i replaced the hotend again with a new one and it works ok now so the problem is solved but i still don't know what's wrong with the other ones thanksby doede3d - Smart_Rap
i build a SmartRap with an Arduino Mega and a Ramps 1.4 board it suddenly stopped heating the hot end up to 200 degrees, only at 140 or lower i reloaded the Marlin firmware i mounted a new hotend i replaced the arduino i replaced the ramps 1.4 board no effect when i use a separate power supply for the hot end it still does not reach the needed 190 degrees i measured the resistance of the thermiby doede3d - Smart_Rap