Sorry, bkay, I'm not sure how much help I can be. I haven't really messed with my 600s, I did get a 626 to chain stitch, but it took a bit of doing as the bobbin case was messed up.
I'm thinking it is the set screw for the stud portion of the tension assembly.that ssorrow is referring to. That screw would be accessed after opening the nose plate. The same oiling or penetrating oil might help. Besides oiling and gentle tap on the properly fitted screwdriver, heat is an option. I'm not sure I would heat after using PB Blaster, maybe just SM oil.
I don't know if
http://site.tandtrepair.com/600Stuff...tuff/600TR.pdf would help or not. I know Terry has been recommended by others, especially when working with slant shank machines. I think that there are two sites, but I believe
http://site.tandtrepair.com/ is the most up-to-date site.
Janey - Neat people never make the exciting discoveries I do.