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四、维修案例[例1]故障现象:不开机。分析与检修:此机器是分公司的返修板。首先接入机架测试故障,发现指示灯亮,但是不开机。初步分析:由于指示灯正常点亮,说明待机电路的5 V待机电压正常。于是测试数字板插座P803的P14脚,发现没有高电平输出。进一步测试待机控制的Q803的基极为高电平,说明数字板的主解码芯片没有工作。检查数字板的待机电路,发现待机电路的U803的DC-DC转换电路没有工作,没有1.26 V的待机电压。测试L807处的对地电阻也没有发现异常。决定先
Fourth, the maintenance case [Example 1] Symptom: do not boot. Analysis and Maintenance: This machine is a branch of the repair board. First access to the rack test failure, the indicator light, but does not boot. Preliminary analysis: As the light is lit normally, indicating standby circuit 5 V standby voltage is normal. So test P143 P80 pin socket and found no high level output. The base of the Q803 for further test stand-by control is high, indicating that the main decode chip of the digital board did not work. Check the digital board standby circuit, found the standby circuit U803 DC-DC conversion circuit is not working, there is no standby voltage of 1.26 V. No abnormalities were found in the resistance to ground at the test L807. Decide first