GB1059125A - Improvements relating to circuits for protecting the luminescent coating of a cathode ray tube - Google Patents

Abstract

1,059,125. Cathode-ray tube circuits; television. TELEFUNKEN PATENTVERWERTUNGS G.m.b.H. Oct. 9, 1963 [Oct. 9, 1962], No. 39738/63. Headings H4F and H4T. The luminescent screen of a cathode-ray tube 1 (Fig. 1) is protected from damage by the provision of an auxiliary bias source (shown schematically as a battery) 10 which, on switching-off the equipment, causes the production of an intense beam to discharge the wall anode fed from the EHT supply 11 before the deflection of the beam ceases. The source 10, poled as shown, is connected in series between the control electrode 6 and the brightness control bias potentiometer 8, the actual potentials on the cathode 2 and electrode 6, Uk, Ug respectively, being shown in Fig. 2 together with the potential Ugk of electrode 6 relative to the cathode and the potential E of source 10. On switching off the equipment at time t 1 the P.D. Ugk decreases negatively until time t 2 after which it increases positively towards the value E and produces an intense electron beam which discharges the tube wall-anode, the decay rate of the deflection circuits being such that discharge is substantially complete before deflection of the beam ceases. In practice, the source 10 comprises a capacitor which is charged (to about 40V) by rectifying positive or negative pulses (such as the blanking pulse 13) derived either directly from the horizontal recurring circuit (Figs. 3 and 4 respectively, not shown) or from separate windings on the horizontal transformer (Figs. 5 and 6, not shown). For automatic contrast adjustment as a function of ambient brighteners, a photo-sensitive resistor (F) may be shunted across portion of potentiometer 8 (Fig. 7, not shown).