Charging system

FYI the difference between the 69/ earlier incorrectly called "single field" (grounded field, one field connection) and 70/ later incorrectly called "dual field" (isolated field, 2 field wires) is that.......
69/ earlier, the VR senses battery and controls amount of current going to the alternator field

On the 70/ later ones, the field was controlled by a transistor collector circuit to ground, and so had to be changed. The light blue wire sends key switched full 12V power to the field, through the field, and back to the VR on the green field wire. The VR then controls, so to speak, "the amount of ground" to control field current and therefore alternator output.