i managed to add switches to switch between the 707 and 727 roms the
other night. it was really simple. it's only really that cool if you
need a small setup like me, but here it is-

what you need:
-wire
-spdt switch (4)
-10k resistor (8)
-razor blade for cutting traces
-28 pin ic socket (4)

the instructions are the same for all the voice roms (i think ic34,
ic35, ic19 and ic22, check the schematics), so i'll just write
instructions for one.

oh, label the roms before you remove them from the machine you are
cannibalizing, otherwise you might get mixed up and all hell will
break loose.

1. solder 28 pin socket on top of the voice rom, connecting all the
pins except pin 22 (pin 1 and 28 are marked on one side of the
board).

2. snip pin 22 off the socket.

3. cut the circuit trace leading to pin 22 rom.

4. place new voice rom in the socket, being careful to pair new ic34
with old ic34, etc.

5. solder 10k resistors between pin 22 and ground (pin 14 i think)
on both voice roms. i soldered to the top of pin 22 of the new rom,
since the bottom of the pin is in the socket.

6. solder a wire from the original point that pin 22 was connected
to (i think it's ic29 pin 8 for ic34 and ic35, and ic29 pin 11 for
ic19 and ic22, double check the schematic) to the middle lug of an
spdt switch.

7. solder one wire from pin 22 of each of the stacked roms to each
side of the spdt switch. i used the resistor legs as a handy point
to solder to.

the end

other notes:
-you don't need to connect separate wires to the middle lugs for
ic34 and ic35, you can just connect the middle lug of the second
switch to the middle lug of the first. same goes for ic19 and ic22.

-you can use less switches for less flexibility, i.e. dpdt for both
ic34 and ic35 if you want. i like it this way because i can have
bongos, hi-hats, quijada and ride all at the same time, etc.

feel free to ask me questions if you are unsure about anything.