ASUS X455LJ Laptop Power On No Display Repair

 Laptop Model: ASUS X455LJ

Fault : No Display

Repair Process :

After disassembling the machine and looking at the board condition, it was a second machine for repair. Open the drawing. This board only has a bitmap. It is measured that the common point is normal at 3V5V and has no output.

     The standby chip is UP1589Q. Linear 3V has no output. Linear 5V is only a little more than 4V. The chip is very hot when I touch it. I measured the working conditions of the standby chip to check. The main supply has 19V going through a resistor 18.6V.

     Then, the linear turn-on is 19V, the common point voltage. I am a novice and have never seen such a high voltage. I remember the teacher said that there is usually a voltage divider here for linear turn-on.

     After checking the bitmap, I found that there should be a missing ground resistor PR8115. , but the strange thing is that there is no information about this resistor on the drawing. It seems that it has not been installed in the first place. I would like to ask everyone here, is it normal to turn on 19V? As shown in the picture:

I used a calculator to calculate that the resistor that should be added here is 200K, so I removed a 200K resistor from the material board and added it, then powered it on and measured it to turn on to 5.7V.

 I thought it should be able to turn on with less! When I tested the 3V linearity again, there was still no output, the 5V linearity was only a little more than 4V, and the chip was still hot. 

 Then turn on the oscilloscope to measure the 3V linearity and there is no movement. When measuring the 5V linearity there is movement, but the 5V rises to 1.48 seconds and then drops and rises again, as shown in the figure.

Now you know why the chip is hot? I wonder if there is a short circuit in the 3V linear power amplifier. The measured 3V linear to ground value was 440, and the 5V linear value was 520.

 There should be no short circuit. I simply removed the upper and lower tubes of the 3V5V and took a look. The result was that the problem remained the same.

 

The same thing happened after changing the standby chip. Finally, I thought of a way to disconnect a node of the 3V linear with a blade (please ask the masters if there is a way to disconnect without damaging the board), and then measure the 3.3V linear.

 It came out, the 5V linearity is normal, the linear turn-on measurement is also normal at 6V, and the chip is no longer hot.

Now I am more certain that the problem is with the 3V linear power amplifier. After checking the power amplifier, there is basically no obvious short circuit problem.

 After the second-level conversion, there are also three-level tubes. Every resistance and capacitance has been measured. Even the EC has been removed. The measurement problem is still the same when the 3V linear point was disconnected.

Suddenly, an idea occurred to me that there seemed to be a 0 ohm passing through during the measurement. Resistor, as shown in the figure:

I didn't know if there was any problem later, so I dismantled it and tried it. I was excited and found that the 3.3 linearity came out, and the 5V linearity was also normal. I checked the 0 ohm resistor and found a BAT54CW.

When the value of the third pin to ground was measured to be 0.07, it was found that there was a short circuit.

This is the common cathode diode used by the battery and the 3.3V linear power supply for the RTC circuit. After tracing, it was found that a 1UF small capacitor next to the keyboard port was short circuited.

After replacing the small capacitor, the standby function was normal. When I measured the 3V linear value to ground, it was still more than 440. This experience told me that a normal value to ground does not mean that the power stage is normal. After installing the power supply and entering the system, the repair was completed.