The string breaks at the bridge. The bridge has a knurled knob, threaded tuner on it for the extra string (the D). The string passes through the knob, which has a washer in it to prevent string twisting (it seems to be moving freely with no tension on it), then over a small roller. As the string is tightened, the point of contact (at the bridge) is the roller (the string is bent slightly at the roller). As the force against the roller increases the string fails at that point, just before reaching the proper pitch. I can send some photos, but I was hoping that someone would be familiar with this conversion bridge and maybe had the same problem.