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You must be reading the old part of the wiki. F2 supplies the logic only on early boards (versions prior to 1.2). Logic supply has been on F3 ever since. Odds are he has a 1.3 board, so look at this page: http://reprap.org/wiki/Rambo_v1.3#Fuses This has been great advice and much needed help. It do...

It's hard to fix the issue if you don't know which wires are which. Figure that out, and what's going on will likely surface. The hotend fan runs all the time, so trouble with those leads could cause a short instantly without having to turn on fans or heaters... Seems like a good place to start tra...