Did you try to reset the logic board by tripping the small cuda switch under the keyboard ?
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Fron the Apple support web site:
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<< PowerBook G4 (DVI) and PowerBook G4 (1 GHz/867 MHz)
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If the computer is on, turn it off.
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Disengage the two keyboard latches located between the Esc and F1 keys, and the F11 and F12 keys. (You may also need to disengage the keyboard locking screw located between the F5 and F6 keys.)
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Lift the keyboard. To ensure no harmful static electricity is transferred to the computer, properly ground yourself before performing this procedure.
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Press and release the reset button located on the upper right side of the logic board near the power button, shown here.
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Wait 5 seconds.
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Press the power button to restart the computer. >>