referring now to the drawing , fig1 illustrates a conventional cathode ray tube 4 of a tv receiver and the like which exhibits a light spot as a result of electrons incident thereon from a tube cathode 52 when a brightness control grid 53 of the tube is subjected to a non - negative potential . the light spot on the screen 51 is conventionally swept to form the horizontal portion of the tv raster by means of a deflection voltage appearing on the output of a deflection circuit represented generally at 54 . as shown , the output of the circuit 54 is applied to a deflection input of the tube 4 , such deflection input being represented at 56 . such voltage is also applied to the input of a light spot suppression circuit 61 , whose structure and operation in accordance with the invention is described below . the deflection voltage , which illustratively is tapped off a transformer ( not shown ) of the deflection circuit 54 , conventionally exhibits a forward sweep portion 62 and a fly - back portion 63 . in the arrangement of fig1 the forward portion 62 is a positive quantity having a magnitude of 15 volts with respect to the ground of the tube 4 . in accordance with the invention , the circuit 61 is arranged to apply , to the control grid 53 of the tube 4 , a negative blocking potential to suppress beam current in the tube and thereby the light spot on the screen 51 , whenever the voltage applied to the input of the circuit 61 from the deflection circuit 54 falls below a threshold value indicative of the occurrence of the forward sweep portion 62 . in other words , the circuit 61 is arranged to maintain a &# 34 ; fail - safe &# 34 ; negative voltage on the control grid 53 whenever the forward sweep portion 62 is absent , whether such absence be exhibited by the occurrence of the flyback portion 63 of the deflection voltage or the disabling of the associated tv receiver , as exhibited by the cutout of the deflection circuit 54 and the resultant stationary positioning of the spot on the screen 51 . thus , although the cathode 52 of the tube 4 will continue to emit electrons for a finite time after the deflection circuit is disabled , the normal negative voltage automatically restored to the control grid 53 will prevent such electrons from hitting the screen 4 and will thereby provide the required light spot suppression effect . in addition , the normally required separate suppression circuit coupled to the output of the transformer of the deflection circuit 54 for blanking the screen 51 during the occurrence of the flyback pulses 63 is no longer required , since the circuit 61 performs this function as well . in the arrangement of fig1 the circuit 51 includes a pnp switching transistor 17 whose collector is coupled via a charging resistor 2 to a positive voltage source and whose emitter is returned to a source of positive potential whose magnitude establishes the threshold switching level of the transistor 17 and which is slightly below the normal amplitude of the forward sweep portion 62 . the base of the transistor 17 is coupled , via a pair of serial resistors 15 and 16 , to the output of the deflection circuit 54 . a zener diode 18 is connected between the junction of the resistors 15 and 16 for current - limiting purposes . a serial path including a capacitor 3 and a diode 5 are connected between the collector of the switching transistor 17 and ground . the diode 5 is poled in its low - impedance direction relative to the polarity of the positive voltage source that supplies the collector voltage to the transistor 17 . the junction of the capacitor 3 and the diode 5 is connected to the control grid 53 of the tube 4 . with such arrangement , when the voltage applied to the input of the circuit 61 is below the threshold value represented by the reference potential on the emitter of the transistor 17 ( i . e ., when the deflection voltage exhibits a value lower than + 12 volts and thereby a condition indicative of either the occurrence of the flyback portion 63 or the inoperativeness of the deflection circuit ), the switching transistor 17 will be in its normal conductive state . under such conditions , the voltage at the collector terminal of the transistor 17 will assume the potential of the emitter thereof , i . e ., + 12 volts . moreover , the voltage across the capacitor 3 , which upon the turn - on of the receiver will initially be charged up to the 230 volt collector supply through the resistor 2 and the diode 5 , will thereafter exhibit such voltage since the diode 5 will remain non - conductive to hold the control grid 53 off ground ; accordingly , no discharge path will be provided for the capacitor 3 . since as a result the voltage at the collector of the transistor 17 when the latter normally conducts will exhibit approximately the + 12 volt value representing the threshold voltage at its emitter , and since the capacitor 3 exhibits a 230 volt drop thereacross , the voltage on the right - hand terminal of the capacitor 3 relative to ground will be a very high negative value during the conduction period of the transistor 17 . accordingly , the beam current and thereby the light spot of the tube 4 will be cut off . during each forward sweep interval 62 , however , when the deflection voltage exhibits a positive jump over the 12 volt threshold at the emitter of the transistor 17 , such transistor will be driven into its non - conductive state and the voltage at its collector will rise from + 12 volts to approximately 230 volts . as a result , the voltage at the right - hand terminal of the capacitor 3 , when referred to ground , will exhibit a large positive jump to approximately ground potential , thereby permitting the passage therethrough of the electrons emitted from the cathode 52 . therefore , the now - moving light spot will be visible on the screen 51 to define the desired tv picture . when the deflection voltage again drops below its threshold , either at the start of the flyback interval or when the deflection circuit becomes disabled , the transistor 17 will again be driven into its conductive state , whereby the voltage at its collector , and thereby on the right - hand terminal of the capacitor 3 , will undergo a sharp negative drop to restore the grid 53 to its normal negative voltage condition , and the light spot on the screen 51 will be inhibited as desired . the arrangement of fig2 is similar to that of fig1 but is arranged to operate when the forward sweep portion of the deflection voltage is negative as shown . the respective components of fig1 and 2 are the same , except that the polarity of the zener diode 18 has been reversed , and the switching transistor is embodied as an npn device 12 whose emitter is returned to a negative potential ( i . e ., - 12 volts ) slightly lower than the normal negative voltage of the forward sweep portion of the deflection voltage . the function of the arrangement of fig2 is to maintain the switching transistor 12 conductive so long as the voltage applied to the input of the circuit 61 is more positive than the - 12 volt threshold level , i . e . during the occurrence of the flyback pulse 63 or during the times when the deflection circuit 54 is inoperative . under these circumstances , a highly negative voltage is applied to the control grid 53 and the now - stationary light spot will be suppressed . when the deflection voltage goes more negative than the - 12 volt threshold level ( i . e ., during the forward sweep interval ), the transistor 12 is driven into its non - conductive state to gate the tube 4 on and to render the now - moving light spot visible on the screen 51 . in the arrangement of fig3 the base of the switching transistor 1 is not directly excited by the deflection voltage , but instead is coupled to the collector of a second , normally non - conductive transistor 6 of the same npn conductivity type as the transistor 1 . the emitter of the transistor 6 is coupled to ground , and the junction of its collector and the base of the transistor 1 is returned through a resistor 7 to a positive voltage supply . the deflection voltage , whose forward sweep portion 62 is assumed to exhibit a positive voltage as shown , is applied through a decoupling diode 9 , a zener diode 10 which is suitably biased to a reference potential by means not shown , and a resistor 11 to the base of the transistor 6 . in the arrangement of fig3 during the absence of the forward sweep portion 62 the transistor 6 will be nonconductive , and the high positive potential of the collector supply of the transistor 6 will be applied to the base of the transistor 1 to maintain such transistor in a conductive state . as explained above in connection with fig1 such conduction of the main switching transistor will cause the right - hand terminal of the capacitor 3 to exhibit a large negative potential with respect to ground , such potential being applied to the control electrode 53 to inhibit the occurrence of a light spot on the tube screen 51 . at the start of the forward sweep portion of the deflection voltage , the transistor 6 will be driven into conduction , and the resulting reduction in the potential at its collector will cause the transistor 1 to be driven into its non - conductive state . the collector potential of the transistor 1 , and thereby the potential at the right - hand terminal of the capacitor 3 , will thereby undergo a sharp positive step , which will remove the inhibiting effect of the electrode 53 on the beam current and will permit the formation on the screen 51 of the now - moving light spot . when the forward sweep portion terminates , the transistor 6 will again be non - conductive , the transistor 1 will again be conductive , and the voltage on the grid 53 will be returned to its normal inhibiting negative value . in the arrangement of fig4 the emitters of the transistors 1 and 6 are returned to a positive potential rather than to ground , so that the zener diode in the base path of the transistor 6 can be dispensed with . in all other respects , the structure and function of the arrangement of fig3 and 4 are identical . in the foregoing , several embodiments of the invention have been described . many variations and modifications will now occur to those skilled in the art . it is accordingly desired that the scope of the appended claims not be limited to the specific disclosure herein contained .