52 - Picture Displays

Abstract

This chapter discusses picture displays. The focus electrode of the unipotential gun, commonly found in monochrome tubes, is intended to operate at about 250 V with a range from 0 to 350 V. The gun is known as unipotential because the electrodes on either side of the focus electrode are at the same potential. The advantage is that the gun is relatively insensitive to changes in focus potential. Deflection can take two forms: electrostatic or magnetic. Electrostatic deflection, used only for small angles of deflection, consists of a pair of plates that when suitably charged can attract and repel the beam to scan a line. A second pair of plates at 90° to the first completes the raster. Magnetic scan, used for most domestic tubes, uses the principle that the electron beam is deflected at 90° to a magnetic field. To complete the current path of the electron beam, the internal surface of the tube bulb is coated with a graphite loaded conductive coating, joining the cavity connector to the aluminum screen backing and the g4 gun electrode. Traditionally, electrons are emitted from a heated cathode. This is normally a nickel tube with a coating of a barium rich material, heated by a tungsten filament. The temperature and coating composition are critical factors of tube life. Attempts have been made to extend the life of the cathode by providing a reservoir of barium compound to constantly refresh the active area.