ASUS FL8000U is connected to the battery flash screen, and the adapter is normal

“Machine model” ASUS FL8000U

[Fault phenomenon] Boot splash screen

[Maintenance process]

I received an ASUS FL8000U sent by my colleague for repair. My colleague said that this computer can enter the system normally, but if it is powered on by a battery, the screen will keep flashing and the screen will be dark, but it will be normal if the adapter is connected or the adapter is used alone to boot the screen. He changed the screen and checked the drawings. The 3.3V power supply tube connected to the screen failed, and it really could not be repaired, so he sent it to us to see if it could be repaired.

Asus FL8000U is connected to the battery flash screen, and the adapter is normal. Figure 1.

Since our peers have changed the external equipment, we can just remove the motherboard to find the problem. After careful observation, we found that there were obvious signs of maintenance near the screen interface. The triode controlled by backlight and the resistance of brightness level have been replaced. When the battery is directly installed and turned on, the screen will indeed flash and the screen backlight is very dark. We found the interface definition seat and measured the backlight control and brightness level. The voltage is 3.3, which is no problem. The backlight power supply is 9V, which is the same as the battery voltage. After reading the schematic diagram, the backlight power supply is directly connected to the screen from a public point through an insurance inductor, which seems to be no problem.

Next, we plug in the adapter. After plugging in the adapter, it can be clearly seen that the screen lights up, and the screen is no longer flashing. After tapping the signal and power supply of the screen interface again, it is found that the backlight control has not changed as in brightness level. The only change is that the screen backlight power supply has changed from 9V to 16V, which seems to be the flashing caused by insufficient backlight power supply.

Asus FL8000U is connected to the battery flash screen, and the adapter is normal. Figure 2.

Asus FL8000U is connected to the battery flash screen, and the adapter is normal. Figure 3.

We disconnect the battery and measure the resistance of backlight power supply. The left common point resistance is 268, and the right backlight power supply resistance is 282. It seems that there is no problem, but the problem is that there is only one inductor between the main power supply and the backlight, and it is impossible for the inductor to have resistance. Directly remove the safety inductor and use a multimeter to value it. It can be seen that this safety inductor has an internal resistance of 18 ohms. It seems that the problem is obvious. Just pick an identical safety inductor from the material plate and install it again.

Asus FL8000U is connected to the battery flash screen, and the adapter is normal. Figure 4.

[Maintenance result] After the machine is installed, the functions of testing sound, network, recording and playing video are all normal, the frame rate of double baking for half an hour is stable, the fault is perfectly repaired, and the maintenance is over.