Because I want *so badly* to help as much as I can the understanding of the Galaxian3 hardware, here's the comparison between the Starblade boards (Namco System 21) and Galaxian3 (Namco System 21 'custom'?).
Fortunately, I own all the boards from both games
Let's start with few of them first (I don't want to have a room full of boards
Note : you can click on all pictures from this post for a 'big size' picture 1. DSP board :
UP : Starblade DSP board
DOWN : Galaxian3 board
Hey, they seem to be ... the same board
Yes, the board is exactly the same... see the reference on it :For Starblade For Galaxian3
Now, the differences, regarding the settings :For Starblade
no EPROM at position 1F, 1K1, 1N For Galaxian3
there are EPROMs at position 1F, 1K1, 1N
Now, see the difference between the jumpers position :
JP3 VCC GND
JP4 GND VCC
all the jumpers are set in the same position. 100% the same board, but not populated the same
(*) : Well, because it's a 'spare' Starblade board, maybe the EPROMs have been removed. I'll check on the DPS board *inside* my working Starblade game.