The machine parts separately by different dc power supply circuit. A switching power supply output a + two dc voltage 108 v and + 5 v. Among them, + 108 v dc voltage supply line output, output and audio power amplifier circuit; + 54 v dc voltage supply line oscillation, incentive and high-frequency tuner tuning voltage circuit. Line output transformer with high voltage and medium voltage, low voltage, three kinds of dc power supply, the + 12 v low voltage power supply for the rest of the small signal processing circuit, high and medium voltage dc power supply output circuit and auxiliary circuit tube. Bqjservice manual download site
 Bqjservice manual download site
From the point of power supply of the machine, when the switch power failure occurs, will lose the whole power supply, will cause no grating, no sound. If trouble-free switch power supply circuit, output circuit, and field work is not normal, can also be through VT704, VS705 protection circuit, make the switch power supply output voltage, also can cause no grating, no sound. In addition, the line scan circuit, such as failure, make the output circuit to stop working, the line output transformer is unable to provide the output dc voltage, the public channel and tube due to loss of working voltage at the same time, also causes no grating, no sound. If all normal switching power supply and line scan circuit, low voltage dc 12 v power supply is faulty, the former lose power supply will cause no audio, the latter has no power can make three color difference signal output dc voltage is zero, tube cut-off, without a grating phenomenon. Bqjservice manual download site
 Bqjservice manual download site
Apart from the above reasons, the machine of switch power supply circuit adopts horizontal frequency synchronously. The whole machine set of overvoltage and overcurrent protection circuit in the circuit. This part of the circuit principle of work is, when the output transformer with high pressure exceeds limit, provided by the sampling line output transformer winding flyback pulse after VD705 rectifier, C730 filtering by a dc voltage also increases accordingly, when the voltage increases to make the thick film circuit CP701 internal voltage regulator tube conduction, the (4) feet of the output control signal makes the VT704 and VS705 conduction. After when V5705 conduction through the resistor R729 + l08V dc voltage of short circuit; When the line of the output current increases, the resistance increases the voltage drop on the R714, increase the voltage through R737, C735 VD708 conduction, cause VT704 and SCR VS705 conduction, also on the output current increases, the resistance increases the voltage drop on the R625, through T704 R725 resistance can also control and SCR VS705 conduction. Because VS705 conduction after all can the short circuit of 108 v dc power supply, then the fault phenomenon is also light, silent. However, at this time the switch transformer will be issued a "creak" sound (power into the protection state). Therefore, from the switch transformer can be issued a "creak" sound can judge the fault of the nature and the location.

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