Sounds like an old-style reg/rec, possibly with a dodgy earth?
If you leave the ignition on long enough it will probably catch fire, so if you're watching you'll see where the problem is!
Check that you haven't missed the loom to frame earth up on the righthand side of the airbox, the engine to frame wire at the rear of the block and that the black wire coming out of the alloy/grey box just behind the bottom yoke (that's the regulator/rectifier) is making a good earth.
If they are all ok, unplug the two connectors going into the the reg/rec (2 or 3 yellow wires input from generator + a double red and black output to battery/fusebox) and turn the ignition on and see what happens. Just don't start the bike without the red and black wires connected or you will fry the reg/rec for sure.
Good luck
Paul
15th year in DD #68 and getting slower by the year
1199, SS800 & 620SSie DD racer 
paynep
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