Abstract:
A color picture tube is improved by positioning shims on adjacent sides of a mask adjacent the mask corners and between the mask and its peripheral supporting frame. Because of the shims, the resonant frequency of the mask is raised.

Description:
This a continuation of application Ser. No. 610,480 filed May 15, 1984 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to color picture tubes of the type having a shadow mask attached to a peripheral frame, which is suspended in relation to a cathodoluminescent screen, and particularly to novel means for reducing microphonic vibrations in the shadow mask. 
     In these color picture tubes, the accuracy with which the electron beams, emitted from an electron gun, strike individual elemental cathodoluminescent screen areas depends, to a great degree, upon the accuracy with which the shadow mask apertures are aligned with the elemental screen areas during operation of the tube. Although vibration of the shadow mask relative to the elemental screen areas is generally undesirable, tube performance becomes greatly deteriorated when the frequency of mask vibration beats with the electron beam scanning frequency. 
     The present invention changes the resonant frequency of the mask to eliminate or greatly reduce any vibration beat with the scanning frequency. 
     SUMMARY OF THE INVENTION 
     A color picture tube is improved by positioning shims on adjacent sides of a mask adjacent the mask corners and between the mask and its peripheral supporting frame. Because of the shims, the resonant frequency of the mask is raised. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view in axial section of an apertured mask cathode-ray tube. 
     FIG. 2 is a back view of the faceplate and mask-frame assembly of the tube of FIG. 1. 
     FIG. 3 is a partially broken-away plan view of a corner portion of the mask-frame assembly of FIG. 2. 
     FIG. 4 is a sectional view of the corner portion of the mask-frame assembly taken at line 4--4 of FIG. 3. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 and 2 illustrate a color picture tube 18 having an evacuated glass envelope 20 comprising a substantially rectangular faceplate panel 22 and a tubular neck 24 joined by a funnel 26. The faceplate panel includes two orthogonal axes; a major axis X--X, parallel to its wider dimension (usually horizontal), and a minor axis Y--Y, parallel to its narrower dimension (usually vertical). The major and minor axes are perpendicular to the central longitudinal axis Z--Z of the tube which passes through the center of the neck 24 and the center of the panel 22. The panel 22 comprises a viewing faceplate 28 and a peripheral flange or sidewall 30 which is sealed to the funnel 26 by a frit material 27. A three-color cathodoluminescent line screen 32 is located on the inner surface of the faceplate 28. The screen 32 comprises an array of phosphor lines 32&#39; extending substantially parallel to a minor axis Y--Y of the tube. Portions of the screen 32 may be covered with a light absorbing material in a manner known in the art. A mask-frame assembly 33, comprising a substantially rectangular apertured color selection electrode or shadow mask 34 attached to a substantially rectangular peripheral frame 35 having an L-shaped cross-section, is removably mounted within the panel 22 in predetermined spaced relationship to the screen 32 by four springs 42. The mask 34 includes a multiplicity of slit-shaped apertures which are aligned in substantially parallel columns with web portions separating the slits within each column. The L-shaped frame 35 includes two flanges 36 and 37. The first flange 36 faces toward the screen 32 and approximately parallels the central longitudinal axis Z--Z. The second flange 37 extends inward toward and approximately perpendicular to the central longitudinal axis Z--Z. The mask 34 also includes a peripheral skirt 41 which is telescoped relative to the frame flange 36. 
     An inline electron gun 39 (illustrated schematically) is mounted within the neck 24 to generate and direct three electron beams 38B, 38R and 38G along coplanar convergent paths through the mask 34 to the screen 32. 
     The tube 18 is designed to be used with an external magnetic deflection yoke 40 surrounding the neck 24 and funnel 26 in the vicinity of their junction. When appropriate voltages are applied to the yoke 40, the three beams 38B, 38R and 38G are subjected to perpendicular magnetic fields that cause the beams to scan in a rectangular raster over the screen 32. 
     FIGS. 3 and 4 show greater detail of the mask-frame assembly 33. The shadow mask 34 includes a central apertured portion 44 surrounded by the peripheral skirt 41. A plurality of tabs 46 extend from the distal edge of the skirt 41. At least one of the tabs 46 is located at each of the corners of the shadow mask 34. The tabs 46 are welded to the first flange 36 of the frame 35. The outer dimensions of the skirt 41 are less than the corresponding inner dimensions of the first flange 36, whereby a gap 48 is formed between the skirt 41 and the first flange 36. A plurality of wedge-shaped shims 50 are located in the gap 48 between and in contact with the skirt 41 and the first flange 36. Two of the shims 50 are positioned on opposite sides of each tab 46 at the corners of the shadow mask 34. The shims 50 are welded to the frame 35. 
     In a color picture tube having a 68.58 cm (27 inch) diagonal screen and which did not utilize the shims described herein, it was found that areas of the shadow mask near the mask corners have a mechanical resonance of approximately 115 Hertz. Such vibration frequency causes an objectionable visible beating with the electron beam deflection frequency. The Z axis motion of the mask is accompanied by a motion of the mask skirt in the X-Y plane. Specifically, as the mask moves up, the skirt moves in. By placing a shim between the skirt and frame on each side of each corner at a spacing A, as shown in FIG. 3, where A equals 38.1 mm (1.5 inch), the resonant frequency of the mask was raised to 145 Hertz. Such raising of resonant frequency eliminates or greatly reduces the aforementioned objectionable visible beating. 
     Although heretofore, the shims have been described as separate elements being located between the frame and mask, the scope of the present invention also covers several variations of shim means that produce the same rise in resonant frequency. In one of these variations, bent-in tabs are provided, instead of the shims, on the frame flange 36 which contact the mask skirt 41 at the same locations as the shims, shown in the drawings. In another variation, there are outwardly extending flutes on the mask skirt 41 which contact the frame flange 36 at the same locations as the shims, shown in the drawings.