The ADSP on the brainboard is the CPU of the brainboard. Mackie used a DSP for that.
Basically the function of the brainboard is like a small Computer. A small program is stored in the EPROM. It is only for communication with the PSU/CPU to download the real program to the console (but in your case you do not get that far).
once you try the new EPROM and it still doesn't work maybe you can make a higjh Resolution Picture of your brainboard for us. Maybe somebody can see a defect via an optical inspection.