Does the new motherboard from Paul work with other things for the tank? If so, I would just buy a new smoker after checking all the wires for any breaks (the small guage makes them very flimsy). Your volt meter should find that out. You might not have the unit plugged into the right ports? I only know about the 7.0v and 7.1v so I'm not familiar with the older boards. Or the heater is saturated inside and needs to be dried out? If the blower part of the smoker makes a humming sound, then that is at least working (maybe turn the volume down or unplug the speaker so you can hear it better).
Those are the only solutions that I know of that might work. Goodluck