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Check driver transistor Q15.
You'll find a chart on the first page of your manual. ipdb.org has one available for download if you don't yet have a paper copy.
You might find the Pinwiki helpful: https://www.pinwiki.com/wiki/index.php?title=Williams_System_9_-_11#Solenoid_problems

If you're unsure how to test the transistor or how to find it let us know and we'll get you going.

I’ll refer you to the experts: https://www.pinwiki.com/wiki/index.php?title=General#Testing_a_Transistor.2C_Silicon_Controlled_Rectifier_.28SCR.29_or_Field_Effect_Transistor_.28FET.29

Some of the traces on the system 11 boards are really small. If you have never done board work I suggest you practice on something less valuable.

A local repair person or sending the board out are other options.

Don’t forget about the pre-driver grantopia mentioned.