Abstract:
Vacuum pump exhaust manifold pressure is employed to control the conversion voltage applied during a CRT conversion process for enabling conversion of CRT cathodes in a nearly constant vacuum environment.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates generally to a cathode ray tube (CRT) and more particularly to the activation or conversion of one or more cathode elements of such a tube during the production thereof. 
     Several prior art cathode conversion techniques have been employed in CRT production processes. One such technique follows a fixed time schedule during which the applied filament voltage is varied in several discrete steps from approximately 25% of hotshot voltage to full hotshot voltage. This procedure is disadvantageous is that it is conducted without regard for tube vacuum, with the attendant result that CRT cathodes are converted at various vacuum levels depending on the amount of outgassing present in an individual tube. Conversion at insufficient vacuum levels has been found to be detrimental to cathode performance and life. 
     It is therefore the principal object of this invention to provide an apparatus for conversion of CRT cathodes in which vacuum pump exhaust manifold pressure is continuously monitored during the conversion process so that cathode conversion is accomplished under nearly constant vacuum conditions. 
     This object is accomplished in accordance with the illustrated preferred embodiment of the invention by employing control circuitry for supplying a steadily increasing voltage for application to the CRT filaments during cathode conversion, for sensing the tube vacuum during the conversion process, for preventing an increase in the applied voltage if tube vacuum falls below a first predetermined level, and for initiating a reduction in the applied voltage in the event tube vacuum falls below a second predetermined level. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of logic and control circuitry comprising a CRT cathode conversion apparatus in accordance with the preferred embodiment of the invention. 
     FIGS. 2A-D are a detailed schematic diagram of a preferred circuit implementation of the CRT cathode conversion apparatus of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the block diagram of FIG. 1, there is shown a CRT 10 having write gun and flood gum cathodes to be converted. The filaments associated with these cathodes are separately connected to programmable power supplies 12 and 14, each of which provides a source of voltage for converting the associated cathode. It is desirable to employ remote sensing of connection with power supplies 12 and 14 to avoid regulation problems that would otherwise occur as a result of the voltage drop in the leads between the power supplies and the CRT filaments. These power supplies are driven by buffer drivers 16 and 18. A pair of gain controls 20 and 22 associated with buffer drivers 16 and 18, respectively, are employed to select any one of several hotshot voltages, depending on the type of CRT filament currently in use. 
     A control input line 24 is provided from an external vacuum pump employed to exhaust the CRT. Control input line 24 carriers a control voltage that is linearly inversely proportional to CRT vacuum. This line is connected as an input to a pair of limit switches 26 and 28 and to a rate sensor 30. The individual outputs of limit switches 26 and 28 and rate sensor 30 are connected as inputs to a binary up-down counter 32. A clock 34 and first and second timers 36 and 38 provide additional inputs to up-down counter 32. The binary output of up-down counter 32 is connected to a 10-bit digital-to-analog converter 40 which in turn controls buffer drivers 16 and 18. The block diagram of FIG. 1 may be implemented by means of circuitry shown in the detailed schematic diagram of FIGS. 2A-B. 
     Cathode conversion is accomplished by first establishing an appropriate level of CRT vacuum by means of an external vacuum pump. During the time that this vacuum level is being established, operating power may be applied to the conversion apparatus, which in turn applies 25% of hotshot voltage to the CRT filaments. This voltage level is not destructive to the CRT regardless of the existing vacuum level. When a chosen conversion vacuum level has been reached, the conversion process may be continued by actuating a start switch 42 illustrated in the detailed schematic diagram of FIGS. 2A-B. Start switch 42 initiates up-counting in up-down counter 32 based on the clock pulses generated by clock 34. The binary countoutput of up-down counter 32 is converted to an analog voltage by digital-to-analog converter 40. The resulting analog voltage is applied to buffer drivers 16 and 18 which in turn control the voltage output of power supplies 12 and 14. 
     As long as the voltage control input line 24 does not exceed a predetermined limit that corresponds to the minimum CRT vacuum at which conversion may safely take place, up-down counter 32 will continue to count pulses supplied by clock 34 until it becomes full. For example, in the production of a certain type of CRT the frequency of pulses generated by clock 34 is adjusted so that up-down counter 32 will become full in about 4 minutes, 35 seconds. During this time, the increasing binary count output of up-down counter 32 is converted to a linearly increasing analog voltage by digital-to-analog converter 40. This increasing analog voltage applied to buffer drivers 16 and 18 results in a voltage that linearly increases from 25% of hotshot voltage to full hotshot voltage being applied to the CRT filaments. When up-down counter 32 becomes full, timer 36 is triggered and disables the up-down counter 32, thus allowing application of the full hotshot voltage to the CRT filaments. 
     According to the illustrated preferred embodiment of the invention, timer 36 is arranged to time out in 35 seconds. At that point, timer 38 is triggered and forces the output of the up-down counter 32 to a value such that 75% of hotshot voltage is applied to the CRT filaments. In the illustrated preferred embodiment of the invention timer 38 is preset so that 75% of hotshot voltage is applied to the CRT filaments for 12 minutes. At the end of that time period an RTT indicator 44 signals the operator to seal off the tube. When the tube has been sealed the conversion process is halted by actuating a stop switch 46. 
     In practice, the CRT vacuum rarely remain constant enough to allow the conversion process to systematically proceed as just described. Due to normal outgassing in the CRT during the conversion process, significant variations in the CRT vacuum level are encountered. The present cathode conversion apparatus has been arranged to insure that the CRT cathodes are converted while maintaining a relatively constant high vacuum in the CRT. If, during the conversion process, the voltage on control input line 24 increases to a predetermined level indicative of a vacuum level that is insufficient to allow the conversion process to safely continue, limit switch 26 is automatically actuated. This halts up-counting in up-down counter 32, thus holding the previously achieved voltage at the CRT filaments and preventing a further increase in that voltage. Due to the time lag betweem the vacuum level in the CRT and that at the vacuum pump, the vacuum level in the CRT may continue to fall after limit switch 26 has been actuated. Such a continuing decrease in CRT vacuum will, at a predetermined level, trigger limit switch 28. Limit switch 28 in turn commands up-down counter 32 to begin down-counting, thereby decreasing the voltage applied to the CRT filaments. When the vacuum has overcome the outgassing action in the tube to the point that the vacuum level in the tube has been restored to the level to which limit switch 28 is preset, limit switch 28 is again triggered, this time halting the down-counting in up-down counter 32. Further improvement in the vacuum level within the tube triggers limit switch 26, thus again beginning up-counting in up-down counter 32. The process is repeated in response to subsequent periods of tube outgassing until up-down counter 32 becomes full, thereby applying full hotshot voltage to the CRT filaments. Rate sensor 30 is provided to monitor the rate of change of voltage on control input line 24. When a very rapid decrease in vacuum level takes place, rate sensor 30 acts to halt the up-counting in up-down counter 32 even before limit switch 26 is triggered. 
     Application of the present apparatus and method for converting CRT cathodes has been found to yield more uniform cathodes than have resulted from the use of prior art conversion methods. Test results have shown the present apparatus and method to be applicable to a variety of different CRT types and to work well with various vacuum systems. While the time required to perform a complete conversion cycle may, depending upon the amount of outgassing present in a particular tube, be somewhat longer than the required under the various prior art conversion methods involving fixed time schedules, the resulting improvements in cathode performance far outweight any time disadvantage.