Method for spot-knocking an electron gun mount assembly of a CRT

Abstract: A spot-knocking method for an electron gun mount assembly in a evacuated CRT (21) comprises applying a spot-knocking voltage between an anode (47,51) and a first focus electrode (45), with the remaining gun elements, including a heater (37), a cathode (39), a control electrode (41), at least one screen electrode (43) and a second focus electrode (49), electrically floating.

Description: This invention relates to a method for spot-knocking the electron gun mount assembly of a CRT (cathode-ray tube) and, for example, to a method of spot-knocking an electron gun mount assembly having six electrodes. In the manufacture of a CRT, it is the practice to electrically process the electron gun mount assembly therein after the CRT has been completely assembled, exhausted of gases and sealed. One step in this electrical processing is spot-knocking, which involves inducing arcing in the gaps between adjacent electrodes, usually between a focus electrode and an electrode adjacent thereto. Arcing removes projections, burrs and/or particles which would later be sites for the field emission of electrons during the normal operation of the CRT. U.S. Pat. No. 4,214,798, issued to Hopen on July 29, 1980, discloses a spot-knocking method that may be applied to a bipotential or a tripotential electron gun structure. A bipotential gun structure typically has a heater and cathode K, a control grid G1, a screen grid G2, a single focus electrode G3 and a high voltage electrode, which is often designated as the anode or G4. Although separate elements may be provided for each of the three electron guns of a color picture tube, recent practice has tended to use common elements for the G1, the G2, the G3 and the anode of the three electron guns. A tripotential gun differs from a bipotential gun in that it employs three focus electrodes, instead of only one, for the focusing action. A tripotential gun typically has a heater, a cathode K, a control grid G1, a screen grid G2, three focus electrodes G3, G4, and G5, and an anode, which is often designated G6. In the method described in the cited patent, the heater, the cathode, the control grid and the screen grid are interconnected, and, in the bipotential gun structure, spot-knocking voltages are applied between the anode and the interconnected gun elements, with the focus electrode electrically floating. The tripotential electron gun is similar to the bipotential electron gun for the purpose of spot-knocking, except that the G3 and G5 focus electrodes are interconnected within the CRT, and two separate stem leads are connected to the G3 and G4 focus electrodes which are electrically floating during spot-knocking. Many methods of spot-knocking electron gun assemblies have been used previously in attempts to improve the electrical characteristics of television picture tubes. Most of these methods involve forcing arcs to occur between two adjacent electrodes to remove projections, burrs, and/or particles, so that the field emission of electrons between the two elements is significantly reduced at the normal operating potentials. In all cases involving spot-knocking between the anode and the focus electrode G3, positive fluctuating DC high-voltage pulses are applied between these two electrodes, with all other electrodes being held at ground potential or allowed to float, as described in the cited patent. An alternative is to ground the anode and apply negative fluctuating DC high-voltage pulses to the remainder of the gun structure. The size, shape and repetition rate of the high-voltage pulses vary widely, depending upon the nature of the spot-knocking equipment used. The voltage pulses used most frequently for spot-knocking are sinusoidal and are derived from the normal variation of the line voltage. They may be half wave, with the lowest portion either at some minimum positive DC level or at ground potential, or they may be full wave, in which case the lowest value is usually clamped at ground potential. Very fast rise time pulses of short duration, sometimes derived from the discharge of a capacitor through a ball gap, have also been used, in which current pulses often exceed 100 amperes. Although the power associated with these pulses is very high, the duration of each pulse (often less than one microsecond) limits the energy of the induced arc to levels which are safe for the tube elements. Regardless of the type of pulses used for the spot-knocking, most users have found it prudent to avoid the application of negative pulses to the anode. In recent years, improvements in the focusing of the electron spot on the screen have been achieved by the use of increasingly higher voltages on the focusing elements of both bipotential and tripotential types. Because of these higher operating potentials, it is often necessary to provide for spot-knocking between the focus electrode G3 and the screen grid G2; for tripotential types, spot-knocking among the various focus grids G3, G4 and G5 is also believed to be desirable. In another spot-knocking method as described in U.S. Pat. No. 4,052,776, issued to Maskell et al. on October 11, 1977, a very high amplitude RF bursts are added to the fluctuating DC pulses of relatively low amplitude which are used to spot-knock between G2 and G3. In this method, the fluctuating DC spot-knocking voltage pulses are introduced through the stem leads to the G3 and G5 of a tripotential gun, and the RF burst is introduced through the remainder of the stem leads, which are electrically connected. Because the stem leads are close to one another, either the peak DC voltages must be maintained at relatively low values, which is of limited effectiveness, or special precautions must be taken to prevent electrical breakdown among the external portions of the stem leads. Yet another spot-knocking method is described in U.S. Pat. No. 4,682,963, issued to Daldry et al. on July 28, 1987. A two-step conditioning process is disclosed for a CRT having six grids. During normal operation, the G2 and G4 are interconnected to a relatively low voltage. The G3 and G5 focus electrodes are interconnected at a higher potential, and the anode, G6, operates at the highest potential. A general conditioning includes applying high voltage DC to the anode and applying pulse voltages to the interconnected G2 and G4 electrodes. The heater, the cathode, and the G1 are interconnected and allowed to float. The G3 and G5 are interconnected to each other and also allowed to float. During the second step of the processing, the heater, the cathode and the G1 through G5 electrodes, inclusive, are connected to the pulse voltage, with a high voltage DC applied to the anode. While several of the above-described spot-knocking methods relate to six element electron guns (in addition to the heater and the cathode), none provides an adequate means for conditioning a double bipotential electron gun or a six-element electron gun having two screen grids and two focus electrodes. A double bipotential gun structure typically has a heater, a cathode K, a control grid G1, a screen grid G2, a first focus electrode G3, a first anode G4, a second focus electrode G5 and a second anode G6. The first and second focus electrodes G3 and G5 typically operate at about 7 kV, and the first and second anodes, G4 and G6, operate at about 25 kV. One type of six-element electron gun structure includes (in addition to the heater and cathode) a control grid G1, a first screen grid G2, a first focus electrode G3, a second screen grid G4, a second focus electrode G5 and an anode G6. The first and second screen grids G2 and G4, typically operate at about 300V to 1000V, the first and second focus electrodes, G3 and G5, operate at about 7kV, and the anode, G6, operates at about 25kV. In accordance with the present invention, a method for spot-knocking an electron gun mount assembly in an evacuated CRT comprising a plurality of gun elements including a heater, a cathode, a control electrode, at least one screen electrode, a first focus electrode, a second focus electrode and an anode, includes applying a spot-knocking voltage between said anode and said first focus electrode, the remaining gun elements being electrically floating. In the drawings: FIGURE 1 shows a schematic representation of a first circuit arrangement for practicing the inventive method on a first electron gun. FIGURE 2 shows a schematic representation of a second circuit arrangement for practicing the inventive method on the electron gun of FIGURE 1. FIGURE 3 shows a graph comparing the stray emission after spot-knocking by the conventional and the inventive methods. FIGURE 4 shows a schematic representation of a third circuit arrangement for practicing the inventive method on a second electron gun. FIGURE 5 shows a schematic representation of a fourth circuit arrangement for practicing the inventive method on the electron gun of FIGURE 4. The spot-knocking method according to the present invention may be applied to any electron gun mount assembly of a cathode-ray tube, CRT, having a cathode and a plurality of electrodes for directing and focusing an electron beam, wherein at least two of the electrodes operate at the same potential. There may be a single electron gun or a plurality of guns in the mount assembly of the CRT. Where there is more than one gun, the guns may be in any geometric arrangement. Where there are three guns, as in a color television picture tube, for example, the guns may be arranged in a delta array or in an inline array, as is known in the art. The method may be applied, for example, to a double bipotential electron gun of the type schematically represented in FIGURE 1. The double bipotential gun structure typically has a heater, a cathode, a G1 or control grid electrode, a G2 or screen grid electrode, a G3 or first focus electrode, a G4 or first anode, a G5 or second focus electrode, and a G6 or second anode. Although separate elements may be provided for each of the three electron guns of the CRT, recent practice has tended to use common elements attached to glass support rods (not shown). In the double bipotential electron gun, the focus electrodes G3 and G5 typically operate at a first voltage of about 7 kV, and the anodes G4 and G6 operate at a second voltage of about 25 kV. The double bipotential electron gun of the present invention utilizes a glass stem (not shown) having sufficient leads (or pins) to permit both the G3 and G5 electrodes to be connected to separate leads, despite the fact that during normal tube operation the G3 and G5 electrodes operate at a common voltage of about 7 kV. The separate leads exiting the evacuated tube envelope permit the inventive spot-knocking method to be utilized. FIGURE 1 includes a schematic, sectional, elevational view of an evacuated CRT 21, including a faceplate panel 23 carrying on its inner surface a luminescent viewing screen 25. The panel 23 is sealed to the larger end of a funnel 27 having a neck 29 integral with the smaller end of the funnel 27. The neck 29 is closed by a stem 31. The inner surface of the funnel 27 carries a conductive coating 33 which contacts an anode button 35. The neck 29 houses a double bipotential electron gun mount assembly. This assembly includes three double bipotential guns, only one of which is shown in FIGURE 1. The mount assembly includes two glass support rods (not shown) from which the various gun elements are mounted. The gun elements of each gun include a heater 37, a cathode 39, a G1 or control electrode 41, a G2 or screen electrode 43, a G3 or first focus electrode 45, a G4 or first anode 47, a G5 or second focus electrode 49, and a G6 or second anode 51. The first and second anodes, 47 and 51, respectively, are internally electrically interconnected, and the second anode 51 is connected to the conductive coating 33 by means of snubbers 53. In the preferred embodiment, the heater 37, the cathode 39, the G1 electrode 41, the G2 electrode 43 and the G5 electrode 49 are connected to separate stem leads 55 which extend through the stem 31. The G3 electrode 45 is also connected to a separate G3 lead 57 which extends through the stem. During spot-knocking, the stem 31 and the stem leads 55 and 57 are inserted into a base (not shown), and the leads 55 are electrically floating. A source 59 of high frequency voltage pulses of short duration and fast rise time is inserted in a socket lead 61 between the socket and ground 63. The pulses comprise between 92 and 150 kilovolts (kV) of AC of about 350 kilohertz. The anode button 35 is connected through an anode lead 65 to a source 67 of about +45 kV potential. The anode potential is applied to the internally interconnected anodes 47 and 51. The base (not shown) comprises an insulating silo which houses and electrically isolates the portion of the G3 lead 57 which is outside the CRT. This type of base is described in U.S. Pat. Nos. 4,076,336, issued to Wardell, Jr., et al. on February 28, 1978, and 4,127,313, issued to Marks on November 28, 1978, for example. The high frequency voltage from the source 59 forces arcing and imparts a high voltage, whereby gas molecules in the vicinity of the electrodes are efficiently ionized, and the gas ions and arcs effectively remove undesirable debris from the surfaces of the facing electrodes. An alternative method of spot-knocking is shown in FIGURE 2. The structure is similar to that shown in FIGURE 1, and identical elements are identified by the same numbers used in FIGURE 1. During spot knocking, the stem 31 and the stem leads 55 and 57 are inserted into the base (not shown), and the leads 55 are electrically floating. Unlike the method of FIGURE 1, the socket lead connects the G3 lead 57 directly to ground 63. The anode button 35 is connected through the anode lead 65 to a source 167 of low frequency pulsed spot-knocking voltage, and then to ground 63. The pulses from the source 167 increase initially from ground to peaks of about minus 35 ± 5 kilovolts, and then increase to peaks of about minus 60 ± 5 kilovolts in about 90 to 120 seconds. The pulses are comprised of half-wave rectified AC voltage having a frequency of about 60 hertz. The positive portion of the AC voltage is clamped to ground. The total duration of the pulses may be in the range of 0.1 to 0.2 second (6 to 12 cycles), and the time spacing may be in the range of 0.5 to 1.0 second. FIGURE 3 shows the results of radio frequency spot-knocking (RFSK) tests. The "regular" RFSK was performed with the G3 and G5 electrodes floating, the heater, cathode, G1 and G2 electrodes grounded, and the spot-knocking voltages of the alternative method applied to the anode button 35. The "enhanced" RFSK was performed according to the alternative method, with the heater, cathode, G1, G2 and G5 electrodes floating and only the G3 electrode grounded. The spot-knocking voltages of the alternative method are applied to the anode button 35. As shown in FIGURE 3, the inventive method permits the G3 and G5 focus electrodes to be operated at voltages up to 29 kV (extinction voltage, V EXT) without introducing any visible (stray) emission, e.g., above about 40 nanoamperes, from the electrodes, whereas regularly spot-knocked electrodes exhibited stray emission at voltages equal to, or exceeding 22 kV. The spot-knocking method described herein also is applicable to six-element electron gun structures (not including heaters and cathodes) of the type schematically represented in FIGURE 4, which shows a sectional, elevational view of an evacuated CRT 121 including a faceplate panel 123 carrying on its inner surface a luminescent viewing screen 125. The panel 123 is sealed to the larger end of a funnel 127 having a neck 129 integral with the smaller end of the funnel. The neck 129 is closed by a stem 131. The inner surface of the funnel 127 carries a conductive coating 133, which contacts an anode button 135. The neck 129 houses a six-element electron gun mount assembly which includes three electron guns, only one of which is shown in FIGURE 4. The mount assembly includes two glass support rods (not shown) from which the various gun elements are mounted. Each electron gun includes a heater 137, a cathode 139, a G1 or control electrode 141, a G2 or first screen grid 143, a G3 or first focus electrode 145, a G4 or second screen grid 147, a G5 or second focus electrode 149, and a G6 or anode 151. The first and second screen grids 143 and 147, respectively, are internally interconnected, and the first and second focus electrodes 145 and 149, which operate at the same electrical potential, have separate stem leads, as described below, to facilitate spot-knocking. The anode 151 is connected to the conductive coating 133 by means of snubbers 153. An electron gun of this type is shown in U.S. Pat. No. 4,764,704, issued to New et al., on August 16, 1988. In the embodiment of FIGURE 4, the heater 137, the cathode 139, the G1 electrode 141, the interconnected G2 and G4 electrodes 143 and 147, and the G5 electrode 149 are connected to separate stem leads 155 which extend through the stem 131. The G3 electrode 145 is also connected to a separate lead 157 which extends through the stem. During spot-knocking, the stem 131 and the stem leads 155 and 157 are inserted into a base (not shown), and the leads 155 are electrically floating. A source 59 of high frequency voltage pulses of short duration and fast rise time, identical to that described with respect to FIGURE 1, is inserted in a socket lead 61 between the socket and ground 63. The pulses comprise between 92 and 150 kilovolts (kV) of AC of about 350 kilohertz. The anode button 135 is connected through an anode lead 165 to a source 67 of about +45 kV potential. The source 67 also is identical to that described in FIGURE 1. The anode potential is applied to the anode 151. The base (not shown) comprises an insulating silo (also not shown) which houses and electrically isolates the portion of the G3 lead 157 which is outside the CRT. This type of base is described in the above-cited U.S. Pat. Nos. 4,076,366 and 4,127,313, for example. The high frequency voltage from the source 59 forces arcing and imparts a high voltage, whereby gas molecules in the vicinity of the electrodes are efficiently ionized, and the gas ions and arcs effectively remove undesirable debris from the surfaces of the facing electrodes. Yet another method of spot-knocking is shown in FIGURE 5. The structure is similar to that shown in FIGURE 4, and identical elements are identified by the same numbers used in FIGURE 4. During spot-knocking, the stem 131 and the stem leads 155 and 157 are inserted into the base (not shown), and the leads 155 are electrically floating. Unlike the method of FIGURE 4, the socket lead connects the G3 lead 157 directly to ground 63. The anode button 135 is connected through the anode lead 165 to a source 167 of low frequency pulsed spot-knocking voltage, and then to ground 63. The pulses from the source 167 increase initially from ground to peaks of about minus 35 ± 5 kilovolts, and then increase to peaks of about minus 60 ± 5 kilovolts in about 90 to 120 seconds. The pulses are comprised of half-wave rectified AC voltage having a frequency of about 60 hertz. The positive portion of the AC voltage is clamped to ground. The total duration of the pulses may be in the range of 0.1 to 0.2 second (6 to 12 cycles), and the time spacing may be in the range of 0.5 to 1.0 second.

Claims: A method for spot-knocking an electron gun mount assembly in an evacuated CRT, said mount assembly comprising a plurality of gun elements, these being a heater (37;137), a cathode (39;139), a control electrode (41;141), at least one screen grid (43,143,147), a first focus electrode (45;145), a second focus electrode (49;149) and an anode (47,51;151); characterized by the step of applying a spot-knocking voltage between said anode (47,51;151) and said first focus electrode (45;145), the remaining gun elements (37,39,41,43,49;137,139,141,143,147,149) being electrically floating. The method defined in claim 1, characterized in that spot-knocking voltages are applied between each of two anodes (47,51) and said first focus electrode (45), said anodes being spaced from either side of said second focus electrode (49). The method defined in claim 1, characterized in that said spot-knocking voltage is applied between said anode and said first focus electrode (145) interposed between two screen grids (143,147). The method defined in claim 1,2 or 3, characterized by said first focus electrode (45;145) being electrically grounded. The method defined in claim 1,2 or 3, characterized by said first focus electrode (45;145) being electrically connected to a source (59) of high frequency voltage pulses of short duration and fast rise time.

Inventors: HERNQVIST KARL GERHARD, HERNQVIST, KARL GERHARD
Applicants: RCA THOMSON LICENSING CORP, RCA THOMSON LICENSING CORPORATION
Application year: 1989
Main classification: H01J9
IPC codes: H01J9