Shadow Mask Cathode Ray Tube Shield

This invention relates to cathode ray tubes of the shadow mask type and particularly to electron shield means therefor.

Shadow mask color cathode ray tubes usually include an envelope panel member comprising a faceplate and a peripheral side wall. A mosaic screen of phosphor dots is disposed on the inner surface of the faceplate. A multiapertured shadow mask electrode is mounted within the panel member adjacent to the phosphor screen and is surrounded by the side wall. The diameter of the apertured electrode is slightly smaller than that of the panel side wall so that a gap of about 1/2-inch exists between the periphery of the electrode and the side wall.

In normal operation of the tube, the electron beam is scanned in a rectangular raster over the shadow mask electrode and phosphor screen. In order to completely cover all portions of the screen, the electrode is over-scanned in some places. This results in electrons being projected into the gap between electrode and the side wall, and eventually onto the phosphor screen. This produces an undesirable luminescent glow around the periphery of the screen.

In accordance with prior art practices, a stiff sheet metal strip electron shield is welded to the mask electrode and extends to adjacent to the panel side wall to intercept the over-scanning electrons. If the stiff sheet metal electron shield is positioned so that it abuts the side wall, then when the tube is finish-processed by a final high temperature bake-out, the mask and shield expand (more than the glass panel) causing the shield to exert a force on the mask which distorts the mask and affects its alignment and spacing relative to the screen. If the stiff sheet metal electron shield is spaced from the side wall so as to prevent mask distortion during bake-out, then some electrons are projected through the space between the electron shield and the side wall and cause peripheral screen glow. Although such glow is indeed much less than when no shield at all is provided, it nevertheless is frequently sufficient to be objectionable.

It is an object of this invention to provide a shadow mask cathode ray tube having improved electron shield means.

This invention may be embodied in a shadow mask cathode ray tube having a shadow mask electrode comprising a generally rectangular frame across which is mounted a multi-apertured sheet metal masking member. According to the invention, an electron shield comprising a double-flanged strip of metal foil is fixed to the mask and completely blocks the gap between the mask and the panel side wall. The foil strip has a first flange extending along the strip at one edge thereof. This first flange is disposed transversely to the longitudinal axis of the cathode ray tube and against the mask frame. The strip may be secured to the mask frame by a plurality of tabs integral with the first flange which are crimped around the inner rim of the frame. A second flange extending along the strip at the other edge thereof is pressed outwardly against the panel side wall so that the foil strip is conformed closely to the side wall, thus blocking the gap between the mask and the side wall. The pressing can be performed, for example, by a person passing his finger around the side wall against the foil strip or by the use of a suitable tool operated in a similar manner.

In the drawings:

FIG. 1 is a longitudinal section of a shadow mask cathode ray tube embodying the invention;

FIG. 2 is a transverse section taken along line 2--2 of FIG. 1;

FIG. 3 is a perspective of one form of electron shield suitable for use in the tube of FIGS. 1 and 2;

FIGS. 4-18 are various enlarged elevation, plan, and section views of another form of electron shield useable in the tube of FIGS. 1 and 2; and

FIGS. 19 and 20 are enlarged sections of a portion of the tube of FIGS. 1 and 2 taken respectively along lines 19--19 and 20--20 of FIG. 2.

Referring to the drawings, a cathode ray tube 10 having a central longitudinal axis A--A includes an envelope 12 comprising a faceplate panel 14 and a funnel 16. The panel 14 includes a faceplate portion 18 disposed centrally and perpendicularly to the axis A--A and a peripheral side wall 20 which extends generally parallel to the axis A--A. The panel 14 is sealed at its open end to the large opening of the funnel 16 by a frit seal 21.

A plural electron gun assembly 22 is disposed within the neck of the funnel 16. The gun assembly is adapted to project a plurality of electron beams toward the faceplate 18.

A mosaic screen 23 of phosphor dots is disposed on the inner surface of the faceplate 18. A plurality of electrode mounting studs 24 are secured to, and extend inwardly from, the inner surface of the side wall 20. A multi-apertured shadow mask electrode 26 is mounted within the panel 14 on the studs 24. The electrode 26 is disposed adjacent to the mosaic screen 23 and is surrounded by the side wall 20 with a spacing gap 27 between the electrode and the side wall. The electrode 26 comprises a generally rectangular frame 28 across which is mounted a multi-apertured sheet metal masking member 30. The frame 28 is of L-shaped cross-section having an "axial" peripheral flange 31 which is substantially parallel to the axis A--A and a "transverse" flange 32 which extends inwardly toward the axis A--A. The masking member 30 is secured to the axial flange 31 near the edge of this flange. A plurality of leaf support springs 33 are attached to the outer surface of the axial flange 31 and are provided with apertures into which the studs 24 are received to support the shadow mask electrode 26 within the panel 14.

A foil strip electron shield 34 is disposed in contact with both the mask frame 28 and the panel side wall 20 so that it blocks or closes the gap 27 between the mask electrode 26 and the side wall 20. The shield 34 includes a transverse (to the axis A--A) flange 36 along one side of the strip and an axial flange 38 along the other side of the strip. A plurality of tabs, 40, which extend from the transverse flange 36, are provided for securing the shield 34 to the mask frame 28.

The foil shield 34 is disposed so that its transverse flange 36 lies flush against the transverse flange 32 of the mask frame 28. The tabs 40 are crimped over the inner rim 42 of the frame flange 32 to secure the foil shield 34 in place. The axial flange 38 of the foil shield 34 is then pressed against the inner surface of the panel side wall 20 so that it lies flush against the side wall 20. This pressing may be accomplished, for example, by a person passing his finger around the panel 14 so as to press the foil shield 34 toward the gap 27 and against the panel side wall 20. Alternatively, a suitable tool may be used to conform the strip shield 34 to the panel side wall 20.

The foil shield 34 may for convenience be provided as four separate pieces, each lying along one side of the rectangular mask electrode 26. The separate lengths of the foil shield 34 may be sightly overlapped and interlocked at the corners of the electrode 26 as indicated at 44.

FIG. 3 illustrates one simple form in which a length of the foil shield 34 may be provided prior to its being installed in the tube 10. In this form, the shield 34 has a substantially right-angle, L-shaped cross-section consisting essentially of a transverse flange 45 and an axial flange 46, both of which are substantially flat. Tabs 40 are provided on the transverse flange 45. After being positioned and pressed into place, the FIG. 3 form of the shield 34 may be considerably distorted from the right-angle shape illustrated, so as to conform to the mask frame and panel sidewall. Furthermore, the shield may be pressed into the gap 27 between the mask electrode 26 and panel sidewall 20 (for a purpose hereinafter described) so that its two flanges 45 and 46 are somewhat separated from each other by a U-shaped section of the shield extending into the gap.

FIGS. 4-18 illustrate a preferred form in which the foil shield 34 is provided prior to its being installed in the tube 10. The shield form of these figures is provided as four separate parts, two of a long part 50 illustrated by FIGS. 4-11, and two of a short part 52 illustrated by FIGS. 12-18. FIGS. 4 and 5 are respectively plan and elevation views of the long part 50. FIGS. 6-11 are various enlarged sections thereof. Similarly, FIGS. 12 and 13 are plan and elevation views and FIGS. 14-18 various enlarged sections of the short part 52. As shown in FIG. 2, two of the long parts 50 are designed to fit neatly along the two long sides of the mask electrode 26 and panel 14 and two of the short parts 52 along the two short sides. Both the long part 50 and the short part 52 comprise the transverse flange 36, the axial flange 38, and an intermediate section 54. The intermediate section 54 varies considerably in shape along the length of both the shield parts 50 and 52. FIGS. 19 and 20 illustrate the purposes of such variation.

As shown in FIG. 19, the intermediate section 54 of foil shield 34 is shaped to dip into the gap 27 in the four corner regions of the panel 14 where electron overscanning is most severe. Each section 54 is disposed within the spacing gap 27 with the sides of the U resiliently engaging the sidewall 20 and the axial flange 31 of the frame 28. The shape of the intermediate sections 54 at these regions results in a cantilever spring-like action which urges the axial flange 38 of the shield against the panel sidewall 20 to insure a good closing of the gap 27. In the central regions of the four sides of the panel 14, the presence of the studs 24 and support springs 33 prevents a similar dipping of the shield 34 into the gap 27. At the side regions the intermediate section 54 has a reverse dip in order to clear the studs and springs, as shown in FIGS. 1, 6, 14 and 20. However, in these side regions (where electron overscanning is less severe) the gap 27 is adequately closed simply by pressing the axial flange 38 of the shield against the panel side wall 20. If desired the foil shield 34 may be provided with an accordion-like fold 56 (FIGS. 6, 14, 20) between the transverse flange 36 and the axial flange 38 to produce a spring action in the side regions to urge the axial flange 38 against the side wall 20. Whether or not the foil shield dips into the gap 27, it is nevertheless shaped so that the axial shield flange 38 lies generally parallel to and against the side wall 20, as shown in FIGS. 1, 19 and 20. Thus, even if the shield does not make a continuous line of contact completely around the side wall 20, the passageway between the axial shield flange 38 and the side wall 20 will be so narrow and long and sufficiently nonaligned with the electron paths that any electrons are prevented from passing through the gap 27 and striking the screen 23.

The foil for the shield 34 may be of any suitable metal, such as aluminum or steel. A preferred material is 0.003-0.004 inch thick aluminum foil having a black ceramic coating on one side thereof. Such blackened aluminum is marketed commercially by Revere Copper & Brass, Inc. The shield may be installed with its black coated surface out of contact with and facing away from both the mask frame 28 and the panel sidewall 20. This provides desirably good heat radiation properties which contribute to maintaining as low a temperature of the mask electrode 26 as possible. The pliability and resilience of 0.003-0.004 inch thick aluminum has been found to be highly satisfactory.

The thickness of the foil should be such that it can be easily permanently deformed by hand to exceed its elastic limit. The resilience of the foil should be sufficiently low so that it can be pressed against the panel side wall and conformed thereto without a significant inward spring-back away from the side wall when the pressing force is removed. Absence of spring-back can best be assured by pressing the foil shield, where possible, slightly into the gap 27 between the mask electrode 26 and the side wall 20, as well as against the side wall. At the same time, the foil, once pressed against the side wall 20, should have sufficient resilience so that when the mask is heated and expands (e.g., 20 thousandths of an inch toward the side wall), the foil will yield to the expansion without exceeding its elastic limit. Thus, when the mask cools and contracts, the foil shield will exhibit an outward spring-back and remain in contact with the side wall 20.

By virtue of the double flange shape of the foil shield 34, a substantial surface portion of the axial flange thereof can be disposed flush with the side wall 20. This assures complete closing of the gap 27 and complete interception of the over-scanning electrons.

By virtue of the shield tabs 40, the foil shield 34 can be secured to the mask frame 38 without the necessity of separate securing means such as a weld or a clamp. Such a self-securing property of the shield 34 also facilitates fabrication of the cathode ray tube 10 because the shield 34 need not be temporarily secured in place (such as by a fabricator's hand) while some separate permanent securing means or force is being applied.

In one embodiment of the invention, the foil shield 34 has been constituted of aluminum foil having a thickness of 0.003 inch. In another embodiment of the invention, the foil shield 34 has been constituted of cold rolled steel foil having a thickness of 0.002 inch.

Aluminum foil may be preferred because of the ease with which it can be formed and the freedom from the likelihood of personnel injury (e.g. cutting a finger on a sharp edge) during the forming thereof which it offers. Also, compared to steel, aluminum foil will more readily yield without distorting the mask when high temperature expansion occurs because of the reduction in its strength at high temperatures.

Steel foil may be preferred because of the ease with which it can be provided with a heat-radiating blackened surface coating which is suitable for incorporation internally of the cathode ray tube 10.