Probe current voltage pin 420*4450 head diameter 5.0 over current current and voltage pin

Photocoupler

Take over a Changhong LCD TV LT32710 three no, after the open test no 5v, internal 300v, measured power supply output Schottky half bridge, found 24v power supply 20200 breakdown, after replacement test machine, all voltages returned to normal However, the backlight is not bright, and there is an image shadow in the screen, indicating that the problem lies in the backlight circuit.
The backlight start signal from the motherboard and the brightness pulse width adjustment signal are all normal. , measuring the backlight chip OZ964 power supply 5v normal. The third pin of the test chip, the voltage is 0, the voltage is controlled by 2 channels, and the ENA from the main board is controlled by the voltage comparator composed of LM358. The ENA of the main board is normal, then the output of the LM358 is estimated to be abnormal.
Q206 is the FET that controls ENA. It has been turned on. The turn-on voltage comes from the thyristor circuit formed by Q1 and Q13, and the low level is triggered. The low level comes from Q14, and Q14 is controlled by LM358, which is already high level. Suspected that 358 is damaged, it can be instantly lit after replacement, and then the protection is extinguished. Note 358 is indeed broken, and there are other faults that cause protection.
Short the first leg of the OZ964 to the ground, cancel the protection, connect the external lamp to the board, and then turn it on. It is found that it can be lit, but the tube can only be illuminated to half.
Using the oscilloscope to measure the 11-pin and 20-pin waveforms of the OZ964, the theory should be the same. The actual measurement found that there is no output at 11 feet, and the 20-pin has a signal of 65 kHz 5Vp-p. There should be a problem with the 11-pin circuit. The multimeter test found that the 11-pin short circuit, down, found that the gate source breakdown of the drive tube Q28. The model is K712. A FET of the same type is removed from the overvoltage protection sampling circuit, and the fault is still found after booting. After measuring the waveform again, it is found that the output waveform of 964 is already normal, but the waveform on the coupling transformer is different. The rising edge of the rectangular wave on the T102 transformer is obviously attenuated and jagged. Indicates that there is a load short circuit.
The multimeter measures the gate-source resistance of the FET Q201 Q202 of the two power outputs inside the power supply and finds that it has broken down. With 2 5N50 replacement test machines, the whole machine resumes normal illumination.
Restore the protection circuit, test the machine on the machine, the sound map is normal, and the whole machine is repaired.