Are you instead meaning a 570 model? We have not made a 571 model sonar unit.
1. Power the unit on and allow it to start up and start working.
2. Press the power button one time. You should see a menu pop up that has the Light menu and the Contrast menu on it.
3. Press the Down Arrow to get to the Contrast menu and then press the Right Arrow button to darken the contrast of your unit.
These are the same instructions for all of our monochrome 300, 500 and 700 series units. So even if you do not have a 570 unit it should work the same.
