Lenovo 15ISK NM-A711 notebook does not start repair

Machine model: Lenovo 15ISK NM-A711

Fault phenomenon: the notebook is not powered on, and there is no response when the power button is pressed.

Maintenance process: disassemble the machine and detect that the 3.3V is short-circuited by 2 ohms, but it can’t be detected that there is no temperature when it is powered on, and the 3.3V voltage burns the current for 2A, and only the redness near PL600 can be seen by the thermal imager.

Lenovo 15ISK NM-A711 Notebook Maintenance without Booting Figure 1

Lenovo 15ISK NM-A711 Notebook Maintenance without Booting Figure 2

I thought the south bridge was short-circuited, but the capacitance below the south bridge was not short-circuited, and the capacitance above the bridge was not short-circuited. I also thought that the south bridge was burned, and if others were burned, it would obviously heat up.

It’s hard to change this bridge with vinyl and it’s expensive. I’m ready to give up, but I always feel strange. Once again, put your thermal imaging finger on the crystal of the south bridge and clearly see that the finger part is not red. Isn’t the bridge broken? It’s still not obvious to remove the 3.3V inductor and burn it, and so is the 3.3V power supply chip. Thermal imaging is this piece, and you can’t tell which one it is.

Lenovo 15ISK NM-A711 Notebook Maintenance without Booting Figure 3

Smoke it with pine incense and then burn it. Sure enough, there is a melted part. When you look carefully, it turns out that the filtering capacitor with 3.3V output is short-circuited. The key point is that this capacitor is generally used for 20V filtering capacitors, and it rarely breaks down, so I didn’t think about it. I took it down and tested it, and it was short-circuited. The 3.3V short-circuit was ruled out. I can turn it on again.