I feel so stupid !
When installing the multi, I've replaced SOU1 PAL chip on the A-board with the one included in the kit, instead of replacing the PRG1 PAL chip
Putting the PALs at their right place solved all my issues, now everything is working fine
Lesson learned: never install a multi when you're tired
Thanks a lot to @Darksoft for his time
When installing the multi, I've replaced SOU1 PAL chip on the A-board with the one included in the kit, instead of replacing the PRG1 PAL chip
Putting the PALs at their right place solved all my issues, now everything is working fine
Lesson learned: never install a multi when you're tired
Thanks a lot to @Darksoft for his time