Someone asked if you were using flat or spiral blades? If you have the problem with spiral blades, they will clamp on edge & after tightning the tensioner, it's just a matter of time & it will pop loose. It turns flat & slips out. That's just spiral blades that I'm talking about. If you have problems with the flat, then you should make sure the left set-screw is even or protruding out of the hole. If it is inside, the you bend the blade & or, it doesn't tighten the blade. Just some thoughts here & hope you can solve the problem. I have the 788 & no problems so far. Good Luck.     Larry