A sudden black screen or inability to power on a phone is a headache for many. It often indicates a deeper hardware failure. On a complex phone circuit board, even a seemingly insignificant component can be the culprit for an entire system breakdown. In this article, Phonefix will help you gain a deeper understanding of the troubleshooting and repair process for such issues through a real-life case study involving an iPhone 13 Pro Max.
I. Fault Phenomenon
The customer sent an iPhone 13 Pro Max for repair, and the core fault was a black screen and could not be turned on. According to the customer's description, after connecting to an OTG adapter cable, the phone first failed to charge, and then the device automatically shut down and could no longer be started. There was no response when trying to force the computer to turn on many times, the iPhone screen did not light up, and there was no recognition signal when connected to the computer. After several failed repair attempts, the iPhone was taken over by an experienced technician.
II. Fault Analysis
After taking over, the maintenance master first inquired and preliminarily inspected the equipment. Based on the key information of "failure immediately after connecting the OTG cable", the maintenance technician judged that this was most likely an overvoltage or overcurrent fault caused by an external accessory. The root of the problem may not be limited to charging itself, but can also spill over:
1. Battery and battery detection line: Overvoltage shock may damage the protection circuit inside the battery or the battery data detection pin on the motherboard.
2. iPhone Power Management Chip (PMIC) and Related Power Supply Lines: Abnormal currents may affect the stability of the core power supply.
3. Accessory Identification Circuit (ACC) and its peripheral components: This is the circuit that communicates directly with the USB port and is the first to bear the burden of current shock.
III. Repair Steps
Step 1: Fine visual inspection and preliminary measurement
Perform a comprehensive examination of the motherboard under a HD Trinocular Microscope, focusing on the area near the USB interface. No visible signs of burnout, water ingress, or component breakage were found. Using a multimeter to measure the VBUS power supply pin of the USB interface, it was found that the ground resistance value was very low, close to a short circuit, and confirmed that there was a serious fault in the power supply path.
Step 2: Short circuit investigation and thermal imaging positioning
Connect a low-voltage power supply to the motherboard and use a Thermal Imager Camera to scan. It was found that there were abnormal hot spots on a tiny capacitor and a TVS protection diode near the U2 chip. These two components are the filtering and protection components on the USB 5V power supply line and are typical victims of overvoltage shocks.
Step 3: Remove the short-circuit component and review it
Use a hot air gun to carefully remove the heated fault capacitors and TVS diodes. The ground resistance value of the VBUS pin is measured again, the short circuit state is released, and the resistance value returns to the normal range.
Step 4: Deep Signal Path Troubleshooting
Although the short circuit is ruled out, the device still fails to turn on. The master recalled that "no battery data" is a deep manifestation of such failures. So according to the Phone circuit diagram, find the detection pin on the battery connection base and measure its ground diode value. The resistance value was found to be abnormally high (almost open), far from the normal 500 ohms. Following the clues, it was found that a precision resistor that provided the pull-up voltage to the detection signal had been damaged and broken, causing the power management chip to be unable to read the battery information and refusing to turn on.
Step 5: Replace components and function tests
Identify TVS diodes, filter capacitors, and pull-up resistors of the same specification from the board and solder them to the motherboard using a hot air gun under a microscope. After soldering is complete, use a digital multimeter to re-measure all relevant test points and confirm their resistance values have returned to normal. Then, after a basic reassembly of the motherboard and connection to a power supply, the ammeter will display normal fluctuations in the startup current, indicating a successful boot.
IV. Repair Results
After replacing the damaged filter capacitors, TVS protection diodes, and key pull-up resistors, this iPhone 13 Pro Max was completely repaired. All functional tests pass perfectly:
- Boot normally and enter the system with intact data.
- Charging function is restored, and fast charging is supported.
- USB data transfer function is normal, and the data can be synchronized when connected to the computer.
- All functions such as touch, display, Face ID, camera, etc. are abnormal.
V. Repair Summary
This case reinforces the meticulous nature of mobile phone repair. From precise diagnosis to precise soldering, every step requires high-quality tools. If you're currently engaged in mobile phone repair or looking to improve your skills, choosing reliable equipment is crucial. China Phonefix is committed to providing a wide range of high-precision repair tools to support your professional journey.
