Re: Depth Finder
Without pictures or model I'm taking a wild guess based on age. If it has a circular dial then I suspect it also has either two or three knobs; Power, Range (maybe), and Sensitivity (or Gain). Turn the Sensitivity/Gain to minimum, then turn on Power. When you turn it on there should be an arm that spins around inside the housing, just like the hands of a clock, only much faster. On the end of the arm is a red LED/light (can't remember if this was an LED). Turn the Sensitivity/Gain until you see the red light at essentially one location on the dial. If you turn it too far you'll see flashes all over the place, you're looking for a pretty solid light at one location on the dial, with darkness the rest of the way around. Depending on the bottom type, depth, and transducer condition this may be easy to achieve and interpret or not so much, these units always took a little operator guesswork. If there is a Range knob it was usually Feet/Fathoms, in Feet position your reading at the red mark is in feet, in Fathoms position the same reading is in Fathoms. Sometimes the Range was more like 60/120 in feet, depends on the unit.
That's all from memory, which is pretty hazy, but we had several units along those lines in boats from the 70s. Give it a try and let us know how it works. And if none of that makes sense take a picture and post it so we can see what we're working with.