Sprinkler wiring to Omni Pro confusion

[Oznavad]

I've had my 8 sprinkler zones run by my OP2 for years in an expansion panel and 8 relay board. Over the years, I've had a few solenoid failures each year. While changing a couple this spring, I tested the leads with the zone off and notice I'm getting 12-15V AC. When the zone is on I get 26V. It made me think there was a wiring issue and while it worked, it might be why I've been burning up solenoids. My current wiring is in the photo below. I had all commons at the relay tied together and connected to the -24V AC input on the board. Each zone's solenoid is connected to its colored wire and goes to the NO contact. The brown and white wires are connected to the +24AC terminal and all solenoids in the valve box are connected to brown in one box and white in the other.

Suspecting a possible issue in the wires from the relay board to the valve box, I disconnected everything out side and wired a zone the same way to see if I got no voltage when the relay was off, but there's always 12v or more when off and 26v when the relay is energized.

I'm assuming there should be no voltage when the relay is off. Can someone help this electronic hack figure this out?
View attachment relay.jpg

 

[McKinneySmart]

Some questions first.
1. How many 'outside value boxes' are there?
2. Normally the white wire is the common in sprinkler systems. What is with the 'brown' wire?

 

[Sandpiper]

Disconnect each solenoid from its NO contact. Then do the following:

1. Check each wire feeding each solenoid for voltage while the wire is still disconnected. If there are no stray voltages, continue.
2. With the solenoids still disconnected in the box, check resistance from one solenoid to each other solenoid lead. With the solenoid lead wires disconnected at both ends, there should be nothing. (looking here for lead wires shorted to each other)

You can also do this:

Take a spare solenoid and hook it up directly to the relay board, one zone at a time. (This will bypass the field wiring and help isolate the problem to either in the field wiring, in the field box, in a solenoid, or in the cabinet.) Check all zones.

BASIC IDEA is this: Isolate and test. One part at a time.