However, the screen remains a blue screen, and the sound is just noise-like.

I've re-soldered and checked various parts, and I think 5V is reaching each section. I've also cleaned the cartridge slot.

The 68k CPU isn't heating up, so I replaced the CPU and RAM, but it's still not heating up. Voltage is coming through.

I feel like the problem might be somewhere else... Since 5V is present, I don't think it's the common regulator failure...

Does anyone know where I should check next?

Thanks.