You most likely need to replace the DC-IN port. For getting the right one and the installation instructions please give us the last three letter of your serial number so we can tell which machine you have. It could also be that the AC plug in jacks outer ring has spread and is no longer making secure contact, This can be squeezed tight. Thanks
UPDATE
Now that I have your model number, here's how to do it:
PowerBook G4 Aluminum 15" 1-1.5 GHz DC & Sound Card Replacement
Here's the part: crwdns2937315:0IF154-072crwdne2937315:0
crwdns2934105:0crwdne2934105:0
crwdns2934113:0crwdne2934113:0
crwdns2915270:0crwdne2915270:0
crwdns2889612:0crwdne2889612:0
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