Patent GB-1271999-A (GB)
Title: Brightness limiters for television receivers
IPC: H04N5/59
Filed: 1969-05-19
Abstract: 1,271,999. Television C.R.T. circuits. R.C.A. CORPORATION. 19 May, 1969 [22 May, 1968], No. 25380/69. Heading H4T. In an E.G. colour television receiver shown in skeleton, wherein a kinescope is excited by rectification of deflection flyback pulses coupled over transformer 65 to halfwave H.V. rectifier 66 for the second anode, capacitor 67 to focus electrode rectifier 68, and halfwave rectifier 70, resistor 71 and capacitor 72 to screen control circuits 73; the secondary return being taken to the emitter of transistor 40 over resistor 75 forming a filter with capacitors 76, 77. The video detector output referenced to a selected D.C. level by voltage divider 16, 17 is connected to base of transistor 15 also biased from the divider, whose emitter is resistor grounded and connected over series network comprising matching resistors 18, 22; delay line 20, and conductor 21 to emitter of transistor 23 grounded over series resistor 25, matched cable line 26, resistors 24, 28 and brightness control variable resistor 29. Base bias thereto is applied over resistor 27 from the emitter of transistor 40 and one end of a voltage divider comprising variable resistor 41 in series with resistor 44 returned to positive reference supply. The collector is grounded over resistor 45, and base biased from divider 42, 43. The base of transistor 23 is RF by-passed by capacitor 29<SP>1</SP> and its collector is resistance coupled to the base of transistor 50, whose collector is coupled to the base of transistor 55, whose emitter is coupled over grounded resistor 56 to video driver 62 energizing the kinescope guns, while feedback is provided over series resistors 57, 58 and a grounded shunt correction path comprising series resonant capacitor 59, inductor 60, and variable resistor 61 to the emitter of transistor 23. It is shown that variations of the D.C. components of the signal or of the operating points of the amplifier 15, 23, 30, 55 together with operation of the brightness control varies the kinescope gun bias and beam current, and to avoid overdriving with consequent picture degradation, tube damage, and possible X-ray emission; the maximum bias in the video amplifier chain must be limited. Initially for kinescope cut-off at maximum setting of resistor 29, zero current in capacitor 67, and zero current in diode 70, with only leakage current in rectifier 66; transistor 40 is biased " on " and reference current in 41, 44 is defined by the brightness limiter threshold set by 41, so that C-E current to earth is equal to the limiting threshold current; base voltage plus base emitter drop in transistor 40 affording the correct bias for transistor 23. Decrease of brightness resistor 29 biases transistor 23 to increase conduction, and to increase positively the kinescope gun bias, whereby increased current in rectifier 66 decreases the C-E current in transistor 40, since the current through 41, 44 is constant. The emitter voltage is held constant until the C-E current is driven to zero, when its emitter voltage decreases, and its base electrode voltage is driven more positive than its emitter voltage. Thus, further decrease of brightness resistor 29 further decreases the emitter electrode voltage of transistor 40 due to its cut-off condition, and to further increase in current over rectifier 66; and the bias on transistor 23 is varied in such sense as to counteract the increase of kinescope current. Resistor 45 and capacitor 77 limit the current in transistor 40 and reduce surges applied thereto to prevent damage from areas across rectifier 66. Typical component values are given.