Lenovo ThinkPad T450 Notebook no booting repair

Machine model: Lenovo ThinkPad T450

Main board version number: NM-A251

Malfunction: Notebook no booting.

Lenovo ThinkPad T450 Notebook no booting repair Figure 1.
Maintenance process:

When the machine is powered on for the first time, the ThinkPad logo is no longer on. Plug in DC power supply, power up to 0.252a, stop for a while, and then auto power off drops to standby (0.004) and no longer switch.Remove the machine, unplug the built-in lithium battery, and then unplug the CMOS battery. After discharging, plug in DC power supply again. You can automatically power on to 0.252a and stop for a while. The following phenomenon is the same.
Remove the motherboard, power on the bare board, the current runs very low (0.009a), and then auto power off no power on(0.004).Open the drawing and measure the common point VCC, there is always 19v VCC.According to the time sequence, rtcvcc VCC is normal, – rtcrst VCC is the post stage of D3 or D6 after R620, – srtcrst is the later stage of D3 or D6 after R250, and VCC is normal after measuring.

Lenovo ThinkPad T450 Notebook no booting repair Figure2.

Crystal probability is too small to exclude.Both dswvrmen and intvrmen are pulled up through resistance through rtcvcc, and are measured at 2.8V + near CPU.After the power is plugged in, the deep sleep standby VCC and L4 Inductor are 3.3V normal, and 3.3vpg is measured, but no.
Look, the standby chip is tps51220a. Normally, it should be able to send out three linear VCC,2v, 3V, 5V, and pwm3 is also measured3V is also found on L4 Inductor, and pwm5v is only 4.2V.

Lenovo ThinkPad T450 Notebook no booting repair Figure3.

The above figure shows that 3.3V is normal; the figure below shows that 5V is only 4.2V, indicating that 5V VCC is not enough and under voltage.

Lenovo ThinkPad T450 Notebook no booting repair Figure4.
After disconnecting pj2, power on (multimeter) to test whether 5V VCC or 4.2V.

Lenovo ThinkPad T450 Notebook no booting repair Figure5.
It is judged that it is the problem of VCC chip and its peripheral accessories. Brush Flux and SMD REWORK STATION to Capacitor resistance around standby chip again. After cooling, the bare board is powered on, and the current is up to 0.5 + and then 0.8 + ~ 1.0 + after a short stay. Plug in the screen and test booting up.