Your manual should have instructions for making that adjustment if it's possible. Pinball Resource is the place to go if you don't have one. http://www.pbresource.com/mansch.html
Take a look inside your backbox at the jones plug near the lower left as shown in the IPDB photo: https://www.ipdb.org/showpic.pl?id=1728&picno=22091
What adjustment is that for?