But the first problem is to find them. Luckily, if the valve is leaking, then the wet spot would be the location of the valve. So we found the center of the wet area and started to dig there. Starting with a 2 foot by 2 foot area, we just intersected one of the boxes. And enlarging it a bit, there was the other box on the other side of the hole. The boxes are 23 inches apart -- center to center or rim to rim -- and the line between them runs basically parallel to the house or the curb in front of the house. The one is 33 feet from the corner of the house (by the gas meter), and the other is 33 feet 4 inches.
Turning the water supply line to the irrigation system back on, we see water beginning to appear in the box for Zone 10.
So we dig up more around the valve, and take it apart. Remove the diaphragm and take it Ewing Irrigation and Landscape Supply to get a replacement. They wanted to know what type of valve it was from -- I don't know -- and they guessed an Irritrol 2400S. $18.79. Take that back home and it seems to fit fine.
However, in taking the valve apart, I removed the solenoid, and apparently lost the moving part of it. I don't remember it dropping out; it doesn't seem to be anywhere in the lawn (I used the metal detector to look for it), but it's missing and the solenoid does not work without it.
Using the solenoid from Zone 11, I was able to get Zone 10 working properly; now Zone 11 doesn't work, since it is missing a (working) solenoid.
There was a solenoid with the Irritrol 2400S, but it while it fits, it does not work. There are apparently 3 different types of irrigation valves (direct, indirect, and semi-direct), and while the diaphragm worked, the solenoid does not. I was able to get an Orbit Replacement Solenoid Model 57041 from Home Depot which does work.
So Zone 10 has a new diaphragm and Zone 11 has a new solenoid.