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

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Use a special method to repair the logic board. A Skyworth 42E321W cool TV set, there is boot music, the backlight is bright, there is a touch of green light, no image.
I believe that people generally look for logic boards. But the logic board of this TV set is the screen board. The whole machine is divided into power board, backlight small board, signal board, and screen board with logic board. The logic board is narrow, at the bottom of the screen, it is not easy to check and operate. Under the magnifying glass, the DC-DC conversion manifold is seen as CM501. Carefully measured, except for the 12V power supply, there is no more than 2V on this board. The 40 lines from the main board, two sets of signal lines, the voltage is about 1.3V, it is obvious that the CM501 does not produce 3.3V power.
So look up on the network, find some of the drawings of 501, carefully analyze, found that this DC conversion circuit, there is no clock line connected to the motherboard, data lines, there is no clock circuit, yes, voltage conversion, what is the relationship with these? That is, the 9-pin is the open pin, and now the 9-pin voltage is zero, so there is no working voltage required for the screen. However, the 9-pin is turned on, which is generally performed by the signal processing circuit CM2801B. It has been proved by practice. Now that the CM2801B has no power supply, how can it be implemented here? Everyone will consider DC conversion CM501. Because 3.3V is also his conversion. 3.3V is the power supply of CM2801B, the main power supply of the screen small board is very important. It seems that CM501 is damaged. However, it is very difficult to replace the CM501. Moreover, the online saying that there are too many fakes, it is rare to buy good goods. I thought, maybe CM501 is 3.3V conversion is broken, otherwise it may be good? This possibility is great. Say dry, put the 3.3V coil off the front, use the external 3.3V to connect to the coil test, measure the current, static 280ma, boot test, the image returns to normal.
Now there is a way to reduce the 12V with 7805, two 1N4007 in series at 5V, power-on test, the voltage is about 3.2V. The image is very normal.
Power calculation, 3.3V power supply, 300ma to 400ma, power 1.2W, DC DC conversion efficiency 100%, 12V requires 100ma current. Now, 12V direct step-down 3.3V, current 300ma to 400ma, power 3.6W to 4.8W, increase power 2.4W to 3.6W, increased power is dissipated with heat. This power waste is still affordable.
Improvement, if the power supply is connected to the motherboard, the heat is reduced, the power is not increased, and it looks good. Consider the logic board 12V power supply should have an electronic switch, so even.