[Drake] T4XB receiver mute

[Andy Bullington]

I posted this to the other Drake list but haven't gotten an answer.
I'm sure its something obvious but I just got this little transmitter
and I have no experience with Drake equipment before this.
I have made up a mute cord that runs from the accessory plug on my SX101A to
the receiver mute jack on my new T4XB. The manual says the jack provides a
closed circuit on receive (which it does fine) and an open circuit for
transmit. (which it does not).  I know the cable works because I can mute
and unmute the receiver by shorting out the T4XB end of the cable. I'm at a
loss. Any ideas? Thanks!
                  Andy W1AWB