a while ago i was trying to stick both the 727 and 707 roms in the
same machine with a switch to select between them. i was trying to
hook ribbon cables to every pin of the roms, and then use those big
serial port switches to switch between all the pins on each rom.
well, i just saw a post in the synth-diy archives at www.buchi.de
and that described simply "piggybacking one rom on top of the other,
and soldering together all of the pins except the "chip enable"
pins, which go to a simple spdt switch with pull-up resistors! i
have a 727 on the way, so once it gets here i'll give this a shot
and post the results. here is the original post:

>>But there is a guy that has put the rom of an 727 on top of the
>>707-rom and then put switches so that he had both machines in
>>one.. . But i couldn īt get a hold of him..

>That īs not very difficult. It īs called piggybacking.

>You solder the same type of ROM (or static RAM) chip on top of
>another,
>connecting all pins except it īs chip enable pin (CE) and wire the
>separate
>CE īs to a switch. (usually with pullup resistors on the chips for
>an active
>low on the CE pin).

>This was a very common trick to expanding the RAM on Jp8 īs, Dx7 īs,
>etc.

>While the 727 and 707 roms are of the same type, as I remember it,
>there
>were some other hardware voicing changes between the two machines.
>So
>swapping in a 727 ROM in a 707 will sound MUCH more like a 727, but
>there
>might still be some audible differences from a true 727.