Lenovo ThinkPad T450 notebook is not turned on for repair

Lenovo ThinkPad T450 notebook is not turned on for repair

It is the problem of judging the power supply chip and its peripheral accessory components. Brush the solder paste, and re-solder the capacitors and resistors around the chip to be processed with an air gun. After cooling, the bare board is electrified, and the current goes up to 0.5+ and then goes up to 0.8+~1.0+ after a short stay. Plug in the screen and test the lighting machine.

Machine model: Lenovo ThinkPad T450

Main board version number: NM-A251

Malfunction: The notebook does not boot.

Maintenance process:

Plug it in for the first time. After the ThinkPad LOGO is displayed on the machine, it doesn’t light up anymore. Plug in the adjustable power supply, power it up to 0.252A, stop for a while, then power it off, and it drops to standby (0.004) and doesn’t trigger again.

Lenovo ThinkPad T450 notebook is not turned on for maintenance Figure 1

Dismantle the machine, unplug the built-in lithium battery, then unplug the CMOS battery, and plug in the adjustable power supply again after discharging. You can automatically power up to 0.252A and stop for a while. The latter phenomenon is the same.

Remove the motherboard, power on the bare board, and the current will run very low (0.009A), and then it will not trigger when power is off (0.004). Open the drawing and measure the common point voltage. There is always 19V power supply. According to the time sequence, the RTCVCC voltage is normal. -RTCRST voltage is obtained from the back stage of D3 or D6 through R620, and -SRTCRST is obtained from the back stage of D3 or D6 through R250. After measuring the voltage, it is normal.

Lenovo ThinkPad T450 notebook is not turned on for maintenance Figure 2

The probability of crystal oscillation is too small, so rule it out. Both DSWVRMEN and INTVRMEN are pulled up by resistors through RTCVCC, and both are measured at 2.8V+ near CPU. After plugging in, measure the standby voltage of deep sleep, and the L4 inductor is 3.3V normal. Measure 3.3VPG, no.

The machine chip is TPS51220A, which can normally emit three linear voltages, 2V, 3V and 5V, and can be measured. The PWM3.3V is also available on L4 inductor, and the measured PWM5V is only 4.2V ..

Lenovo ThinkPad T450 notebook is not turned on for maintenance Figure 3

Above, it shows that 3.3V is normal; below, it shows that 5V is only 4.2V Explain that the 5V voltage is insufficient and undervoltage.

Lenovo ThinkPad T450 notebook is not turned on for maintenance Figure 4

After disconnecting the rear stage of node PJ2, power on (use multimeter) to measure whether the voltage of 5V is 4.2V

Lenovo ThinkPad T450 notebook is not turned on for maintenance Figure 5

It is the problem of judging the power supply chip and its peripheral accessory components. Brush the solder paste, and re-solder the capacitors and resistors around the chip to be processed with an air gun. After cooling, the bare board is electrified, and the current goes up to 0.5+ and then goes up to 0.8+~1.0+ after a short stay.

Plug in the screen and test the lighting machine.

Lenovo ThinkPad T450 notebook is not turned on for maintenance Figure 6