The most common cause of this symptom is that there is a mismatch between the speed of the console port and the speed specified in the terminal emulator (Hyperterm, TeraTerm, SecureCRT, or whatever emulator that you are using). I suggest that you set the speed of the emulator starting at 1200, and working up through 2400, 4800, etc. Be aware that with some emulators that when you change the speed you need to stop and restart the connection to pick up the new speed.
HTH
Rick
HTH
Rick