Sounds like you and I have the same problem. Mine in a tz. I've replaced all the amps, the RAM and made sure the roms work and confirmed this board does not work in another machine.
I've made some observations with the logic probe. Pin 1 of u2 on a working board shows no signal, whereas on the dead board it shows low. Also the all signals on a working RAM (u9 I think) are way different to the dead one even after putting new RAM in. I assume any 6264 RAM will work?
I've also reseated, swapped ribbon cables to make sure they are not the problem.
