The hydromatic propeller was a simple and reliable system.
This would seem to be at odds to your prevoius comments, thus...
A common problem with the hydromatic prop was that the button wouldn't pop out. The solenoid would hold the button in, and the propeller would feather, then with the pump running, continue to drive itself out of feather again.
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That 'solenoid' was properly called a 'holding coil' and its operation was very straightforward.