low/no charging

I'm not 100% sure witch alternator I 've got. I belive It is something the last owner bought from ebay. All I know is that it is not original. Chromed nice looking thing. It might be a "squareback".. I'm not able to confirm that now, but I'll check tomorrow. But It sure looks like the one on the left :-)

When you say "field wire" is that the negative wire?