It is found that there is secondary power supply in front of R4001. After passing R4001, there is no power. If the resistance value of the measured resistor is good, C4023 will be short-circuited, replaced decisively and repaired.
Check "About this machine" and find that there is no baseband, make up the dropped inductance and install the motherboard into the machine. Boot test shows that the version of "Modem Firmware" in "About this machine" is normal, indicating that the baseband has been working normally.
IPhone XS Max mobile phone screen is not displayed, and it is found that the resistance of PMU_TO_DISPLAY_RESET_CONN_L line at pin 5 of the cradle is unstable. Scrape the PCB board, reconnect the lead of PMU_TO_DISPLAY_RESET_CONN_L, and repair the fault.
Firstly, the step-down circuit was measured, and it was found that there was a resistance between the capacitors connected at both ends of the charging step-down inductor, so a rich inductor was replaced.
The mobile phone can't be charged, and the fault of cleaning up the tail plug is still the same. By measuring the resistance of MIPI bus to ground, one group is 156 and 567, and the other four groups are all about 567.
IPhone12 Pro Max does not display when it is turned on. The measurement shows that the 15th pin is short-circuited by touching the cradle, and rosin is smoked. After replacement, the boot display is normal.
Measuring the resistance value of one end of C1226 main power supply is also about 400, basically locking the main power supply or middle layer virtual welding. Welding the main power supply in layers. Start-up test, there is a signal, and the maintenance is over.
When measuring the related power supply of the charging chip, it is found that the AVDD voltage of the G8 pin is 3.9V, which is a little higher than the normal 3.3V..
The iPhone XS Max does not start up. After relocating the CPU, it is found that there is no signal in the base band, and the power supply is short circuit. Use the infrared diagnostic instrument to confirm that the capacitor is short circuit.
