ULM100USB set lock function procedure: 1. Insert the USB receiver to mixer, check the USB receiver LED light is flashing, if the LED is not flashing please press the button until LED light flashing, refer to below picture. 2. Press the microphone volume key " " at the same time, when the LED light is flashing please release the button. 3. Check both LED lights flashing, please quickly press the USB receiver key to lock them. 4. Check the microphone and USB receiver LED lights are not flashing, that the lock is successful.