Lenovo LA-B091P (LA-B091P )Automatically On & Off Problem

Laptop Model : Lenovo B50-70

Version number: LA-B091P

Fault: Standby is normal, the Power On Indicator Light Is On, The Current Is 0.02, And It Automatically Powers Off After 3 Seconds.

 

Maintenance Process:

Disassemble the machine and measure the standby voltage of 3V5V, which is normal. The waveform of the oscilloscope's rear stage is stable and there is no clutter. The measured 3V5V secondary conversion voltage to ground value is normal.

I checked that the board is fully integrated. With this kind of fully integrated bridge, the current does not come up and the timing does not continue. First think about the RTC part. The measured button battery is 0.34V. This is wrong, as shown in the picture above:

In the picture, you can see that the battery voltage passes through a resistor PR106 and goes directly to the dual diode PD101. Change a new battery and test the resistor on the left to 3V and the resistor to 1.35V on the right. 

It is wrong. It seems that something in the rear stage lowers the voltage. Looking at the drawing, there is only one capacitor in the rear stage, and it goes directly into the CPU.

After thinking about it, I removed the capacitor first, but the test is still the same. It should be that the CPU is pulled low, which is troublesome. Later, I did a small experiment. 

I plugged it in and directly shorted the resistor on the standby voltage side. I measured the RTCVCC voltage at 3V and it was normal. The standby current did not change. When I pressed the switch, the current went up and was displayed.