As has been said a million times before, check your soldering. Your problem is 99.9% caused by your soldering. You either have a dry joint, solder splash, or damaged something on the board. Look very closely until you find it, if you can't find anything (cause there is something), then desolder everything and start all over. If after desoldering everything (before trying again) your Xbox doesn't work, then you damaged something in your attempt.