Well, when I got my new (Secondhand) Epic 5, I had a problem somewhat similar to this. I had to plug half-way to get any sound. It was one of the internal tips of the (modular) jack. As stated, it is modular, and you can disassemble it (each tip is part of a thin metal piece that slides in and out of the square plastic box that makes the jack casing). I happened to have a couple similar jacks lying around, from some onboard bass preamps I bought, so I got the replacement piece from one of those, and installed in its place, and voila!. You have to be careful, though, because one of these pieces, that has a small plastic round tip at the end, is the switch itself, and if bent or damaged, it might result in an always on circuit, which would drain your battery quickly. It has been a year since I did this, and it has been working flawlessly ever since.