Older Dixie Narco units utilized an adaptable vend relay.
NOTE: BEFORE WORKING ON ANY ELECTRICAL COMPONENT, ENSURE THE UNIT IS UNPLUGGED FROM THE POWER SOURCE.
These modifications will not require the use of the coin mechanism, so the first step is to unplug the coin mechanism. DO NOT PLUG IT BACK IN. You can leave it in place in the machine in the event you wish to convert it back to pay.
There are 2 specific types of relays that I will address here, the first is the open type 6-select vend relay.
On this relay, look at the wiring harness (if you have the original) and you will find a wire identified with the numbers G37/G18 ... unplug this wire and tape the end up to prevent shorting out against the cabinet. Facing the terminals, it should be located on the very top, left terminal.(HINT: The terminals with the smallest gap between them are the top terminals).
Ok, you have unplugged the G37/G18 wire and made it safe. Now, locate the G11 wire, it should be on the second pin (top to bottom) in the right column. Unplug it and plug it on the terminal where G37/G18 WAS plugged.
Plug the machine into a power source and you should now have free vend set. Reverse the procedure to put it back into pay mode.
The second type of relay is used on both 5-select and 6-select machines and is the one common to Dixie Narco 240/138-6. It is identified by having a clear plastic case covering the entire relay.
As with previously, ensure the unit is unplugged from the power source and the coin mechanism is unplugged.
Carefully remove the vend relay from the socket. It may be stubborn or it may appear as though the socket is part of the relay. Carefully rock it back and forth and it should come out of the socket. If it is really stubborn, you may want to remove the 2 screws holding the socket in place and then carefully pry the socket from the relay using a small screwdriver. Remember, if you damage the relay or socket, it may not work properly or at all.
Ok, great ... now you have the relay unplugged. You will need a paper clip that has been straightened out or a stiff piece of wire for the next step. A safety pin will work too.
Find the wire identified as 71. It should be in the bottom left terminal of the socket. From the socket side of the terminal body, insert your paperclip into the small notch under the terminal. carefully apply a little bit of downward pressure while pulling on the wire. The paper clip will disengauge the terminal lock and allow the wire terminal to be removed easily from the socket. After removing wire 71, make it safe by taping it to prevent shorting.
Next, find wire 10, remove it in the same manner as you did wire 71. Place it back into the socket where you removed wire 71. Plug in the vend relay and plug the machine into the power source. You will now have free vend.
I've attached drawings of both of the relays for easy identification.