The Hakko iron you have is awesome. I have one myself. When I remove old wire from my esc, I usually have the iron set to about 700-750 degrees F. That temperature works good for me at least. You say that the iron won't melt the solder? Well, I use to to have that problem too. I then found out that you have to tin the end of the iron with a little solder otherwise the solder on the speedo won't melt. Yeah. Try melting a little solder to the tip of the iron before trying to melt the solder on the speed control. I don't know if that will solve your problem. Sorry if it doesn't help, and sorry if I'm just repeating something that someone else said already.