battery draining rapidly without anything switched on

I have a XJ 650 4KO 1981. I had the motor apart last year, and it started, but ran badly. I have just got over a bit of an illness, and picking up where I left off.
1 - the battery had deeply discharged, dried up an declared deceased.
2 - On testing the circuits with the new battery, I found the battery draining rapidly without anything switched on
3 - By process of elimination, disconnecting at the fuses, I found the current drain stopped when I disconnected the big multi (7) pin plug at the reg/rec.
4 - A car mechanic friend suggested the alternator to be at fault
5 - I did notice a wee spark when I pulled the connector apart
6 - I just remembered the lights being dim/yellow last year when I tried them to see if they were working
Has anyone had/fixed a similar issue? - (I have done a search, but I am not very good at that type of thing)
Thanks

 

The alternator is behind the bridge rectifier and regulator so if the rectifier is working power only travels towards the battery and the excitor wire is only live when the main switch in on (stretching my memory on that one though)
In saying that, does it charge properly when running?
Could be a fault in the regulator or alternator that caused the regulator to fail. Gotta troubleshoot from the manual.
Disconnect the alternator and check resistance etc.

 

Hi and thanks, not tried to start it due to the possibility of my new battery getting hammered. I have a multimeter, but not used it due to lack of determination....
Just found this dutch guy

who is good at explaining the basics of testing components, so here goes