Hay Guys, i have a week old 255ext and when welding on ac the arc starts off hard sometimes then you let off the pedal and the arc starts to sputter and spit ,cuts off then you give it more pedal and then it comes way up with the heat way to hot and melts a hole in the aluminum". Tried it over and over same thing but 2 day ago it worked but still had HF start problem . It has very horrible arc on sine wave, square wave it don't matter . Welding 1/8" aluminum 120 HZ ac balance is 67 #5 cup and 140 amps argon 15 3/32 tungsten . then on DC some times the arc wont start but the arc seems stable on DC . I think its a bad board on the HF side. any help thanks