Just for testing purposes, all you have to do is swap the line that is giving you trouble. For instance---if it is line 3 --take the mod plug out of the 3/4 line port and swap it with the plug in the 1/2 line port or the 5/6 line port. What you're trying to do here is find out if its the port or the line. If you find out that it is, indeed, the port, then you have to do some rewiring of the mod plug--or of the surface jack (that you may or may not have) or you'll have to switch some wires on your 66 block (if that's what you have.) It's sort of hard for me to tell you what to do as far as the wiring goes, because I don't know how it is set up. But first---find out if it's a bad port, or a problem with the line.

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