If it's a true 120Hz hum, then PSU is a good thing to look at, but it's rather more likely to be the internal buss amplifiers, which increase gain as they are terminated with more and more modules.
Scope the rails loaded and unloaded. If you see no difference, look to a better internal greounding scheme, something like the 200B mod that Eddie Ciletti details on his site. many Ameks of the era had similar problems.
I should add that when you go noise testing to test the guts of a console, the only things that should be connected are the console and the monitors. Everything else should be disconnected. If the buzz isn't there under those circumstances, your installation should be investigated, with special attention paid to the grounding philosophy.
Good luck,
Keith