A S&S schematic is findable on getlofi.com but besides being nearly unreadable, it has obvious errors (bus lines on the ROMs not matching for example).

Since it didn't inspire me much, I took the time to draw a schematic from the PCB I had, with the help of my multimeter's beep and Photoshop, checking each connection more than once.

As the diodes of the power board tend to go out (or darken/break) easily, I also added its schematic.

Not having the pin of the microcontroller, I have not dared to name the pins from the source code comments. Some connections are however named, such as those used to scan and read the VFD keypad.

It is now clearer that the ROMs are wired in parallel and have no independent Chip Select signals. First surprise. It is also visible that the microcontroller isn't wired directly to the ROMs, but only to the synthesizer. Second surprise.

Having the cartridge port pinout, it is noticeable that some pins are present but not used. In this case, the PD (PowerDown) signal is routed to the connector but all the cartridges I'm aware of are only 10 pins, not 12.