The downgrade iphone 3G is used to fix the problem after update of the GPS baseband iPad 06.15.00,
or to block the release of the operator of the iPhone 3G.
A good system if you backup  by 4.1 SHSH is:

1.Download 4.1 stock firmware from apple.
2.Restore 4.1 directly using iTunes.
3.Backup 4.1 SHSH using TinyUmbella.
4.Make a custom firmware using PwnageTool_4.1.3.
5.Restore 4.1 custom firmware using iTunes.
6.”Exit Recovery” using TinyUmbella if need.


TinyUmbella 5.00.07…la-5.00.07.pkg

iPhone 4.1 stock firmware…7_Restore.ipsw

If you have not saved SHSH.
I found this video on youtube.

         Downgrade iPhone Baseband
         Downgrade any Baseband 3GS



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