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Received a Xiaomi Redmi 12 for repair with customer complaint: no display image.
Under bright light, you can see the display is actually working (image is present), but there's no backlight!
This indicates a completely different type of failure ☝️
1️⃣ Disassembling the phone - standard procedure, nothing unusual here 😊
Moisture indicator is red - likely liquid damage.
First step: test with known-good display.
After swapping displays, confirmed the issue isn't in the panel - problem is on the motherboard.
2️⃣ Inspected display connector and surrounding components - visually everything looks normal.
3️⃣ Removed motherboard from display frame and took out cameras.
4️⃣ Carefully cut away part of the metal shield covering the backlight driver ICs.
No visible corrosion or damage found in the area.
5️⃣ Pulled up the schematic and started continuity testing backlight circuits.
Immediately found an open circuit at pin 4 (WLED_SINK2) of the controller IC.
Two options: trace where the break occurs in the board, or jumper the connection.
Since no visible damage present, simplest solution is to bridge with enameled copper wire 😊
6️⃣ Notched part of the frame for better access.
7️⃣ Soldered one end of wire to the controller pin, secured with UV-cure mask.
8️⃣ Soldered other end to pin 4 on connector, again securing with UV mask.
9️⃣ Reinstalled board for testing.
Backlight works!
🔥 After verifying all other functions, ready for final assembly and return to customer 🙃
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