I recently purchased this used AB1. [img:1kx5gi16]http://img169.imageshack.us/img169/6002/ab1z.jpg[/img:1kx5gi16] Everything is great except for when I put the pickup selector switch (or whatever it's called) in the third position (first being toward the bridge and fourth being toward the neck). When I put it in that position, there is no output. It used to be that it was intermittent, but it recently seems to have gotten worse to the point that there is no output. The other positions seems to work fine (meaning only that they all produce an audible signal). Any ideas on what I can do to get it working again? Is this a DIY job or does it need professional attention?