New electronics on old transponder
I bought a new unit including a new through-hull transponder last summer. It was clear I was going to have to cut an access port into the liner to mount the new transponder, so I was looking for an easier approach. A discussion with another sailor reminded me that the transponder is a passive device. It converts an electric signal from the electronics into a sound pulse, and then converts the sound return from the bottom back to an electrical signal.The key is that the transponder (or head) be tuned to the frequency of the electronics. The documentation on my new unit said it operated around 200 kiloherz. The original Datamarine was a 200 DL or something like that. So I bought some connectors at Radio Shack, and wired the old transponder (a big beefy through-hull) to the new electronics. The only trick was that the new transponder had 3 wires - two signal, and one ground. I figured the old unit didn't have a ground, since you need a loop to send the signal. So I wired the shield and core of the old coax lead to the signal contacts of the new electronics, and "voila" it works just fine.Then I bought a piece of teak, and cut a circular plug to mount the new electrinics in the old housing, and it has worked reliably since.DavidLady LillieBy the way, there are plenty of positive reports on new through hull transponders in the archives - so either way will work if you follow the directions to mount the through-hull unit.