I have also been pondering these issues for a few years as my original ST50 depth display gradually dimmed. Most salespeople will tell you that you have to buy the transducer of the display manufacturer (e.g. Garmin). False. As someone mentioned above, most transducers are made by one manufacturer and the only thing that makes it a Garmin is the unique connector on the end.
I found the right salesperson at West Marine who knew what he was talking about. Start by looking for the tag on the transducer or on the cable a few inches from the transducer. The key piece of information on mine was P19. It is a standard 50/200 kHz depth transducer. The second key piece of information was that instead of trying to sell me a new transducer, this gentleman reached under the counter and pulled out the Garmin black box. This is literally a black waterproof box about 3 inches square. One side has a cable with the Garmin 12-pin connector. I believe there is also an 8-pin available. The other side has a waterproof gland for the transducer cable. I cut off the Raytheon transducer connector, fed the wire into the box, stripped the two wires plus shield and connected them to the terminal points inside the box as directed for the P19 in the manual. The manual lists dozens of transducer models and can be downloaded from the Garmin website.
The box is nothing more than a very expensive (~$70) break-out box with a 12 point terminal strip. Sort of a punishment for not buying the Garmin transducer. To do again, I would buy the 2 foot 12-pin extension cable, cut off the female end, and simply solder or crimp the three wires together.
Important tip that wasted two hours:
The manual mentions this for other models, but not the P19. The chart shows the shield connected to pin one and the temperature wire to pin two. If your transducer does not have the thermometer feature, then you have to jumper pins one and two (temp and shield), or it will not work.
I found the right salesperson at West Marine who knew what he was talking about. Start by looking for the tag on the transducer or on the cable a few inches from the transducer. The key piece of information on mine was P19. It is a standard 50/200 kHz depth transducer. The second key piece of information was that instead of trying to sell me a new transducer, this gentleman reached under the counter and pulled out the Garmin black box. This is literally a black waterproof box about 3 inches square. One side has a cable with the Garmin 12-pin connector. I believe there is also an 8-pin available. The other side has a waterproof gland for the transducer cable. I cut off the Raytheon transducer connector, fed the wire into the box, stripped the two wires plus shield and connected them to the terminal points inside the box as directed for the P19 in the manual. The manual lists dozens of transducer models and can be downloaded from the Garmin website.
The box is nothing more than a very expensive (~$70) break-out box with a 12 point terminal strip. Sort of a punishment for not buying the Garmin transducer. To do again, I would buy the 2 foot 12-pin extension cable, cut off the female end, and simply solder or crimp the three wires together.
Important tip that wasted two hours:
The manual mentions this for other models, but not the P19. The chart shows the shield connected to pin one and the temperature wire to pin two. If your transducer does not have the thermometer feature, then you have to jumper pins one and two (temp and shield), or it will not work.
