On my H-260, I have a standard horison depth finder which I just today got rewired and working. My problem is: I don't know how to operate it. The read out head is a white plastic box about 1.5" X 4" with a screen for value readings and three buttons on the right side: Top one is labelled Depth, middle one is Alarm On and bottom one says Alarm Off. How do you turn it off? How do you set the depth levels for alarming. I have no book on the device.