Hisense LCD TV TLM4236P (1138 board) the first illness: boot light is off. Carefully observe the moment when the screen is bright, and the left side is bright to the right. It is estimated that the problem of the high voltage board on the right is likely.
Power-on test, the high-voltage board on the left has high-voltage output instantaneously, and the high-voltage transformer on the right side has no output. After protection, the high-voltage transformer primary and the driver IC 1 pin (ground) have 7--8V voltage. Obviously, the ground loop is unreachable. After careful measurement, I found that the short-end solder joint of JP68 was soldered.
The second symptom: the sound hoarse is seriously distorted, and the audio signal line is still rustling. Want to take a shortcut to see if there are any precedents on the Internet, check for a long time without any clues.
There is no DC voltage difference across the measuring speaker, and the temperature of the power amplifier TFA9810 is also normal. The fault did not change after 20 minutes of booting. The oscilloscope measures the sound buffer and amplifies the U45 input section with clutter. The input waveform of the MST6X89 73\74 pin is normal. The problem is obviously on the MST6X89.
Measurement 70 feet (AVDD-AU) 3.3V is normal. There is no voltage on the 69 feet, which is obviously not normal. First blow down C435, start measuring 69 feet voltage rise to 1.6V, plug in the audio signal line, the sound is very bright and pleasant.
Regarding AUVREF, AU represents a sound circuit, VERF: reference voltage. The external capacitor is the reference voltage filter.
This reference voltage is equivalent to the base bias of the triode amplifier circuit. When the bias voltage is too low, the negative half cycle of the AC signal cannot pass. When the bias voltage is too high, the positive half of the AC signal saturates the transistor and cannot pass, so the AC is small. The reference voltage of the signal amplifying circuit is usually about 1/2 of the operating voltage of the circuit. If it is too high or too low, the output will be distorted or even output. This is the basis for this fast judgment.
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