My charger is very loose when I plug it into cells' USB port. S3

It's a simple fact, thin metal bends.

Over time through the stresses of what goes on, the metal bends and it makes the port loose.

The best way to fix it is to just replace the port, which on this phone requires very good soldering skills.

The easiest way is to open the phone and then using some pliers, carefully squeeze the port back to a smaller size.

You can follow the guide below on how to remove the motherboard, which you dont need to do, just open up the back until you can see the micro usb port and carefully bend it back.

Samsung Galaxy S III Motherboard Replacement

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