Just in case anyone still needs a way to adjust output voltage of an original powerjack inverter there is a very non-invasive way and very cheap. For those of you wondering why you would want to do this.... in USA at least, 120v is the most common appliance voltage and powerjack design targets 110v with small drift. 108 is just too low for many appliances, it drives up current and causes poor starting of compressors.
Solution, remove toroid from inverter and wind 14-15 turns 12or14awg wire evenly spaced. Solder/crimp terminals on ends and reinstall toroid. Remove red wire connecting to 110v outlet. Connect one of the leads of new winding to outlet and the other lead of new winding to the red wire you removed earlier. Now double check your work, make sure everything is exactly as it was when you took it apart with exception of the new winding added in series with outlet. Connect dc power and switch on. Measure outlet voltage, if it is 100v power off and swap the leads of the new winding. If it is 122v. Then power off and put lid back on inverter, you are done and your main control board wont know the difference but you get 120-122 v at the outlets.