When I got the board, I measured all the inductors across the board and found no short circuits. I powered it on directly and found that voltage couldn't even pass the first isolation MOSFET. Because this board is from a very niche brand, I couldn't find schematics or boardviews, so I could only look up its charging chip model to find a similar circuit for reference.

The reason it didn't pass the isolation MOSFET is either a problem with the isolation MOSFET itself, an issue with the charging chip, or insufficient working conditions. Pin 20 VCC has power coming in, and the 6V linear voltage is also present. However, the ACDET pin has 4.2V, which is incorrect. According to the chip's datasheet description, this pin must be between 2.4V and 3.15V; it cannot experience over-voltage or under-voltage. Therefore, the problem is pinpointed right at this series voltage divider!

Don't underestimate the role of this series voltage divider; it will directly prevent the chip from boosting voltage. At first, I thought the chip was bad, but after replacing it with another one, the situation remained the same. Only then did I start paying attention to this specific point... So, beginners must understand the working conditions of the chip; only then will your future repairs be more accurate. (My drawing isn't great, so please just bear with it to understand the principle).

Remember that this chip uses internal boosting. If you encounter a situation where it doesn't boost, first check if the voltage at that voltage divider point is too high or too low, and whether the chip's conditions are met. If it still doesn't boost, only then should you replace the chip (this is my personal experience). Here is a picture showing you the locations of those two resistors.

Without a schematic, you can only use a multimeter to measure and find those two resistors. Finally, I will provide a few internal circuit diagrams for you to look at, and the chip datasheet has also been uploaded. I hope everyone makes a little progress every day...

Machine Model: Deep Sea Titan X1 1050TI

Fault Symptom: Does not power on during initial testing (after pressing the power switch multiple times, it turns on occasionally; there is a standby current of 0.25A).

Repair Summary: The EC soldering was inadequate, causing intermittent triggering. Being stuck on the LOGO was because the previously replaced CPU was not reballed from lead-free (high-temperature solder) to leaded (medium-temperature solder). (It is recommended to reball all newly purchased CPUs with medium-temperature solder). It was put on the BGA rework station twice. The first time, it was just reflowed without being removed, which allowed it to enter the system, but freezing issues occurred. The second time, it was completely removed and reballed; testing showed no further freezing. During the process, heating on the BGA station twice caused two component issues: 1. The power supply resistor R3331 for the protection and isolation circuit (this resistor is prone to failure, seems to be a common fault) showed abnormal resistance. Pin 1 had 20.5V while the other pin only had 17.5V, causing under-voltage in the BQ chip so it didn't work. 2. The feedback resistor R3382 was damaged, causing the 5V chip to work abnormally; the 5V linear output was only 3.7V. Because this board generates 5V before 3V, the subsequent circuits could not function properly without the 5V. (Below is the repair process. Experts can leave now; beginners can read the repair logic for reference during repairs - multiple images).

Repair Process: Let's look at the pictures first to see the condition of this machine.

Looks high-end, right? Enough nonsense, let's inspect it without disassembling to see if there are any obvious damaged components and if we can get an easy fix. After looking carefully for over 10 minutes, it seems luck is not on my side today; no obvious issues found. Let's use the multimeter and test step by step. Friends who have read my first post should know how to operate, right? First, measure if the power switch voltage is above 3.3V. Since the machine itself has a standby current, our logic can skip the protection and isolation circuit. The basic startup sequence of a laptop is generally: connect the power, current goes through the protection and isolation circuit to generate the common point. With the common point, the linear voltages in various circuits will turn on (sometimes 3V-5V turn on directly; the actual situation needs to be determined according to the schematic's timing).

Thereby causing the components in each circuit to generate standby current. Is this clear to the beginners? Experts can just ignore my nonsense above. Well, this board has a physical button. I really like this design, it's convenient and easy to use. The multimeter measures the voltage of this power button to be over 9 volts.

Trying to press the switch, we find the voltage is significantly pulled low. At this time, we need to rely on the schematic or boardview. (Inserting a small ad here — Xinzhizao schematics, how many of you use it like me? Raise your paws). Let's see which EC pin this switch current goes to, and check if the EC receives this high-low-high transition.

The EC only has a high level, and when we press the switch, the voltage is pulled low, but the EC does not issue a transition to the bridge (PCH), so it might be an issue with the EC. The EC of this machine had been worked on by someone else. (Some repair details weren't photographed, please forgive me; repairing while photographing inevitably leads to forgotten photos.) Some pins were not aligned properly. So we desoldered it, reinstalled it, and tried again. After reinstalling as shown above, the 3.3V measurement was normal. Then we tested if the conditions for the bridge were met, like:

All power supplies are normal.

(Crystal oscillator waveform), so I didn't take many pictures and just skipped ahead. Through the above measurements, we found it might be an EC soldering issue. After re-soldering the EC, the issue of occasionally not powering on was resolved. However, after booting, it got stuck at the Windows interface when entering the system. From previous repair experience, getting stuck at the WIN LOGO is mostly a CPU issue. Moreover, this CPU had been replaced. Actually, it was replaced by my own hands 3 months ago, but I was a bit lazy at that time and didn't remove the lead-free solder, directly installing it with the original high-temperature solder. So I suspected the problem was here... Then I put it back on the BGA operating table again.

First, reflowed it to see if the problem could be solved. Took a picture on the operating table as shown. Assembled for testing, it passed the WIN interface and directly entered the system for testing. Played a movie to see... As expected, the problem still appeared. After the reflow, it froze while playing a movie after 1 day of use. Continued to disassemble it.

This time, I removed the CPU, reballed it with medium-temperature solder, and tried again. Put it on the BGA operating table once more. After reballing, I found it completely failed—it couldn't be triggered anymore, and even the standby current was gone. Immediately checked the protection and isolation circuit and found the issue shown below. I found on the forum that R3331 on this board is a common fault.

This resistor is prone to failure. Actual measurements also found that the voltages on the left and right sides of the resistor were indeed abnormal. But I really don't know the exact reason why the component failed; it might have been caused by the high temperature when using the BGA. After passing the protection and isolation, I still found it couldn't trigger. Looked at the schematic again to see why 5V wasn't coming out. Still followed the international convention: power supply - enable - linear output - 5V output. The power supply was normal, the enable signal was present, but the 5V linear was only 3.7V.

As for why this happened, I don't know the exact reason either. Is there an expert who can kindly explain?

Finally, attached are the boot pictures.

In the very end, from the actual repair process, there were actually many good measurement points indicating abnormal voltages, but due to my carelessness, I didn't take pictures immediately. I just dealt with it directly and only remembered to take photos after it was fixed... so the photos might not be complete, but the logic and the actual measured conditions have been marked on the schematic boardview... Please be understanding, fellow repairers!