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一台红梅WJD-18型35cm黑白电视机在收看节目过程中经常出现光栅突然暗下去的情况,过了几秒、几十秒或几分钟,再去调节亮度电位器,又能收看,但故障消失后又嫌亮度太亮。这一故障变化无规律,已送修几次,均未解决问题。开机检查,发现故障时如迅速测量显像管的阴极、聚焦极、加速极等电压,都基本正常,只有栅极电压随光栅明暗变化而波动较大。当荧光屏很暗时,栅极电压为25V,正常时则为65V。该机亮度控制部分的电路如图1所示。故障时进一步测量400V整流二极管负极,电压并无波动,而8R_(21)(1.5MΩ)和8R_(23)(200kΩ)之间及8W_2亮度电位器端1及端2的电压均随光栅明暗跳变而波动,即光栅暗时电压降低。分别检查
A Hongmei WJD-18 35cm black and white TV programs often appear in the process of grating sudden dark situation, after a few seconds, tens of seconds or a few minutes, then go to adjust the brightness potentiometer, but also watch, but After the fault disappeared too bright too bright. The failure of this irregular change has been repaired a few times, did not solve the problem. Boot check, found that the fault, such as rapid measurement of the CRT cathode, focusing pole, accelerating pole voltage, are basically normal, only the gate voltage fluctuations with the grating light and shade fluctuations. When the screen is dark, the gate voltage is 25V, while normal 65V. The machine brightness control part of the circuit shown in Figure 1. When the fault is further measured 400V rectifier diode negative, the voltage is not fluctuating, and 8R_ (21) (1.5MΩ) and 8R_ (23) (200kΩ) and 8W_2 brightness potentiometer terminal 1 and terminal voltage are 2 with the grating dimming Variable volatility, that is, when the grating diminished voltage. Check separately