Question
I have one sprinkler that seems to have a small bit of water seeping out of the head.  I've replaced the head and it's still doing it.  Where else can the water be coming from?

Answer
Most likely it is your control valve for that zone.  You will need to locate the valve, and then you will have to open it up by removing the screws on the top of the valve, and then clean out the parts that are inside.  The rubber disk is called the diaphragm, and it needs to be inspected to make sure there are no tears or punctures in it, or that it is not worn out.  I would just replace it anyway since you have the valve open.  Reassemble the valve and it should solve the problem.