i connected to it the ground and the signal control only and the power from external power supply
"and the power from external power supply" ?
Is that the power to the RPi system, via the microUSB connector? Or something related to your servo control subsystem?
A wiring diagram or clear focussed picture of your wiring might help.
Connecting any external voltage greater than 3.3volts to an RPi GPIO pin is likely to cause permanent damage with the symptoms you describe.