Method of correcting deflection defocusing in self-converged color crt

Abstract

This disclosure pertains to the production of self-converged color CRT display systems, and in particular to a method of reducing the effects of off-axis deflection defocusing of electron beams in such systems. The disclosed method comprises installing in the neck of each color CRT bulb a three beam in-line-type electron gun whose mechanical and electrical design parameters are such that the beams in their free fall state have a selected nominal value of underconvergence at the screen which is such that substantially the entire population of production tubes is underconverged. On the neck of each tube is installed a self-converging yoke which establishes, in addition to main deflection magnetic field components, an astigmatic field component which self-converges said beams, but which undesirably introduces astigmatic deflection defocusing of the beams when deflected off the tube axis. Static convergence of said beams at said screen is effected by establishing a static astigmatic quadrupolar magnetic field component common to all three beams and opposite to said astigmatic yoke field component which, while accomplishing the desired static beam convergence, deliberately introduces an astigmatic distortion of said beams. The distortion is a function, for each tube, of the free fall underconvergence value for that tube and at least partially compensating the deflected beams for the deflection defocusing of the beams by the oppositely directed astigmatic yoke field component.