I just got my first soldering iron, a pen style weller. It was a real cheapo at $20 from home depot. When I go to tin the tip all the solder just beads up on itself and won't touch the iron. The iron turns black almost instantly and will smoke even if nothing is touching it. I have tried two different solders, one that came with it and some electric solder I bought separately, both are lead free and rosin core. The iron is 25 watts and gets up to 750 degrees fahrenheit. Any ideas?