This link may help.

Here is a link that might be useful: Toro Super 600 instructions

Take a shovel and cut a ring of sod around the head pull the "donut" of sod out and excavate around the head down to its point of attachment, I would guess from what you have said that you will find a soft plastic riser. Once you see the point of attachment you will have a better idea of what must be done. If the threads are buggered up you may need to replace whatever assembly is used. You may find the problem is the sprinkler body; bad threads of cracked base, in which case replace the head and save the old internals as a spare. Shouldn't be much to it. You could always call an irrigation contractor.