HP 15-fh0013dx LA-M798P ENLDO 3V5V does not rise.

Hello everyone, I need help in trying to make sense of where the fault could be.

1. TPS65994 has no separate bios chip, which suggest the firmware could be in the main bios chip.
- It draws only 5v/.027A. there are no shorts to the capacitors around it and caps read way above 1000 in doide mode.
- there are 1.5v, 3.4v, and 13.5v on 3 caps around the chip.

2. ISL9538BH has 5.1v and 4.8v on pin 16(ADP) and 17(DCIN) respectively.
- pin 19(ACIN) has 0.938v.
- pin 24(ACOK) has 3.2v.
- both 21(SDA)/22(SCL) have .092v. there is no short on both pins.
- pins 1(CSON) 2(CSOP) 3(VSYS) all have 16.77v

3.SY8388BRHC has no shorts anywhere.
- 16.77v on pins 2,3,4.
- pin 8(ENLDO) has 0.241v
- diode mode reading on pin 8 is in excess of 2000
- voltage divider circuitry resistor values of 499K and 150k to pin 8 (ENLDO_3V5V) are accurate.
- pin 13(LDO) which should output +3VLP is only .047v and there is no short on that voltage rail.
- as a result there is no +3VALWP, which also has no short on that rail.

4.SY8370CTMC also has no shorts anywhere.
- pin 1(IN) is 16.77v
- pin 5(ENLDO) shares the same voltage divider with SY8388BRHC and also has 0.241v
- diode mode reading of pin 5 in excess of 2000.
- no +5VLP on pin 9 and no short.
- no +5VALWP on pin 12 and no short.

The obvious issue here is +3VLP, which must be available to initiate everything. so I decided to experiment and replaced SY8388BRHC with SY8386B, which has same pin values and the readings were same as above which suggests that SY8388BHRC seems ok. so I went a step further and applied 3v from Lab power to pin 8(ENLDO) and lo and behold, +3VLP, +3VALWP, +5VLP, +5VALWP all shot up as well as the power led indicator and USBC voltage also shot up to 20V and battery is charging.

My issue here is why the value of the resistors are the same yet LDO is not enabled. I desoldered the resistors and checked and even replaced them. I have to mention that the schematics for LA-M798P is unavailable so I had to fall on LA-G341P,0P5A,LA-K971P and LA-K061P. checks with voltage divider apps online suggests that the voltage should be approximately 3.9v but I'm reading 0.241v on the motherboard, and there is no short on that line.

Any suggestions will be greatly appreciated.

siddiq wrote:My issue here is why the value of the resistors are the same yet LDO is not enabled.

Do you mean that you have 16V on pin 1 and 0,241V on pin 2 of 499kΩ resistor, its pin 2 is connected to pin 1 of 150kΩ resistor, whose pin 2 is connected to ground? If all of the above is TRUE, then something is overloading ENLDO_3V5V signal rail. What is the actual resistance on that 150kΩ resistor while it is soldered onto the PCB?

Please also post the clear picture of those two resistors with the surrounding area.

Hello and thanks for the response... before I read your post I decided to remove both 3V and 5V chips and resistance on ENLDO_3V5V rail was 142k and when I apply power, I get 3.458V. I will resolder one after the other and see what comes up.

I strongly suspect that SY8370CTMC may be faulty as you have already replaced SY8388BRHC with SY8386B and nothing has changed.

So I took a closer look at the circuitry and saw interconnectivity between EC_ON,EN1,EN2(ENLDO_3V5V) so I desoldered the EC controller and retested all the resistors and caps again and by chance noticed that PC522 was not shorted but had a slight leakage, and then it all made sense... so after replacing PC522 and resoldering back the EC controller , the laptop came back to life. Thanks for your response and suggestions.