The jack is on a little 1" square circuit board held by two screws inside the recorder. Unfortunately, Panasonic no longer stocks the whole circuit board, so it is irreplaceable. The jack itself is a standard item carried by Panasonic. To repair this you'll need some soldering skills, particularly in "unsoldering". You'll need to open the lid, remove and unplug the circuit board, and unsolder and replace the jack. Whether you or a friend can do it depends on your experience.
And there's no guarantee that will fix the problem. Look closely at the jack with a glass and see if it looks like this
. If it do, then that's your problem.
Analog -- Always out of adjustment.
Digital -- Always one more bug.