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

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

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I encountered a Skyworth LCD TV model 32E55HM (8R28 movement) with a noticeable speaker noise issue during maintenance. Initially, the sound was normal, but after a few minutes, a rustling noise became apparent, which gradually decreased in volume. When the TV volume was turned down, the accompanying sound reduced, but the background noise remained constant. Interestingly, when the volume was set to zero, the noise disappeared completely.

During startup, I checked the power board output voltages: 5V, 12V, and 24V were all within normal ranges. I also tested the 24V supply to ground with a 1000uF capacitor, but no improvement was observed. This led me to eliminate the power board as the source of the problem.

Visually inspecting the motherboard, I noticed that the main chip had a heat sink attached, though the exact model wasn't clear. The fact that the sound was audible at startup but gradually faded suggested a possible temperature-related issue. I used an alcohol swab and a hair dryer with cold air to cool the main chip, and the noise improved slightly, although it didn’t fully disappear.

I found four three-terminal regulators on the board: U14 = 5.07V, U13 = 2.64V, U10 = 1.26V, and U12 = 1.33V. Based on experience, I believed U10 was functioning properly as the core voltage for the chip. However, I suspected that U12 might be intended for a 1.8V rail. Measuring the input to U12 gave 3.3V, so I connected two standard diodes in series between the input and output. While this improved the sound quality, the diodes got hot, so I disconnected them.

I then tried connecting four diodes in parallel before using them in series across U12’s input and output to increase current capacity. However, the TV failed to boot (the red light didn’t turn green). After measuring U12's output at 1.67V, I soldered the negative diode back in, and the TV booted normally again. It seemed like the adjustment wasn’t quite right.

Upon closer inspection, I realized that U12 was an adjustable three-terminal regulator. R187 (330 ohms) was connected between the output and control pin (upper adjustment resistor), while R186 (unknown value) was connected between the control pin and ground (lower adjustment resistor). I replaced R187 with a parallel 330-ohm resistor, which resulted in a stable 1.44V at U12’s output. After testing for five hours, the sound remained clear and loud.

Although I couldn’t confirm the exact target voltage for U12, I was confident it was between 1.33V and 1.67V. This repair was somewhat of a guesswork approach, and I’m open to corrections if needed. This case reinforced my belief that troubleshooting often requires both technical knowledge and some trial and error.

Finally, I examined R186 under a magnifying glass and identified its value as 22 ohms. Using a soldering iron, I confirmed it measured 22 ohms. I replaced it with a 20-ohm and 10-ohm resistor in series, resulting in a U12 output of 1.35V. The TV functioned perfectly after this fix. The issue was resolved successfully.