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[例1]开机无光栅、无伴音、无图像,但开机/待机指示红灯亮。分析检修:指示灯亮,说明滤波板上辅助电源+5.6V直流输出正常。测量光电耦合器N_(903)①～②、④～⑤脚电压正常,待机控制电路没有起控。断开L_(915)、L_(913)(拆去140V过压保护),在L_(915)右端与地之间跨接一只100W灯泡作负载,测量+B140V、+30V两组电源无输出,证实开关电路未起振。用示波器在开机瞬间观察开关管V_(901)基极,正常时该点应有1.52V_(p-p)50Hz起动脉冲,而屏上实际显示启动脉冲的幅度约0.45V_(p-p),这说明启动电路或正反馈回路异常。检查R_(901)、R_(902)、C_(907)、VD_(903)、C_(901)、C_(902)、V_(901),发现C_(907)严重漏电,更换新品电容后V_(901)起振,声、光、图恢复正常。开关电源正常振荡的两个必要条件是:启动电源提供
[Example 1] There is no raster on, no audio, no picture, but the power on / standby indicator is red. Analysis and maintenance: indicator light, indicating that the auxiliary power supply +5.6 V DC output on the filter board is normal. Measurement Optocoupler N_ (903) ① ~ ②, ④ ~ ⑤ pin voltage is normal, the standby control circuit is not controlled. Disconnect L_ (915) and L_ (913) (remove 140V overvoltage protection) and connect a 100W bulb between the right end of L_ (915) and ground for load measurement. + B140V, +30 V No output , Confirmed that the switching circuit has not started. Use the oscilloscope to observe the base of the switch V_ (901) at the moment when the power is turned on. This point should have a start pulse of 1.52V pp (50Hz) normally and the actual start pulse amplitude on the screen is about 0.45V_ (pp) Or positive feedback loop is abnormal. Check the R_901, R_902, C_907, VD_903, C_901, C_902 and V_901, and find that there is a serious leakage of C_907. After replacing the new capacitor V_ ( 901) Start-up, sound, light, map returned to normal. The two necessary conditions for the normal oscillation of the switching power supply are: Startup power supply