Dual mode color switch for beam penetration crt

Abstract

A high voltage color switch for beam penetration CRT has two operating modes, the first of which is well suited to displaying a sequential group of colors on the faceplate of a CRT, and the second mode of which is well suited to displaying a random color that varies from a preselected baseline color. A high voltage power supply has an output voltage level which is selected to provide a predetermined baseline color when presented through the secondary winding of a transformer to the anode of a CRT. The primary winding of the transformer is connected through a first switch to the output of a driver. A feedback loop from the output of the driver senses the DC voltage level associated with the input waveform and biases the input of the driver to force the level of DC voltage presented to the primary winding of the transformer to zero. At the same time in the first mode the second switch is open so that the input waveform to the driver circuit is integrated in a low-pass filter into a DC level and then presented to the reference input of the high voltage power supply. In response to changes on its reference input the high voltage power supply changes its output voltage level so that no perceivable change is apparent to a viewer. In its second mode, the color switch can provide a color write period in a random color and the second switch is closed so that the high voltage power supply does not respond to changes in the DC level of the input voltage waveform. In this mode a reset period is necessary so that the magnetizing current in the transformer is returned to the center of its dynamic range.