. 1, is centrally mounted within the neck 14 to 
generate and direct three electron beams 28 along initially coplanar 
convergent paths through the mask 24 to the screen 22. 
The tube of FIG. 1 is designed to be used with an external magnetic 
deflection yoke 30, such as the self-converging yoke, shown surrounding 
the neck 14 and funnel 12 in the neighborhood of their junction. When 
activated, the yoke 30 subjects the three beams 28 to vertical and 
horizontal magnetic flux which causes the beams to scan horizontally and 
vertically, respectively, in a rectangular raster over the screen 22. The 
initial plane of deflection (at zero deflection) is shown by the line P-P 
in FIG. 1 at about the middle of the yoke 30. Because of fringe fields, 
the zone of deflection of the tube extends axially, from the yoke 30 into 
the region of the electron gun 26. For simplicity, the actual curvature of 
the deflected beam paths in the deflection zone is not shown in FIG. 1. 
The details of the electron gun 26 are shown in FIGS. 2 and 3. The gun 26 
comprises two glass support rods 32 on which the various electrodes are 
mounted. These electrodes include three equally spaced coplanar cathodes 
34 (one for each beam), a control grid electrode 36 (G1), a screen grid 
electrode 38 (G2), a first accelerating and focusing electrode 40 (G3), 
and a second accelerating and focusing electrode 42 (G4), spaced along the 
glass rods 32 in the order named. Each of the G1 through G4 electrodes has 
three inline apertures therein to permit passage of three coplanar 
electron beams. The main electrostatic focusing lens in the gun 26 is 
formed between the G3 electrode 40 and the G4 electrode 42. The G3 
electrode 40 is formed with four cup-shaped elements 44, 46, 48 and 50. 
The open ends of two of these elements, 44 and 46, are attached to each 
other, and the open ends of the other two elements, 48 and 50, are also 
attached to each other. The closed end of the third element 48 is attached 
to the closed end of the second element 46. Although the G3 electrode 40 
is shown as a four-piece structure, it could be fabricated from any number 
of elements, including a single element of the same length. The G4 
electrode 42 also is cup-shaped, but has its open end closed with an 
apertured plate 52. A shield cup 53 is attached to the plate 52 at the 
exit of the gun 26. 
The facing closed ends of the G electrode 40 and the G4 electrode 42 have 
large recesses 54 and 56, respectively, therein. The recesses 54 and 56 
set back the portion of the closed end of the G3 electrode 40 that 
contains three apertures 60, (center aperture shown), from the portion of 
the closed end of the G4 electrode 42 that contains three apertures 66, 
(center aperture shown). The remaining portions of these closed ends of 
the G3 electrode 40 and the G4 electrode 42 from rims 70 and 72, 
respectively, that extend peripherally around the recesses 54 and 56. The 
rims 70 and 72 are the closest portions of the two electrodes 40 and 42. 
Located on the bottom of the shield cup 53 are two magnetically permeable 
coma correction members or shunts 74 and 76. The bottom of the shield cup 
53 includes three apertures, 82, 84 and 86, through which the electron 
beams pass. The centers of the undeflected electron beam paths are 
designated R, G and B. The R and B paths are the outer beam paths, and the 
G path is the center beam path. 
FIG. 3 shows the shunts 74 and 76 in greater detail. Each shunt is a flat 
plate having a rectangular outer periphery and a square, centered aperture 
78 therein. Two of the sides of the aperture 78 are parallel to the inline 
direction of the inline electron beams, and two of the sides are 
perpendicular to the inline direction of the inline electron beams. The 
shunts 74 and 76 are centered on the two outer or side apertures 82 and 86 
in the shield cup 53. 
Typical dimensions for the shunts 74 and 76 when used in an inline electron 
gun having a center-to-center aperture spacing of 5.08 mm (200 mils) are 
as follows. 
Outside Dimensions 
6.045 mm.times.7.620 mm 
(238 mils.times.300 mils) 
Aperture Dimensions 
4.115 mm.times.4.115 mm 
(162 mils.times.162 mils) 
Thickness 
0.508 mm (20 mils) 
The use of shunts with the foregoing dimensions can provide a raster 
correction of approximately 46 mm/gauss. 
The shunts 74 and 76 have certain common characteristics that are shared 
with other shunt embodiments to be described hereinafter. Each shunt is 
longer in a direction perpendicular to the inline direction of the 
electron beams than it is in the inline direction of the electron beams. 
Each shunt is symmetrical about both a vertical axis and a horizontal 
axis, and the shunts do not overlap the center aperture in the shield cup. 
FIGS. 4 and 5 show the effect that the shunts 74 and 76 have on the 
horizontal and vertical deflection fields, respectively. In FIG. 4, the 
vertically extending field lines of magnetic flux of the horizontal 
deflection field H are attracted by the shunts 74 and 76, and most of the 
lines are bypassed around the two outer beams R and B. The shunts 74 and 
76 also bypass a portion of the horizontal deflection field around the 
center beam G. In FIG. 5, the horizontally extending field lines of 
magnetic flux of the vertical deflection field V are attracted by the 
shunts 74 and 76, and most of the lines bypass around the two outer beams 
R and B. At the center beam G, the shunts concentrate or enhance the lines 
of flux. However, because of the straight facing sides of shunts 74 and 
76, the lines of flux are evenly distributed in the area of the center 
beam G. Although most of the magnetic flux lines are bypassed around the 
outer beams, some flux lines also pass through the apertures of the 
shunts. The shapes of these flux lines within the aperture are, to some 
extent, affected by the shape of the apertures in the shunts. Since the 
shapes of the flux lines can distort an electron beam, it is important to 
utilize an aperture in the shunt that is both vertically and horizontally 
symmetrical. Square, rectangular and circular shunt apertures have been 
found to be approximately equally effective. 
FIG. 6 shows a second embodiment of coma correction members or shunts 88 
and 90. Each shunt has a rectangular outer periphery and a rectangular, 
nonsquare, centered aperture 92 therein. The short sides of the aperture 
92 parallel the inline direction of the inline electron beams. The shunts 
88 and 90 perform their coma correction function essentially as do the 
shunts 74 and 76. However, because of the shape of the rectangular 
apertures therein, which are narrower horizontally but longer vertically, 
the horizontal flux lines in the apertures are straighter at the beam 
paths, and the number of vertical flux lines at the beam paths are 
slightly reduced. Such tailoring of aperture shape can be used as a 
trimming technique to compensate for minor variations in electron beam 
distortion. 
FIG. 7 shows a third embodiment of shunts 94 and 96. These shunts 94 and 96 
have the same external rectangular configuration as the foregoing shunts 
but include circular apertures 98. It has been found that the effect of 
the circular apertures on electron beam quality is very close to that of 
the square apertures. 
FIG. 8 shows a fourth embodiment of shunts 100 and 102. These shunts 100 
and 102 have oval external peripheries and circular apertures 104. The 
shunts 100 and 102 collect the same amount of horizontal flux lines, but, 
since they are closer at the center beam, concentrate more of the vertical 
deflection field (horizontal flux lines) at the center beam G.