Patent GB-766542-A (GB)
Title: Improvements relating to circuits for cathode ray tubes or television receivers
Inventor: TAYLOR JOHN HARRY
IPC: H04N3/20
Filed: 1953-08-29
Abstract: 766,542. Television. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Aug. 23, 1954 [Aug. 29, 1953], No. 23926/53. Addition to 731,679. Class 40 (3). [Also in Group XL (c)] Relates to a television receiver cathode-ray tube circuit of the type disclosed in the parent Specification, in which the cathode of the C.R.T. 8 is connected to the anode of a pentode 1 whose resistive anode load 6, 7 is connected to the normal +H.T. supply, the control grid of pentode 1 being connected to a tapping on a potentiometer 4 connected between earth and the capacitor 5 (corresponding to capacitor 9 in the parent Specification) across which is produced a "boost" voltage by means of an " economy " diode magnetic line deflection circuit such that under normal operating conditions pentode 1 is conductive but in the event of a time-base failure there is no " boost " voltage and pentode 1 is cut-off thereby reducing the intensity of the cathode beam of tube 8 and preventing damage to the fluorescent screen thereof. According to the present invention a triode 2 is cathode coupled to pentode 1 by the common cathode resistor 3 and additional signals for controlling the beam of tube 8 are applied to the grid of triode 2; such additional signals may comprise (1) video signals and/or black-out pulses, or (2) signals for producing automatic brightness limitation; alternatively, as shown in the Figure, the brightness limitation signals are applied to the cathode of pentode 1. Frame black-out pulses may be applied to the suppressor grid of pentode 1. When positive modulation video signals are applied to the grid of triode 2, potentiometer 16 is adjusted so that signal voltages up to peak white are insufficient to overcome the positive cathode bias developed across resistor 3 by the current through pentode 1 but any unwanted signals of amplitude exceeding peak white, e.g. ignition noise, render triode 2 conductive and the resultant current through resistor 3 cuts-off pentode 1 and extinguishes the beam in tube 8. The level at which such " black spotting " is initiated may be controlled by potentiometer 16. As shown, the E.H.T. is derived from a free-running oscillator 10, 11 and the E.H.T. is stabilized by applying to the screen grid of valve 10 a positive control voltage which increases as the drain on the E.H.T. increases. Variable resistor 7 in the anode circuit of pentode 1 controls the brightness of the display and it is found that on adjusting resistor 7 to increase the brightness a point is reached at which the E.H.T. stabilization circuit is no longer effective to counteract the drain on the E.H.T. due to the increased beam current. Such an effect is prevented by connecting the screen grid of valve 10 via a potentiometer 13 to the control grid of a valve 14 which is cathode coupled to valves 1 and 2 by resistor 3. The grid of valve 14 is also connected via a diode 17 to potential divider 18, 19 such that under normal conditions diode 17 is conductive and valve 14 is non-conductive. When the drain on the E.H.T. is excessive, the resultant increased potential at the screen of valve 10 cuts-off diode 17 and renders valve 14 conductive thereby increasing the cathode potential of pentode 1 which counteracts the effect of any further adjustment of resistor 7, thereby preventing a further increase in brightness. As described in the Provisional Specification the screen grid of valve 10 is connected directly to the cathodes of valves 1 and 2 via a diode.