So with something said earlier, RS 3.0 is known to be different slightly. We know the PIC and ROM use 5V likely generated by the RS3.0 power board, however not on pins 5 and 6. Does someone have an updated diagram pinout specific to RS3.0?? Just wondering what voltages are present (maybe 8V or 9V for audio amp?) and what pins are 5V. Just trying to complete my little book of knowledge for troubleshooting and repair or mod purposes. Also potentially produce modern repair PCBs as replacements become exhausted. Again, it has been noted and understood RS3.0 is unique. You can put RS3.0 PIC and ROM into other RS versions- they all are 5V "systems".
It's been awhile since I've looked it up, but RS3 systems have both 5v and 12v lines. You correctly surmised that the bigger voltage is for the pair of audio amps.
The two diodes and large caps found on the RS3 power board act as a voltage doubler to get the system to function under 12 volts. One-piece RS2.5 boards function in the same manner.
For RS4 and above, the system was redesigned with more efficient audio amps needing lower voltages.