DELL 15-7559 (AM9AMB8D0) Laptop Does Not Turn On & No 3V5V

Laptop Model: DELL 15-7559

 

Version Number: AM9AMB8D0

 

Fault: No Response When Pressing The Power Button. Standby is 0.001

 

Maintenance process:

 

    Disassemble the machine and test the motherboard, there is no 3V5V, the chip is TPS51225, measure the 3V5V inductor resistance and there is no short circuit, check the working conditions of 51225, EN1 20 pin has no voltage, EN2 6 pin 3.3V, 3 pin RTC LDO 3.3V, 12 The main power supply is 19V.

 

    No matter how many things there are, I tried changing the chip to 3V5V first. After changing it, I tested the 3.3V5V chip and thought it would be too easy!

 

 

    After testing for a while, I notified the customer to pick up the machine. The customer came back 2 hours after taking it away and said it still wouldn't turn on. I took it for testing and found that the fault was the same as before.

 

    I looked at it and it was troublesome. I asked the customer to go back and wait for news. There is no voltage on EN2 pin 6. It seems that this needs to be repaired. I measured that EN2 comes from EC 83 pin ALW_ON, but there is no voltage.

 

    EC pin 3 +3V-RTC has 3.3V. This voltage is the RTC LDO provided by 51225. With this voltage, EC should work, but ALW-ON does not. There was no short circuit in the EC test, so I ran out of ideas, so I put it aside and fixed other things first. After a day, it is not that complicated to look at the timing diagram. At this time, 3V5V should come out.

    Suddenly it occurred to me that 6-pin EN2 has 3.3V. Shouldn't 3.3V come out, but it doesn't? Could it be caused by poor 3.3V filtering? I took a desktop capacitor and tested it on the original filter capacitor. Sure enough, the voltage of 3.3V came out. Now I figured it out. It turns out that 3.3V comes out first and then gives this voltage to EC. EC then sends out ALW-ON 5V and then comes out.

    I found a tantalum capacitor to replace the original 3.3V small capacitor, and the installation test was OK.

Happy Repairing !!