I'm working on a Grand Lizard system 11 pin. I was getting a lot of loud hum from the speakers at all times. I replaced both C2 (470uf 16v) and C3 (100uf 35v) on the MPU board as both had really high ESR as measured with my ESR meter. I also installed a couple screws that were missing for securing the MPU board to the back plate. That fixed the loud hum in the speakers that was present all the time. Now I just get feedback/hum when the CPU lights are activated.

I'm seeing a lot of AC ripple across the 30,000uf 25v filter cap for the lamp matrix so it looks like it needs to be replaced. I've got a new Big Blue that is used on the Atari power supply bricks. It's a 27,000uf 25v capacitor. Is that close enough to work?

Already did that.

Yes. I temporarily swapped out that 30,000uf filter capacitor but no change. Power supply has new caps and I replaced two bad caps in the sound circuit on the MPU. All boards are properly secured in the backbox and the cabinet ground wires are secured at the backbox.

Any other ideas?