Jar Top Irrigation Valve problem

I have a yard irrigation system that uses jar top valves and one of them is sticking open. If I turn the main water valve on one zone will turn on and I can't get it to turn off.
Before I knew it was a jar top valve someone who knows about such things showed me how to take a different type of valve apart to make sure there was no debris stuck inside. When I took a look at my valve this morning it didn't leave even remotely like what I was expecting and I have no idea how to proceed. I didn't even see anything that looked repairable. At least with the other type there was a removable diaphragm that could be cleaned if there were debris present, but I don't see the same design here.
Any ideas? I've been scouring the web looking for details but I'm not finding any. I'm starting to wonder if I should just replace the valve and be done with it. Since I am the anti-handyman, though, that might be pretty hazardous!
Any help is greatly appreciated!

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