Davinci AIO extruder temp issues

Ok,
Thanks everyone for thier posts this far on this topic. I had snapped a wire on my heater while cleaning and ordered a new 40w heater.  Installed new heater and removed connector above extruder and wired directly to rule out connector.  Extruder wont heat up.  Thermistor reads correctly when manually heating with lighter. Went and checked voltage at connection above extruder , nothing. Checked plug on board for 12v dc nothing. Checked continuity across r271 zilch.  Soldered across r271 ... bingo . Heats up, problem is it wont stop. Goes straight up until faults out at 0014 and is smoking hot. Burning smell etc.  unplug it. Have removed solder and went back to no heat, resoldered and overheats. 
Wth?
Any help would be greatly appreciated. 
Bought second hand for my daughter and printed some great stuff.  Now she is looking at me like, why can't you fix it daddy?