Dell Latitude E7450 (LA-A961P) Short Circuit Repair

Laptop Model: Dell Latitude E7450

Board Number : LA-A961P

Fault: There is a short circuit when powering on a large current of 1A, and there is a power outage inside.

Repair Process:

After disassembling the inductors, a short circuit was found. The common point short circuit resistance was 00. Run the line from the adapter to find the initial common point isolation tube. It was found that the first self-conducting isolation tube had broken down and replaced it. 

　　It was found that the 5V upper tube broke down and was replaced, and the resistance of the common point returned to more than 200. There must be other places.

I also tested the battery and found that the resistance of the battery charge sense and the common point was 0. After the PQ705 composite tube broke down and was replaced, the common point was broken down. The point resistance value returned to more than 400 normal.

 It is abnormal to find that the resistance of the battery inductor is only a few tens of ohms. When the front end of the disconnected inductor is disconnected, the resistance is a few hundred, but the back end is only a few tens.

          There is something wrong with the back end. I ran the circuit and found that there was nothing abnormal around it. I burned it directly and connected it to 3V and found that a filter capacitor on the back was hot.

The five parallel capacitors were all normal at first. It should be a leakage. Remove and replace, the inductor resistance returns to a few hundred normal, and the inductor is soldered.

The common point and various places are not shorted, and there is nothing when it is powered on and in standby mode. The common point is only 18V when measured, and the adapter is a false common point of 19.5V.

When measuring the isolation, it is found that the PQ700 G pole voltage is the same as the common point voltage, 19.5V. How can it be turned on? Continuing to run the line, I found that the PQ700 G pole is controlled by PU700. 

The PU700 chip drawing was not found. I found a drawing in Google. DELL uses this chip. I checked that the chip condition is normal, that is, the internal output is 19.5V.

After replacing the chip, PQ700 is turned on, the common point voltage is normal at 19.5, and there is standby current. When the trigger is pressed, there is no 3V5VPWM output.

The value of the inductor is more than 400, which is normal. When testing the standby chip conditions, the power supply is normal. The EC normally sends out a start signal, and the wired core has a benchmark. 

Replace the standby chip and press the trigger 3V5V to come out. Turn off the power immediately after powering on, install the CPU cooler and memory, run the code directly and turn on the Laptop.

This is the end of the repair!

Happy Repairing !!