I think that fix is used more often in the sense of repair and fix up used in the sense of arrange. "I fixed my computer because it was broken" and "I fixed up my apartment because my girlfriend was coming over."

They are not interchangeable. See these examples: He got a plastic surgeon to fix his crooked nose. (meaning repair) This pimp specialises in fixing politicians up with prostitutes. (meaning supply)

Interesting Thanks !

And it´s used also with the meaning of castrating (an animal) : This cat has been fixed up.