ASUS UX303UB REV2.2 WORKS ONLY BATTERY

[ivangserrano]

Hello

I have an ASUS UX303UB that only turns on with the battery connected, it doesn't turn on only with the charger connected.
With the battery and charger connected, the computer turns on but does not charge and I can see some consumption at the source, it seems to me that the charger is taking over.
In the work environment I can see that the battery is at 24% and does not charge or discharge.
I checked the 24780s load ic and I have the following voltages.


24780s

vcc - 18.85
24v acdriver
acok 1.3v
acdet 2.6v
regn 6v

Everything seems ok except the acok has a low voltage, it should have 3v and not 1.3v...

Do I change the 24780s load ic?

I didn't find any shorts in lines near the load IC or shorted mosfets in both the input and the load circuit.

[mon2]

Link a schematic if available. Yes, the ACOK line is not ok with a low voltage of 1v3.

Remove all power. Meter in resistance mode. Trace the ACOK pin to find the pull up resistor. Measure the resistance to ground of each side of the pull up resistor. The always on 3v3 ldo may be defective.

[ivangserrano]

Shouldn't the 3v on the acok come out of the load IC? I have the schematic attached and a photo here

[mon2]

No. ACOK is an open drain output from the charger ic. If the adapter is OK, then this pin will float like an open switch. Then the pull up voltage must be present to park this line to the 3v3 state. The ACOK line is not supplying the 3v3 so an outside regulator is used. If the adapter is not ok then this charger ic will force this line to be pulled down to the 0 volts level. All open drain signals work like this.

[ivangserrano]

so possibly it won't be the ic and load damaged but something next...

[mon2]

Remove all power. Measure the resistance to ground on each side of the resistor @ PR8911. Also, what is the resistance across PR8911?

[ivangserrano]

values are rising little by little

[mon2]

What is the resistance of PR8911?

[ivangserrano]

RES 1M OHM is the value it must have, it says on the boardviewer, but measuring on the circuit I have 58k

[mon2]

To me, a lower resistor values is welcomed here to guarantee that the floating line is pull-up to 3v3.

Flux and remove this part. Then test the resistor while it is off the board.

Power up without this part, what is the voltage to ground now on ACOK without this component?

[ivangserrano]

915k resistor off the board.

1.3v still measured at ground without the PR8911 resistor

[mon2]

There are 2 of dual mosfets (6 pins each) located @ PQ8901 and PQ8904 - both are the same, both are symmetrical on how they are mounted. You can mount them in either direction.

Consider to flux and remove both off the board for testing. Once the parts are off the board, power up again to measure the voltage to ground of ACOK. We are removing each load on ACOK to confirm if the fault is with the charger IC or an external component that may be leaking.

[ivangserrano]

Do I put the PR8911 resistor in place before removing the PQ8901 and PQ8904?

[mon2]

Leave it off for now. Later we will need to place it back to create the parked state of this open-drain signal. That is, without this resistor, we cannot have the ~3v3 logic level. We are only checking if this ACOK line is capable of being without a voltage.

[ivangserrano]

yes, the acok line has voltage with 2.8v decreasing the value....

[mon2]

Place PR8911 back on the board and test the voltage to ground of ACOK again. Checking if the voltage is stable.

[ivangserrano]

with pr8911 in place, I already have the 3v acok =)

[mon2]

Very good. This implies the charger IC is OK with the external power adapter. Now the suspicion is on the 2 x mosfets that follow and are also connected to this signal line.

Consider to clean up the mosfet pads where you removed each earlier in this diagnostic -> flux and solder one of the mosfets back onto the board. Then power up and confirm the ACOK is still ok.

If the ACOK line is no longer ~3v after soldering on the mosfet(s), then we need review why that is an issue.

[ivangserrano]

As incredible as it sounds, I soldered the 2 mosfets and started working..... it charges the battery and only turns on with a charger, how is that possible? cold solder?
Thank you very much, fixed!!

[mcplslg123]

Keep fingers crossed and wait for next 7 days. Sometimes due to heat,a defective pars starts working and fails again after a couple of days. Wiser to replace the suspected mosfets.