大家帮忙看一下这张图

NEC传真机开关电源故障
控制集成块是UC3842,刚开机电源灯闪亮后即灭,测7脚电压只有3.3V,8脚0V,启动电阻完好.更换3842故障依旧,部份电路图如下:
8050和8550组成的电路起到什么作用?
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