Use the following procedure to transmit note numbers from the MIDI out 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 INSTRUMENT GUIDE.
While in this mode, the TR-707/727 will not play its own internal sound source.
Use the same procedure to change back to not transmitting data and playing the internal sounds.
Owner's Manual: https://www.roland.com/us/support/manual_archive/