How does the Roland TR-909 reset circuit work
The first thing that may help you understand what is going on is to put a scope on C704 and see how long it takes for the voltage to reach equilibrium.  I'm going to guess that it takes several cycles.

The cause of this voltage ramp is the winding resistance of the power transformer.  

Now think about what the function of the reset circuit is.  You want to hold the controller in reset while the 5V rail is stabilizing.  The 5V rail ramps up as the voltage on C704 is ramping up.  Starts off at zero and ramps up to 5V.

By holding the reset RC network discharged until the 5V rail is approximately stable, you ensure a clean power-up for the controller.

That's what the reset circuit does.  Q701 is OFF until the voltage on C704 reaches about 6.2V.  This allows Q702 to hold the reset RC timing network discharged.

When the voltage on C704 reaches about 6.2V, Q701 turns ON.  In actual fact, it probably turns ON and OFF once or twice but the reset RC timing network doesn't let the controller start.

The 5V rail is fairly close to 5V when Q701 begins to turn ON.  The drop-out voltage for a 7805 is typically about 1.5V.  When C704 reaches 6.2V, the 5V rail is sitting somewhere near 4.7V.  This is safely above the 4.5V minimum that most 5V devices will operate at.