What do you mean with "output of that sounder"? The signal to/from the
transducer? This wouldn't contain the *amplified* echo ..., no? Sure, I
could take that signal but then I have to take precautions that the
amplifier (for the echo) can handle high power hat the input (from the
main bang). I was hoping to find a way to let the original box do the
amplifying part and then work only with pre-shaped signals.

On reading your post a second time: Do you mean I should look for the
signal that is sent to the LED on the rotating handle?

Anybody with a schematic of an old Seafarer or an Echopilot?
Cheers,
Stefan