1980 xs250se low miles as it has been sitting. 13.6v at 4000rpm, 12.9v at 4000rpm with the lights on. Changed the reg/rec to single unit from the 2 solid state items (xs750/850 item). 4ohm resistance between the white wires, 0ohm between brown and green.

I am not sure how these generate power, but is the rotor supposed to be a permenant magnet, or is it charged by the inner stator? (brown/green wires).

anything else I can check before I go nuts with a sledge hammer?

Sledge Hammers are your friends, oh and Fire, they love fire

Haa"  jim has had this experience 😅.   is it like the RD250,  where the rotor is inside the stator,  with brushes  on the slip rings,, if so   the brown wire needs 12volt ,  to magnatize rotor,,  

no, it's the later type with 2 sets of static windings and a rotor in between

Do a search , as it’s been discussed loads of times. 400 are the same 

sorted. Duff rectifier. Thw cheap chinese replacement was new but also duff. I replaced the stock rectifier with with a regulator / rectifier from a late model vfr800 honda and it works fine now. I had to modify the mounting to fit the new reg/rec in the same place.