Hate to be a wet blanket but without an actual schematic and some reasonable test equipment you are asking for more problems than you are starting with. Replacing a bad opto isolator or other micro miniature electronic component is not plugging in a new tube like the by gone days. Should be done in proper static control environment with appropriate anti static tools. 1 Meg ohm to real earth ground +/-10% for the work surface and 40 - 60 % relative humidity.