一台熊猫3402型44cm黑白电视机,其故障现象为:通电试机,只有喇叭有较轻的噪声外,无光栅、无伴音。检修时,打开电视机,测得+B的12V已变为+16.7V。查电源调整管7V_4(3DD303A)的c、e结击穿。测电源其他各元件及主负载无明显异常。所以将7V_4换新后,+12V电压正常。但故障依旧,仍无光栅和伴音。测行管c极为+12V,基极为0V,无负压。表明行 A panda 3402 44cm black and white TV, the symptoms are: power test machine, only the speaker has a lighter noise, no grating, no sound. Overhaul, turn on the TV, measured + B 12V has become + 16.7V. Check the power adjustment tube 7V_4 (3DD303A) c, e junction breakdown. Other components of the power supply and the main load no obvious abnormalities. So after 7V_4 renewal, + 12V voltage is normal. But the fault still, there is no grating and sound. Bank measured very c +12 V, the base is 0V, no negative pressure. Indicate the line