Here are some bits that are occasionally found online but I don't think they are all in the same place:
Incremental adjustment (>= OS1.2): Hold [0] + [1]=down, [2]=up
Functions accessed by holding [Enter] + :
[0] = Basic Patch
[1] = Resets joystick mix level mod points
[2] = Random patch
[5] = Default global parameter settings
[6] = Does nothing but is in all versions of the code. (Just "returns")
[7] = Check cartridge NVRAM checksum
[8] = Write RAM cartridge header
[9] = Display and clear any saved error codes:
**** ERROR: ****
1 - UART OVERRUN
2 - BUFFER OVERFLOW
3 - BAD NV CHECKSUM
4 - BAD WRITE VERIFY
5 - BAD CART CHECK
Here is a copy of the text section of the service manual. The schematic section is easy to find online but I don't think this is.