Need help removing old disc brake
 

Hi all!
I have an old Sears and Roebuck "Free Spirit" 10 speed that has an old rear disc brake. (The entire brake unit alone weighs about 8 pounds, sweet!) I'm trying to remove the rotor from the old hub but it won't budge. (Lockring is already off, before you ask.) The rotor is attached to what appears to be a run-of-the-mill thread + lockring setup. I'm trying to keep the rotor intact and none the worse for wear, any ideas how to get it off?

I'm not worried about the hub being damaged.

Thanks!
-J