Model no. SV600S Hydrostatic LT1045 - serial no. 3804419573 - built 2.13.2008. PTO/blade engage knob does not engage. Pull out - nothing happens. Was working fine, went into reverse as usual blade disengaged and normally when stationary and about to go forward again, the knob is pushed in and then out to re-engage blades. Tried the only fuse near the back of the PTO knob, 20 amp. no problem fuse intact. What else can I try ????