Hey, One
I don't have the 707 manual, most of this info is only my experience with the machine...
With the machine stopped and in Track play mode, press shift and Tempo/Meas/Sync Mode. The display will show "S-d" (Din Sync??). Keeping Shift pressed, press Sync Mode several times; the display will cycle: S-d,S-t,S-n,S-d...
In S-n, the machine will sync to external MIDI clock, and if you want, you can now switch to pattern play mode.
Mgomes