ASUS X571 no power : battery issue?

[mon2]

Review if there is a coin cell battery on the board. If yes, test the voltage to ground of that battery. This battery may need to be replaced with a fresh one. Often, the ec is powered by the coin cell battery when the main power rail is off.

[mkdj]

Dear mon2,

Thank you for your help. There is no coin cell battery on this board.

Regards,
mkdj

[mon2]

Remove all power. Meter in resistance mode. Measure the resistance to ground of this J5 trace for the EC power rail that is absent. We are testing to see if the rail is being shorted so the voltage is missing.

[mkdj]

Dear mon2,

Value of resistance to ground is 7.5 kOhm. So there is no short to ground on the +3V power rail.

Regards,
mkdj

[mkdj]

Dear all,

I am still trying to figure out what the problem on this specific board is. There is no short on the +3V power rail. But it is at 0V. Also, I don't know how the +3V signal is created, so I can't diagnose anything. Schematics are not available, and I am stuck with the boardview, which does not help me to figure out how the +3V power rail is created.

I am keen on having any information on what to do now. It is a nice machine, and I would like at least to understand how the +3V signal is supposed to be created on this particular board.

Regards,
mkdj

[mon2]

Review the actual regulator and its pinout for the +3V power rail. There should be an EN (enable) pin on this component. This EN signal will be created by either a resistor based voltage divider OR driven by the EC controller. The EC controller is often backed by a coin cell battery or similar. Just this week alone, had 3 cases of where the coin cell battery caused the logic board to appear to be dead. Replacing the battery fixed each of the 3 cases. Do investigate this if you see a coin cell battery.

[mkdj]

Dear mon2,

I can't find the actual regulator for the +3V rail on this board, which is why I am stuck. Also, there is no coin cell battery on this motherboard.

However, I have news. As the board when plugged in is taking 20mA, I tried to use my thermal camera to see if there was a chip heating. And I have found that something small is heating next to a PWM controller. Reference of the chip is MP2884AGU, and datasheet is here: https://www.alldatasheet.com/datashe...MP2884AGU.html

I don't know exactly what is heating (the chip itself or something next to it) because my thermal camera has really low resolution and parallax issues. I tried to use alcohol and a freeze spray to locate precisely the heating element, but without success. I guess it is not heating enough to be detected.

Could this chip be the one creating the +3V signal? And because it is heating, it would mean that it does not work properly, and can't create the signal?

Or is the fact that the chip is heating a normal behavior?

Regards,
mkdj

[mon2]

No, this regulator is not the one responsible for the +3V always on rail. The datasheet shows this regulator as a multi-phase regulator meaning it is intended for much heavier loads such as the CPU or GPU. Much like humans, more manual labour can be performed if multiple people are involved instead of a single person, this regulator takes turns turning on one power leg of this multiphase output then turns this one off and turns on the next and so on. Each take a quick on/off action to not burden the single leg. It may be normal to see a temp rise on this regulator since the current draws are heavy.

The board is only consuming 20mA so very little current draw. As a comparison, a LED will be 5-10mA for a signal led.

Locate a known component that makes use of the 3v3 rail. Place one meter probe there. Meter in diode mode. Touch your meter probes to confirm there is a tone beep. Be sure there is no power to the board. Now probe each inductor on the board till you hear a tone beep. The +3v3 regulator should be near by. Try this approach.

[mkdj]

Dear mon2,

Thank you for the explanation of the role of the multi-phase regulator. I believe I have found the 3V regulator. I have used the boardview and checked all the inductors. There is one that has a +3V_S5 signal, and that is connected to PU22. This chip is SY8286BRAC, and according to the datasheet, it is a stepdown DC/DC regulator : https://www.alldatasheet.com/datashe...GY/SY8286.html

I have checked the pins of this regulator. And I have noticed VIN is absent...

Back to square one : without battery, I have checked the voltage on the first mosfet, and I have 19V on drain, but nothing on the source. Gate voltage is no more than 60mV. So I have checked the charging IC as you suggested before.

REGN voltage @ pin #24 is 6V : ok
Voltage at the ACDRV pin (pin #4) is no more than 70mV : not ok

I wanted to plug the battery and see if the values on the charging IC were different. Turned out to be a bad idea: fire and smoke went out of the 3.3V regulator. I immediately disconnected the battery, but this regulator is now dead...

I think the reason why this happened is because, when measuring the voltage at the 3.3V regulator, I probably created a short somewhere because of my multi-meter probes. Because I think I saw 19V on the VIN pins of the regulator for a few seconds, then it disappeared. I could not read 19V anymore at this location. Then I noticed that I did not have 19V on the first mosftet anymore.

Now that this board is probably dead, I will try to learn from it. I will try to find shorts, remove them, maybe replace the charging IC, and see what happens. I may come back to keep you updated, as this thread may help other people.

Regards,
mkdj

[mon2]

Goto AliExpress. Order a few spares of all linked parts. Order replacements for this 3v3 regulator, each of the mosfets in the power path from the power adapter to the main board, mosfets for the charger ic and and also the charger ic. Be sure the ics have the same suffix as the original part number. The total bill will be under $10usd with spares. Then replace this dead 3v3 regulator first. Do purchase fine tipped meter probes as well.