GB1212552A - Improvements in and relating to devices for limiting beam current in television picture tubes - Google Patents

Abstract

1,212,552. Cathode-ray tube operating circuits. BLAUPUNKT-WERKE G.m.b.H. 13 March, 1968 [22 March, 1967], No. 12129/68. Heading H4T. In a television receiver in which the average beam current is limited by conventional means, the magnitude of rapid variations is controlled by deriving a signal from the resulting rapid voltage variations at the tube anode and utilizing it, via an automatic gain control circuit, to control the level of the video signals modulating the tube beam. Thus, in response to a sudden positive peak in beam current the resulting negative peak in the EHT voltage at the tube anode operates via the gain control circuit to hold back the gain and thus the total contrast so that maximum brightness values correspond to lower values of beam current. This arrangement prevents the release of a large number of secondary electrons from the tube screen which would disturb the composition of a colour picture, cause heating of the screen and by affecting the deflection, lead to a blurred picture. Fig. 2 shows the described embodiment in which the EHT voltage variations are derived from the metallized coating 11 (capacitatively coupled to the anode inner coating 10) and fed via a protective choke 14 (blocking pulses of excessive magnitude), a capacitance voltage divider 15, 16 and a R.C. filter 4 (blocking momentary pulses, such as line fly-back pulses) to the cathode of a diode 5 which is biased by network 7, 8 into its cut-off range. Thus, negatively going voltages which exceed this bias cause the diode to conduct and the voltage developed at its anode is supplied to a biasing resistor 17 in the circuit of the video detector 18 to control the level of the video signal supplied to the video amplifier 19. This controls the level of the feed-back voltage from a gated automatic gain control circuit 20 which holds back the amplification and thus the total contrast until the modulation level returns to its previous value. The picture contrast is thereby reduced so that maximum brightness values correspond to lower values of beam current. The average beam current is controlled by a negative control voltage developed across a resistor 21 in the EHT generator 2 and supplied via a control amplifier 22 to the stages 23 which determine picture brightness. The pulses developed across the protective choke 14 may also serve for other purposes such as a control voltage for a ballast tube.