Happy Xmas!
I remember now, "The Mighty Midget", but you know how it is, some Forum members change bikes or hord bikes faster than a nine pin bowling ball knocks down the skittles!
The fact that the black wire on the replacement unit has a ring terminal attached, would confirm to me that it is the earth (-) lead. Therefore I would assume that the red wire is the live (+) feed for the battery, supplying 14v DC at maximum output.
As for the 3 yellow wires, therein lies the conundrum, as I don't know how the alternator coils are wired, but I think it might be 3-phase, rather than the single phase that I am used to.
Can the beastie run without the battery connected, or does it rely on the battery to supply power to the coil?
I think it might be best to see if Rufus/Santa/Spooky drops by on this thread, as he has owned these wee beasties for years and I would hope, know what to do with the 3 yellow wires! I would hate to advise only for the whole sheebang, alternator and new regulator/rectifier to go PHUT in a puff of blue smoke!
In the meantime I will see if I can find a GN250 wiring diagram and if it aids diagnosis.
I'm off for some shut eye in sheet alley and then a slice of fried Xmas pudding, bacon, mushrooms and egg, followed by my home made marmalade, wi' butter and toast, all washed down wi' a mug of 'Gunfire', in about 8 to 9 hours time. Happy Xmas!
Bill