[Collins] muting a 51J4?

[Eugene Hertz]

Hi All. I am getting ready to get my 51J4 up and running as my receiver for my shack.  I am getting my TR relay and other stuff set up. I am trying to understand how to mute the J4 upon transmitting. I read the manual and was wondering if someone could confirm that in order to mute, I need to apply 12V between contacts #2 and #3 on the strip at the back?

I was really hoping that simply grounding one of the connections would do the trick. That sounds so much more civilized than having to make a source of 12vdc.

Anyone confirm? 

thanks
Eugene