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例6 故障现象:屏幕上部有回扫线。分析与检修:根据图2电路结构和原理可知:屏幕上部出现回扫线的原因有二:一是场输出级STV9379②、③、⑤脚和外围D1557、C1556组成的泵电源输出特性不良;二是变焦垂直消隐脉冲形成电路工作异常不能产生完整的变焦垂直消隐脉冲。更深入地追溯:变焦垂直消隐脉冲不完整,只在变焦放大模式时屏幕顶部或底部出现回扫亮线,其他屏显方式的逆程消隐由VP/100(常规消隐脉冲)执行,VP/100脉冲丢失,会导致满屏回扫亮线。但场输出级
Example 6 Symptom: There is a flyback line in the upper part of the screen. Analysis and Overhaul: According to the circuit structure and principle of Fig. 2, we can see that there are two reasons for the flyback line in the upper part of the screen: first, the output characteristic of the pump power supply composed of the STV9379 ②, ③, ⑤ feet and the periphery D1557 and C1556 in the field output is not good; Zoom vertical blanking pulse forming circuit abnormal work can not produce a complete zoom vertical blanking pulse. Going even further: The zoom vertical blanking pulse is not complete. The flyback bright line appears at the top or bottom of the screen only during zoom-in mode. The reverse blanking of other screen modes is performed by VP / 100 (normal blanking pulse) Loss of VP / 100 pulse will result in full screen flyback bright line. But the field output level