How to Fix a Nikon SB-600 Flash Stuck Zoom
If you Nikon SB-600 Flash powers up, makes funny whizzing noise and then the LCD display shows blinking “–” (instead of the lens focal length), then most likely the motor inside the flash has broken off from its original place. This can happen if you drop your flash or bump it against something. If you are facing this problem with your flash then you can probably fix this easily. This is happening because the bevel gear setup, inside the flash is disturbed. The motor that drives the zoom functionality of the flash through a bevel gear setup is dislocated, you need to reseat the motor at its correct location and if the motor has broken off its assembly then you would also have to glue or tape it strongly to its correct location. Do take out the batteries before you do so, and take all the precautions, as the internal circuits might have high voltage. The procedure to fix this Stuck Zoom problem has been well documented here.