故障现象:屏幕2/3处光栅正常,下部有卷边,在无信号状态下,也无字符出现,在馈入有线信号后,有图像、伴音和字符出现。分析与检修:从故障现象看,故障应在场输出电路,而且与 I~2C 总线数据的改变关系不大,理由是:假如 I~2C 总线数据的改变,只影响幅度或线性不良,而不应伴有卷边出现。据此思路,先对场输出电路 LA7840的各脚电压(见表1)进行检测,从表1中发现,LA7840 Symptoms: the screen 2/3 normal grating, curling the lower part of the no signal state, there is no character appears, after feeding the wired signal, there are images, sound and characters appear. Analysis and overhaul: From the breakdown phenomenon, the fault should be present in the output circuit, but also has little to do with the change of I ~ 2C bus data. The reason is: If I ~ 2C bus data changes only affect the amplitude or linearity, and should not Accompanied by curling appears. According to this idea, the first field output circuit LA7840 feet voltage (see Table 1) to detect, from Table 1 found, LA7840