Use a special way to repair the logic board - android - Phones Developers

Probe domestic switch needle KG-300K, with a head diameter of 3.0mm, normally open switch needle

100V, 250mA, surface mount, high-speed switching diode

Shunluo Inductance Power Inductor SWPA Series – First Class Agent

High-power LED lamp beads, 1W white light, Sanan 30mil high power LED lamp beads, imitation lumens

Brand AVX TPSE226M035R0125 – Low impedance tantalum capacitor, AVX 22

I used a special method to repair the logic board of a Skyworth 42E321W LCD TV. The TV had boot music, the backlight was on, and there was a slight green light, but no image was displayed. Most people would immediately think about the logic board, but in this case, the logic board was actually the screen board. The TV is divided into several parts: power board, backlight board, signal board, and the screen board (which contains the logic board). The logic board is narrow and located at the bottom of the screen, making it hard to access and inspect. Under a magnifying glass, I found that the DC-DC conversion chip was CM501. After checking, I noticed that except for the 12V power supply, there was no more than 2V on the board. The 40 lines from the main board included two sets of signal lines, with a voltage around 1.3V, which clearly indicated that CM501 wasn’t producing the required 3.3V. I looked up some schematics online and carefully analyzed the circuit. I found that the DC conversion circuit didn't have a clock line connected to the motherboard or any data lines. However, there was a voltage conversion involved, and I wondered what the connection was. Then I noticed that pin 9 of the 9-pin connector was left open, and its voltage was zero. This meant that the screen wasn’t getting the necessary working voltage. Usually, pin 9 is powered by the signal processing IC CM2801B, and after testing, it was confirmed that CM2801B had no power supply. At first, I thought the problem might be with CM501, but replacing it was difficult due to the risk of counterfeit parts. I considered whether the 3.3V conversion might be broken, but not necessarily the whole chip. So I tried removing the 3.3V coil and using an external 3.3V source to test. The current measured around 280mA, and during the boot test, the image returned to normal. To make it more stable, I used a 7805 to step down 12V to 5V, then used two 1N4007 diodes in series to get about 3.2V. The result was a very clear image. Calculating the power usage, the 3.3V power supply needed 300–400mA, which is about 1.2W. If using DC-DC conversion, the efficiency is nearly 100%, so 12V would need about 100mA. But when directly stepping down 12V to 3.3V, the current would be 300–400mA, resulting in 3.6–4.8W. That means an extra 2.4–3.6W of power is wasted as heat, which is acceptable for this application. For improvement, connecting the power supply to the motherboard reduces heat and keeps the power consumption stable, making the setup cleaner and more efficient. I also considered that the 12V power supply on the logic board should have an electronic switch to ensure even distribution.