Hisense LED26K316 LCD TV black screen with sound maintenance - Home Appliances

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Symptom: Black screen, backlight is not bright, there is sound.
Overhaul process: Test the power supply backlight (RSAG7.820.4561/ROH) 12V voltage, normal. The C853 (250V/47uF) has 101V at both ends, which is also normal. After the voltage across C932 (350V/10uF) rises to 170V, it slowly drops. It is found that the resistance values ​​of the two 510K chip resistors of R993 and R996 are reduced by about 10k, and replaced by a 1M and a 20K resistor. The voltage of the 5 pin (power supply pin) of the backlight control chip N901 (HL3051M-G1) is 11.9V, which is normal. Monitoring at start-up, connected to the G-pole of the FET V903 (U3N50) of the negative pole of the LED strip, the voltage of about 9V is immediately changed to 0V; its S-pole voltage (R974, R973, R972 three chip resistors in parallel) Both ends) are always 0V.
Reasoning: The G pole of V903 has a voltage of about 9V instantaneously, which is enough to make it conductive. If everything is normal, the S pole of V903 to the ground (both ends of R974) should also be able to measure the instantaneous voltage value, even if this voltage value Very low, but the actual measurement is 0V. Therefore, it is judged that the light bar has an open circuit or the V903 performance is not good.
Try to short-circuit the D-pole and S-pole backlight of V903, but the brightness is dark, because the power supply is lower at this time, so the brightness is darker, so that the LED light bar can be judged to be basically normal. Remove the V903 measurement, normal, reinstall, the backlight lights up, and the test reappears after half an hour. Try 10N60 for U3N50 and troubleshoot.
Summary: Due to the poor performance of V903, N901 can not measure the voltage on the S pole of V903, that is, the current of the light bar, and enter the protection state.