Originally Posted by
bassiusmaximus
Found this on the site as well.
To set alarm on these units pull the small knob out. This will turn the marker light on. Rotate the small knob until the marker light is at the desired depth setting. If desired the knob can be pushed in to turn the marker light off. This does not turn the alarm off, just the marker light. If any echoes appear between the marker light setting and six feet, the alarm will sound. Adjust the large, inner knob for the desired volume.
I guess you might try em and see if either of these is what your model does.