How to mute Tx ?

You should be able to enter at the console:

tx enable off
**this will switch off your tx carrier

When you have finished your maintenance you can enter:

tx enable on
**this will turn your carrier back on

Also just a note if you are in the console of the idirect modem you can type 'help' and it will give you a list of commands.

Note:
you could also disconnect the Tx cable from the modem. THe idirect modem is setup that it will only transmit once it has receive lock. so you could disable the receive also. i.e. rx enable off
But this could cause the antenna to try to search.