On the screen of the radio you will see the letters ST..but it dissapears and goes on again.
so i have bad quality of sound when i turn on the radio. Maybe someone can help me ?

This letters ST will go on and off very quickly.

 

I'm not certain, but would guess that the "ST" is the indication that your FM radio is receiving a stereo FM station (that is, "all of them").

If the indicator is going on and off rapidly, and sounds terrible (on ALL stations) then your head unit is most likely bad - probably a lot easier to swap it out with a good used unit.

Otherwise, if it doesn't happen on some stations it could be because you're trying to listen to an FM radio station that's simply too far away.