Disconnect the main flex cable and connect the charger.

Use a multimeter to measure the voltages at the large end pins of the FPC connector of the charging board. (See the second image)

If the reading is okay (~5V) continue with the next step, if not - change the Charging board.

Attach the main flex cable and measure the large end pins of the Main flex FPC connector - pic.3.

If the reading is okay, continue with the next step, if not - change the Main flex cable.