Use the following procedure to change the MIDI channel and OMNI mode of the TR-707/727:
- Enter Track Play mode.
- Press the STOP button to stop the rhythm if it is running.
- While holding SHIFT, press MIDI CHANNEL and the desired MAIN KEY (1-16) to select the desired MIDI channel.
- Press ENTER to change between OMNI ON and OMNI OFF.
Owner's Manual: https://www.roland.com/us/support/manual_archive/