The depth sounder

The depth sounder was one of the many ‘small’ things to fix when I came across Fringe. The LCD display was clearly damaged, and I could not make out any text.

After a while, I happened to notice that at night, when the instrument backlights come on, I could just about make out a number! That was big, because it meant that the transducer was working, and also that the instrument was largely working too… apart from the display.

The unit is a Signet SL172, and almost certainly came with the boat in 1977. Surprisingly, although Islander went out of business somewhere around 30 years ago, Signet is still around and even repairs old units. They were helpful on the phone and advised me to send the instrument to them by mail. They would then examine it for free and let me know whether it is fixable, or if I need a new unit. Apparently they do sell used units too depending on availability, but the person I talked to wasn’t keen to talk about it.

Now I ‘just’ need to remove the depth sounder from the bulkhead it’s mounted in. Of course, as I’m learning, taking things apart on an old boat is never easy. I unplugged one wire, cut a few more, and unscrewed a few fasteners only to find that the instrument is mainly held in place by adhesive sealant. It takes a few hours of digging with a hack saw blade and prying with a small flathead screwdriver before it finally does come free.

The difficulty with work like this is you have very little feedback until the end… assuming you do get it free without resorting to other measures. I dug away quite a bit of sealant, tried to push… nothing. Tried to twist… nothing. Wrapped a big strap wrench around the back of the instrument and tried to turn it… nothing. Ran a screwdriver around the other side, at least disturbing the seal even if not getting much sealant out… nothing. Another round of digging then, another push, and finally the tiniest bit of movement! It is possible. I’m not going to have to run out to find some special polyurethane removal chemical that hopefully won’t dissolve the instrument plastic along with it. I’m not going to have to break the case just to get the electronics away from the bulkhead. With a firm push on the back I’m able to produce an opening just barely wide enough to work my smallest flathead down the beveled edge on the front. A little bit of leverage and I’m rewarded with the glorious sound of the sealant coming unstuck.

Once the instrument was free I could remove a few more screws and take a peak at its internals. Amazingly, I find the LCD display mostly intact… but the plastic film on top of it is in terrible shape.



Maybe I won’t have to send it anywhere after all. The rest of the internals look pretty good too.

I twist a few wires together and connect it directly to one of my batteries to try out. I can make out faint lines on the display, and when I move the dark film over it I can see the digits clearly, albeit distorted by the wrinkled film. Apparently this is called a linear polarizing filter and is needed due to the particular magic that LCD displays utilize to display characters.


And you can buy some polarizing film online, complete with adhesive backing, for a grand total of $12.

After a few days waiting for the Amazon gods to materialize said film on my doorstep, I get the depth sounder back out and try out the film. After trying out a few different orientations I found the one that works and cut an appropriately sized rectangle:


Sorry for the dark picture, the days are depressingly short right now. I’m holding the film in my hand and covering half of the display, which is why you can only see half of the “18.8” (which means, ‘uh oh I can’t talk to my transducer for some reason’ — because the transducer isn’t plugged in).

Unfortunately the film didn’t have adhesive backing as advertised, so I used a little bit of glue on the edges and corners to hold it in place. If I ever have so repair a similar display I will try harder to find some film with good adhesive backing as using glue is tricky and there is not much room around the edge of the display to tape the edges or cover with clear adhesive film.

But, after letting the glue set overnight, putting the electronics back in the case, and mounting it back on the boat with a fresh bead of adhesive sealant (3M 4200), I have a functional depth sounder!


This is why I’m a fan of older, commoditized technology. I doubt I would have been able to fix a more modern, or fancier, device so easily.

4 thoughts on “The depth sounder

  1. Too late now but a heat gun may have been effective in loosening the adhesive. Anyway very well done in your persistence. Nothing more rewarding than fixing things yourself.


    1. Yeah a bit of heat might have helped. Thanks, I was especially happy with this one because I was expecting to have to send it away for a repair.


  2. Maybe it would not work with a boat as old as yours but we have used whipping twine with some success. It works when there is an overlapping of a housing over the surface where the instrument or its housing is mounted. Working the twine in a kind of sawing action may cut through the gunge. It worked when our boat was about 15 years old.



    1. Good idea. There was overlapping housing as you say… of course, I didn’t realize that was the case until I got in there with the hacksaw blade.


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