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[例1] 故障现象 开机三无,机内“吱吱”叫。去掉机内负载,在+B输出端接一只60W/220V灯泡做假负载,开机测+B,高达200V,且逐渐降至160V左右固动不动。调整取样电位器VR501,+B变化范围很小,不能调整到正常值110.5V。 分析与检修 开机三无“吱吱”叫,说明机内保护电路发生作用。保护电路启动的原因是+B偏高,因此故障出在开关电源部分。开关电源有电压输出,表明开关管及电源振荡电路基本正常;调整VR501,+B有变化,表明取样电路也基本正常。+B偏高的直接原因是开关管导通时间过长,而开关管导通和截止受脉宽控制电路的控制。测脉宽控制电路元件,大都正常,唯反馈启动电容C506(47μ/25V)容量偏小且有漏电现象。试用100μF/50V新电容更换C506后,整机供电立即恢
[Example 1] Failure phenomenon boot no, the machine “creak” called. Remove the machine load, a 60W / 220V bulb in the + B output to do a false load, power measurement + B, up to 200V, and gradually reduced to 160V or so fixed. Adjust the sampling potentiometer VR501, + B changes in a very small range, can not be adjusted to the normal value of 110.5V. Analysis and troubleshooting boot three no “creak” called, indicating the role of machine protection circuit. Protection circuit starts because + B is high, so the fault is in the switching power supply section. Switching power supply voltage output, indicating that the switch and power supply oscillation circuit is normal; adjust VR501, + B have changed, indicating that the sampling circuit is also normal. + B is the direct cause of the high switch-on time is too long, and the switch is turned on and off by the pulse width control circuit control. Measuring pulse width control circuit components, mostly normal, only the feedback capacitor C506 (47μ / 25V) capacity is small and there is leakage phenomenon. Trial 100μF / 50V new capacitor replacement C506, the whole power supply immediately restored