It can be due to many reasons
1. Water damage
2. Fall down
3. Not used for a long time ( Mostly battery is down)
(1) Check the Battery. It should show 3.7 volts or as written on the battery.
(2) Check the Battery Connector
Check two things (a) Voltage coming or not (b) shorting
Note : If there is shorting then clean the circuit in ultrasonic cleaner (Remove display, camera, etc before putting it in ultrasonic cleaner )
Clean the circuit with a soft brush and make it dry with a hair dryer or ****er.
If the shorting does'nt go it may be due to shorting in an ic,capaicitor or circuit. if shorting is in ic u can check it by attaching a battery then press the on-off switch keep the battery for 10-15 seconds. after that remove the battery and check which ic bacomes hot. Reball or replace that ic. If shorting is in circuit it must be removed by ultrasonic cleaner
Note - The circuit is made of three layers. if there is shorting in the middle layer it cannot be removed
(3) Check the On/Off switch & voltage at on-off switch
Note - Connect a charger. If charging is shown on display then the problem is with the on/off switch or software problem
(4) Try flashing the mobile.
(5) If flashing cannot be done then there is hardware fault we first dry solder the Flash IC & RAM IC and check in the Software. if flashing cannot be done then we dry solder/reball/replace flash ic & RAM ic.After that dry solder/reball/replace Power IC, Network IC, Crystal, CPU
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