ACER 4741G Wistron JE40- CP

I haven't posted a notebook repair post for many days, and I am busy with data recovery all day long. The year is coming to an end, and the apprentices are going back to set up shop. They are usually responsible for the maintenance of these machines, but now I have to do it myself. Basically, I have to repair 5 boards every day. Today I will choose an interesting one and write a post. Let's get down to business. Model: ACER4741G Motherboard :

Wistron
JE40- CP
Architecture: I3+HM55+NV independent display

Fault description: There is a standby current when it is powered on, and it does not trigger. It does not trigger on the surface, but in fact there is already a trigger action, and the current jumps from 0.02 to 0.05 repeatedly.

To repair this kind of failure, I usually don't look at the drawings, because it is better to have a picture in my heart than to look at a picture! From the previous fault description, we know that the standby condition of the machine is Basically Met. The Oscilloscope Can Directly Measure the 3V 5V Standby or The Pin1 Pin of the EC ROM Has Data Transmission. O 0.05 After Pressing the Switch, Which FURTHER ProVES that there is no problem why can't the current rise? We know that Wistron has a 3V 5V standby-to-run voltage conversion MOS similar to Quanta. We first measure whether the MOS outputs 3V 5V correctly after triggering. As shown below:

The picture shows the voltage after the 3V voltage has been switched by the MOS. It is obviously wrong. The maximum value is only 2.48V and this waveform has been maintained. It can be seen that his control level waveform is also wrong, as shown in the figure below:

The control stage and the output stage are equal, and the same is true for the measurement of the 5V voltage. We often encounter this in the maintenance of Wistron and Quanta boards. 90% of them are short circuits in the rear stage, but there is no serious short circuit. We can check it with a meter. Use the ground resistance to judge which path is short-circuited. Here we only use an oscilloscope for the whole process. How to use an oscilloscope to determine whether this short circuit
is a 3V load short circuit or a 5V load short circuit, in fact, you only need to measure the 3V 5V standby voltage waveform. As shown below:

The figure shows the waveform of the 3V standby voltage, which is normally flat and does not appear in the picture, but the waveform of the 5V standby voltage is flat, which proves that it is a short circuit caused by a 3V load. Next, use the burner directly The position can be accurately judged by using the method, and the adjustable power supply is connected to the 3V switching MOS, and the high current is directly adjusted to 3.3V, and the fault point can be found quickly by touching it with hands.

The final failure point of the machine in this case is caused by the leakage of the C359 small capacitor in the above picture. This failure can be repaired by directly removing it. The whole board repair takes less than 5 minutes. Faults are accurate to a single point.