Due to the brightness of the CRT projection tube demand is high, so the anode high pressure is much higher than ordinary picture tube, if the anode high voltage change with image content, it will cause the picture size is not stable, image and grating has a shrinkage. The main reason for the high pressure changes in addition to the X board related IC771 within the high voltage circuit, but also associated with the level of line + 140 v power supply, and + 140 v voltage is high and low change, more is the main regulating circuit of switch power supply has a problem, or switching power supply load ability. Ac input voltage with the coupling voltage regulator mediation, + 140 v voltage, the voltage changes over voltage. So doubt there is something wrong with the main switch power voltage regulator circuit, the main switch regulated power supply voltage control circuit is shown in the appended drawings. It is by error detection amplifier IC808, photoelectric coupler D807 and IC801 internal circuit composed of 6 feet. Main power after start up work, switch transformer windings T801 Vl, V2 induced by D887 pulse voltage, 22.1 V dc voltage is formed after C812 rectifier filter, by photoelectric coupler D807 subprime photosensitive tube collector and emitter to IC801 6 feet, make it 6 feet C4 internal capacitance charging, change D807 sensitive resistance pipe set, shoot, the inside can control the size of C4 charging current, so as to control the field effect switch tube conduction time, make the output voltage stability. If for some reason the + 140 V dc voltage increases, the variation of the voltage increases, the circuit detection by IC808 (1) feet after partial pressure, add to the error amplifier tube base, after comparing with its emitter, the base voltage amplification, make through the photoelectric coupler primary light-emitting diode current increases, the luminous intensity increase, the second level of photosensitive tube sets, and internal resistance is reduced, causing IC801 6 feet of current increases, its internal C4 charging voltage up to 0.75 V time speeding up, the field effect switch tube conduction time shortened, switch pulse duty ratio decreases, and T801 main winding the energy stored in the P1 and P2 is less also, + 140 V dc voltage to set value.

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