phil...
i had a similar issue with my X-15. i would press record, but the head would not move far enough up. it would still record, and play back great, but the recording would be OVER the previous recording instead or replacing it. ie: i would record to track #1 and, while recording over the part i played, would hear the original recording. playing it back played TWO recordings on track #1. it sucked. i opened the thing up and found that there is a metal arm that pushes a button that triggers playback and recording. the arm was pushing the button, but not far enough. i souldered a small piece of metal to the arm on the side that contacts the button and it pushes in just fine now. if this is the issue, make sure you dont use too large a metal piece. dom is right about making sure everything fits back the way it was, but its not too difficult if you pay attention...
funny how, when all the BS of the PC gets to me, i can take a little vacation with a 4-track tape machine and some outboard processers...