[ARC5] muting

[RAY FRIESS]

I have an ARC 5 receiver/transmitter pair for 40 meters.  The receiver is all ready to go, having had the usual
mods done to it.  The transmitter is still in original condition, ready to be modded to be put on the air.
 
I have a question that I've been trying to find an answer to, but have been unable to find it addressed anywhere,
such as in the handbooks or other ARRL publications.

The question I have is, how do I mute the receiver when I'm transmitting?