I'm trying to adjust my father-in-law's Stanley Garage door opener model 3220.50. I can't seem to find a manual online. There are three nobs on the bottom of the opener. One big one that says - safety; the other two are 1)white- close 2) black - open. I've tried turning all these nobs but the garage door keeps opening back up when it goes down to the ground. I need help in troubleshooting. Thank you. Vickie