Its been years since I had a sprinkler, but I seem to recall that there
is a screw that keeps it from popping out. I would guess your screw
fell out. If you see a hole without a screw, you could probably take
the unit to a hardware store and get a replacement screw. I think the
screws fall out because they cannot be tightened all the way, as it
would bind the parts.

Elaine wrote:

Hottest driest summer in a long time and today the Nelson brass oscillating
sprinkler I bought last summer just died. Were the cog wheel is the heart
shaped machination keeps popping out causing the arm to stop oscillating.
Any way to fix it or just bite the bullet and get a new one?
Any one have a suggestion on a good brand?
TIA.