Q750 and its peripheral components constitute a line current protection circuit, and the output current detection resistor is R758. +B voltage (140V) of the power output is sent to the line output circuit via R758. Line output circuit work normally, less current to flow through the R758, R758 at both ends of the voltage is small (less than 1.4V) to enable d750 and Q750 conduction, Q750 collector output low level, circuit protection not action. Adeptness output circuit over-current, the current flow R758 rise, R758 at both ends of the voltage is increased, and higher than 1.4V. Thus d750 and Q750 conduction, Q750 collector outputs high level, the D752 sent to Q502 base and trigger Q502. Q502 and Q501 are in the form of a phase locked loop, which is equivalent to a unidirectional thyristor. When the Q502 base is triggered by a high level, the Q502 is turned on, and the collector voltage drops, so that the Q501 can also be turned on. After the Q501 conduction, the base current of Q502 increases, the Q502 conduction is strengthened, which makes the Q501 conduction also follow the result of this cycle (positive feedback), so that the Q501 and Q502 saturation. Saturated and Q502 Q501, Q501 shot very output low level, is sent to the power supply circuit, the power circuit is placed in the standby state, various output voltage is decreased about half, three machine. At this time with the remote control also unable to boot.

R755, R756 and D751 constitute the +B voltage protection circuit, when the +B voltage is normal, the 10V point voltage is less than A, while the D751 value is 10.4V. D751 is in the off state. Voltage exceeds the 155V, a voltage more than 11V, and the D751 conduction, and trigger Q502, enable Q502 and Q501 quickly saturated, shoot Q501 outputs low level, the power to enter the standby state, the whole appearance of the three. D530 Q502, Q502 and Q501, D531,, C530, R531, D530, R532 and other components of the X - ray protection circuit. As we all know, adeptness flyback pulse rate is too high, picture tube anode voltage is too high, so that the CRT to produce harmful to human health of X ray. In order to prevent the phenomenon of this phenomenon, the X - ray protection circuit was designed. The working process of the X - ray protection circuit is as follows:

When the normal work of the circuit, inverse range pulse amplitude is also normal, at this time, the d531 rectifier, C530 filtering, generated by the DC voltage is small, the DC voltage after partial pressure of R53l r532, formed in the B point of the voltage is less than 1O.4V, is not sufficient to make the D530 conduction. When for some reason (e.g., inverse process capacitance decreases) caused by the inverse process of pulse rate is too high, DC voltage produced by d531 rectifier, C530 filtered increased. After r531 and r532 partial pressure, the point B voltage higher than 11V, so that the D530 conduction, and trigger Q502, Q502 and Q501 quickly saturated, shoot Q501 pole output low level, the power to enter the standby mode, three in the machine, thus avoiding the generation of X ray.

Components such as R458, R452, D456, D457, and so on. When the scan circuit is normal, the R458 / / R452 on the / / on the saw tooth wave pressure drop is also small, is not enough to make the D456 and D457 conduction. When some reason caused IC451 output sawtooth wave current is too large, sawtooth voltage R458 / / R452 on also will too large, so that d456 and D457 conduction, and trigger Q502, enable Q502 and Q501 quickly saturated, shoot Q501 outputs low level, the power to enter the standby state, the whole appearance of the three.

When the protective circuit, the machine will enter the standby state, appear three phenomenon. This phenomenon is the same as the remote control is the same, but after the remote control, remote control can boot, but after the protection circuit operation, with the remote control can not boot.

As the Q501 emitter output voltage is to be sent to the CPU (IC1101) 26 pin output voltage to the power supply circuit. Therefore, when the machine has three non phenomenon, and the power in the standby state, first of all have to understand is caused because of the failure of the remote control system, or action of the protection circuit caused by the. Judging method is very simple, just disconnect the R1143, you can easily distinguish between the causes of failure. If after disconnecting the R1143, protected immediately relieved, Q501 shot extremely variable for high level, it is indicated that the fault is caused by remote control system; if the disconnection of R1143, the machine is in state protection, Q501 shot very still low level, indicating that the fault is caused by the action of the protection circuit. At this point, it can be used to determine the fault location more accurately. Because it is low emitter and, row shows that high emitter D752 caused by its because it shows that high emitter presence shows that high emitter, for example if the disconnection of d456 (or D457 after protected lifted Q501 become level fault output circuit; if the disconnect D530, protection of the state lifted, Q501 become level fault line retrace pulse amplitude is too high or X-ray protection circuit problem; if the disconnect, the protection of the state lifted, Q501 become level fault output circuit and over-current protection circuit itself; if the disconnect IM56 D530 D752, protect state has not lifted, Q501 remains level explain fault B voltage caused by excessive. In the process of judging the fault, to pay special attention to the following points:

(1) the use of open circuit method, it is best not to cut off the D751, in order to prevent the +B voltage is too high and damage the line output circuit.

(2) to Q750 (C point to the collector voltage or voltage diagram) as a judge for the over-current protection circuit on the basis of whether the action. In doing so, it is highly likely to cause miscarriage of justice. Because the adeptness overcurrent, Q750 conduction and collector outputs high level, but when the action of the protection circuit. The unit standby, line scanning circuit stop working, the current flowing through the R758 greatly decreased, Q750 turn for cut-off collector into low level. If at this time, the Q750 collector is still low and the protection circuit is not the same as the protection circuit.

(3) not to B voltage X ray protection circuit for judging whether the action on the basis of. Because the B point voltage is only in the presence of the X ray protection circuit, when the circuit enters the protection state, the line scan circuit stops working, and the B point voltage is changed to 0V.

(4) D751 and D530 models are MA4104J voltage value for 10.4V, once the damage, should choose the same voltage value of the voltage regulator tube replacement, if voltage deviation is too large, the protection circuit becomes too sensitive or too slow.

(5) C750, C50l, C752 and other capacitive delay, can make the protection circuit of the anti - interference ability, can prevent the power on the power surge current or interference pulse caused by the protection circuit error. If these capacitors fail, the machine may appear in the process of the work is not time off phenomenon.