First thing you need to do is get an ohms reading from pin 9 to ground using a DMM(Digital Multi Meter) Now take that value lets says it's 2K ohms multiply it by 20 which will give you 40k. If I was a betting man I would say you are probably going to need a 50k VR (Variable Resistor). Turn the screw on the VR clockwise to you hear it clicking then hook your DMM to the VR and make sure it's at maximum value (close or more than 50K) If you close to 0 then turn the screw other way and re-measure. Now solder a wire from pin 9 to middle leg of VR. Then solder a wire from third leg of VR to ground i.e. screw hole or equivalent. This is why I asked in the beginning if you had all the proper equipment.
