I had a 77xxx board, GH051-51, and i left both the Y and SX wire off.
I read here on forum that Y had to be solder to flash the chip.
Thats not the cause.
OK, i tried
PS2 backup - autoboot
PS2 DVD - autoboot
Matrix Flash cd - Autoboot
Flashed firmware OK
Tried homebrew CD (own made, just burned with nero) also works!
So what the hell is Y for then? Only PS1 with SX?