I am attempting to change the power cord on a TW transformer. To get to the terminals I will have to remove a bracket above it which requires disconnecting a couple of wires. I first tried to heat the solder with a pen and then a Weller 8200 100-140 watt gun. I still have not melted the solder to break the connection. How much power does it take? The tip on the gun has been cleaned and re-tinned for proper heat transfer. I get a little smoke but, no melting. Do I need more power? Thanks!