Skyworth LCD TV 32E55HM (8R28) has noise repair experience - Home Appliances

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I encountered an issue with a Skyworth LCD TV model 32E55HM (8R28 movement). The speaker was making a loud noise during the boot-up process, but after about three minutes, the noise would reduce and become less noticeable. However, even when the sound was normal, there was still a faint rustling noise coming from the speaker. I tested the machine by connecting an HD set-top box via the TV’s HD port, and the same issue occurred as with AV1 and AV2 inputs. When I reduced the volume, the accompanying sound decreased, but the background noise remained unchanged. When the volume was turned to zero, the noise disappeared entirely.

During startup, I measured the power board output voltages: 5V, 12V, and 24V were all within normal range. I checked the 24V supply to ground and tested the 1000uF capacitor, but found no issues. At this point, I ruled out a power board failure.

Visually inspecting the motherboard, I noticed that the main chip had a heat sink attached, but its model wasn’t clearly marked. Since the TV produced sound when first turned on, and the noise decreased over time, I suspected a temperature-related issue. I used an alcohol-soaked cotton ball and a hair dryer with cold air to cool the main chip. After cooling, the noise improved slightly, though it didn’t disappear completely.

I observed four three-terminal regulators on the motherboard: U14 = 5.07V, U13 = 2.64V, U10 = 1.26V, and U12 = 1.33V. Based on experience, I believed U10 was likely the core voltage for the chip and appeared normal. I suspected that U12 might be supposed to provide 1.8V. I measured the input of U12 at 3.3V, and using two standard diodes in series (3.3 - 0.7 - 0.7 = 1.9V), I connected them between the input and output of U12. This resulted in clearer sound, and the measured voltage was 1.41V. However, the diodes became hot, so I disconnected them. I then connected four diodes in parallel and then in series to increase current capacity, but the TV wouldn’t start up—no green light. Measuring U12 output gave 1.67V, and after resoldering the negative terminal, the TV booted normally again. It seemed like the adjustment wasn’t quite right.

Upon closer inspection, I found that U12 is an adjustable three-terminal regulator. R187 (330 ohms) connects the output to the control pin (upper adjustment resistor), while R186 (unknown model) connects the control pin to ground (lower adjustment resistor). I replaced R187 with a parallel 330-ohm resistor and retested. The TV started normally, and the U12 output was measured at 1.44V. I ran the test for five hours, and the sound remained clear and loud. Although I don’t have exact specifications for U12’s output, I’m confident it should be higher than 1.33V and lower than 1.67V. This repair was somewhat of a guess, but it worked.

Later, I examined R186 under a magnifying glass and saw the marking “220,” which should be 22 ohms. I replaced R186 with a 20-ohm and 10-ohm resistor in series. The new measurement of U12 output was 1.35V, and the TV functioned perfectly. At this point, the issue was resolved.