Pfaff 230- front dial screw will not move- ?

I have had this machine since the early 70's new- its been a work horse- I have faithfully oiled it and now for some reason I cannot get it to stitch straight- I can only go in zig zag- the front dial (white knob) moves which we used to either make button holes or straight stitch moves- had repair person look at it said this is common on the 230 frozen- can't be fixed- can't believe that this old timer can't be fixed can anyone help me- PS I have sprayed the screw holding the knob with wd40 and can't get screw off.