Sting Ray Wiring

[dur]

I am thinking of resurrecting an old Nasa Sting Ray echo sounder but don't have the correct power plug. As a replacement from Nasa is £ 8.00 I thought I would make my own plug or solder straight onto the pins.
There are three pins - I think the power goes on the two which are side by side but does anyone know what the third (bottom) pin is for?

 

[hubert]

The connector is a 3 pin 180 degree din free socket. Looking in the back of the instrument the pin on the right and the pin at the bottom are connected together and are negative. The pin on the left is positive 12 volts. I made one up for a friend last year.
Best wishes.

 

[dur]

Excellent - Many thanks!