I have an old Delta DJ-20 Jointer. I'm guessing its around 15-20 years old. I placed a 48" straight edge across the top starting near the endof the outfeed table.The outfeedtable is perfectly flat. There is a dip of .007 from the beginning of the in feed table closest to the cutterhead for about 8''. How do I correct this distortion on this machine? I cant get a manual that's old enough for it