what do you propose to do with the nitrogen pressure wise? Need to know so I can tell you which path of repair to take.
Not sure you're going to hear a small leak. One option is to charge to 200-300 psi and go around with soap bubbles to find the leak. That will only work on fittings, but not the coil. The other is to add a 1/2 pound of charge, add the N1 to 200-300 psi and then use an electronic leak detector. If this is a small leak you may need to be pretty close with a detector. It seems leak detection is part science, art and luck.