GB1047515A - - Google Patents

Abstract

1,047,515. Cathode-ray tube protection circuits. BRITISH BROADCASTING CORPORATION. May 7, 1964 [May 14, 1963], No. 19136/63. Heading H4T. A control voltage produced on failure of either the line of field deflection waveform sources of a cathode-ray tube is utilized to render the beam innocuous to the tube screen. The control may be effected by the application of a suppression bias to a suitable electrode of the tube together with, if desired, a " followup " protection effected by controlling the EHT voltage and the beam focus current. Alternatively, protection made be effected by deflecting the beam off the screen. In the described embodiment the control voltage which is utilized as a beam suppression bias on the control grid 23 (Fig. 1) is developed as a negatively-going pulse (Fig. 2, not shown) at point 24 during the stable states of monostable multivibrators 19, 21 the latter being triggered to such states by pulses derived from the line and field drive pulses respectively (which occur at the commencement of the flyback periods) and then triggered to the quasistable states, during which the control voltage is absent, by pulses derived from the trailing edges of the fly-back pulses produced in the line and field deflection coils. Thus, in normal operations the beam is un-suppressed during the forward stroke of the scanning saw-tooth and during the fly-back periods the control voltage is effective to suppress the beams, i.e. to effect normal blanking. However, upon failure of either the time-base circuits or the drive pulses therefor the multivibrators revert to their stable states and effect suppression of the beam or operate in the other manners referred to above.