There could be a design difference issue here. They've recently redesigned the solenoids. The plunger is much larger now. I'm not sure what the brand new solenoids will do on an old Richdel. I know I stripped the threads of a solenoid because of that retainer on an old Richdel.
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Is the solenoid's all black on top or is there a silver round thing right in the middle? If it's all black that might be the problem. I don't know. You might need the old style.
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Have you tried a solenoid from a valve that works?
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What you should probably do is just buy a
205 and replace all the parts. You'll get a new diaphragm, bonnet and solenoid. That should fix it since you say there's power to the valve.