| 摘要 |
PURPOSE:To require no selection of a time constant, to receive no influence of stray electrostatic capacity or the like and to surely erase a spot by providing a switch part and a capacitor and applying a negative voltage to the first grid of a cathode ray tube at the time of extinguishing the driving power source of the cathode ray tube. CONSTITUTION:At the time of interrupting the heater power source 30 of the cathode ray tube 1, a heater 1H has a low resistance, so that the voltage -6.3V of the heater power source 30 is rapidly extinguished and the base voltage of a transistor 22 goes to zero and is elevated. Then, the base voltage is higher than the zener voltage Vz of a zener diode 21 to interrupt the transistor 22 and a resistance variable terminal 23c is held to the negative potential according to the capacitor 24. This negative potential is applied to the first grid 1G of the cathode ray tube 1 and the anode current of the cathode ray tube 1 is immediately cut off to prevent the generation of the spot.
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