While the bolt that goes thru the center of the tensioner is loose now look for a 13 mm hex that is in the vertical position that is behind the tensioner. I don't remember exactly which way it goes but you turn it and that should start allowing the tension on the belt to relax enough to allow you to replace it. Draw out the pattern for the routing of the belt before you remove it. The way the tensioner works is kinda whacky until you have used it a time or two.

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Jim