Old Transducer Coax Cable
Preface: This article assumes you’re vessel is not yet into the 21st century with regard to your marine electronics and that you’re not using NMEA 2000 transducers.
It’s mid season, your boat is in the water, and your old depth sounder dies. What are you to do? There is often a good chance that your transducer is fine and it’s only the display that has died. Despite the transducer being old there is a chance it may be the same frequency as the new depth sounder. If it’s a 200Khz transducer it will likely work with Raymarine and some others that are 200Khz.
I’ve done this a number of times and it works. While certainly not an ideal, or the perfect solution, mixing old and new, this can save you many hundreds of dollars in a short haul expense if you need the changing out of a drilled through the hull transducer. Either way, if you’re in a pinch, it is worth a try.
The new transducer that came with the display can always be installed when you haul for paint, or for the winter, if you are in an area where the water gets hard for half the year.
To use an old transducer you will want to ascertain what the model number or brand is and then hopefully you can track down the frequency. Often times the old depth sounder manual will tell you the frequency in the specs. The transducer frequency must match that of the display head to make it work.
In order to do this you will likely need to cut the old connector off the end of the coax. Many older depth sounders used an RCA connector and some used a BNC type. The center conductor of the coax cable is hot or the signal send wire and the shield becomes ground or the signal return wire.
To confuse everyone the new Airmar transducers, Raymarine, Garmin, Furuno and many others use Airmar as their transducer supplier, are often three wires, hot, ground and a shield.
When using an old coax transducer just make the center conductor hot and the shield the ground. On the Raymarine ST-60 for example you can just ignore the shield input and use ground/black and hot/blue and it will work fine if it is a 200 kHz unit.
Most of the old transducers for depth were either 200 kHz or 210 kHz but there were also some other frequencies used as well. If if your old transducer is a 210 kHz it won’t work at all or not accurately with a newer 200 kHz display.
For this post I used a 31 year old Radarsonics 200 kHz transducer with a 2010 Rayamarine ST-60+. This transducer also worked with a 2006 ST-60 non + model before it got hit by lightning. Interestingly enough the ST-60, and every other piece of electronic gear, was wiped out by that lightning strike. The only thing still working was this trusty 31 year old Radarsonics transducer. I also have the Raymarine/Airmar P-79 unit that came with the ST-60 mounted as a “shoot through” for back up, but currently disconnected at the instrument.
STEP 1 – Strip the jacket back about 3″ and be careful not to cut the shield wires of the coax cable.