Hisense LCD TV TLM4236P (1138 board) backlight bright and off with sound murmur hoarse one machine two disease repair - home appliance repair - Huaqiang Electric

Photocoupler

When I first tested the Hisense LCD TV TLM4236P (1138 board), the boot light didn't come on at all. However, during a brief moment when the screen lit up, I noticed that the brightness started from the left and gradually faded toward the right. This made me suspect an issue with the high-voltage board on the right side.

During a power-on test, the left-side high-voltage board briefly produced output, but the right-side high-voltage transformer had no output. After triggering protection, I measured the primary side of the high-voltage transformer and found around 7–8V at the driver IC’s pin 1 (which should be grounded). This indicated a broken ground loop. After careful inspection, I discovered that the short-end solder joint of JP68 was not properly connected. I re-soldered it, and the boot screen no longer triggered protection.

I then connected the signal test machine, and the problem still persisted... The second symptom appeared: the audio was distorted and hoarse, with a noticeable rustling sound even when no signal was present. I tried searching online for similar cases but couldn’t find any relevant information. I decided to dig deeper instead of taking shortcuts.

After checking the speaker, there was no DC voltage difference across it, and the temperature of the power amplifier TFA9810 was normal. The fault remained unchanged after running the TV for 20 minutes. Using an oscilloscope, I observed that the input section of the sound buffer and amplifier U45 showed noise. However, the input waveform at pins 73 and 74 of the MST6X89 chip was normal. This clearly pointed to an issue with the MST6X89 itself.

I checked the voltage on pin 70 (AVDD-AU), which was 3.3V—normal. But pin 69 had no voltage at all, which was definitely abnormal. I first discharged capacitor C435 and measured the voltage on pin 69 again, which rose to 1.6V. Then, when I plugged in the audio signal line, the sound became clear and pleasant.

The pin 69 is labeled as AUVREF, where AU stands for the audio circuit, and REF means reference voltage. The external capacitor connected to this pin acts as a filter for the reference voltage.

This reference voltage functions similarly to the base bias in a transistor amplifier circuit. If the bias voltage is too low, the negative half-cycle of the AC signal can't pass through. If it's too high, the positive half-cycle may saturate the transistor, causing distortion or even no output. In most signal amplifying circuits, the reference voltage is usually around half of the operating voltage. If it's off, the output will be distorted or cut off. This understanding helped me quickly identify the root cause of the issue.