if it was the ribbon you would be seeing a rug pattern. what I see from your picture is a monitor with the Screen brightness cranked up so high all you see is retrace lines. start by turning the flyback down until the lines disappear.
the switcher additionally won't mean much if the filter capacitors on the MPU are trash. not you, but I don't know why people don't grasp this, you can lose up to 3/4 of a volt across those caps.
in the PSU guide in my signature is a picture of an IC, it shows how to measure voltage on it. don't do this at the 24 ram chips because undoubtedly that's the only time people's hands get unsteady and they wind up destroying numerous ram components. choose another chip close to the headers, that's how you will determine where your +5 is at. your game appears dead, so I'm going to venture a guess that the voltage is morbidly low.
also verify that you have 3 capacitors by the power plug on the MPU, 100 uF/25V. if they're somehow missing like they were on one of my boards then you will have no voltage going to the chips.
the power plug has a variety of different colored wires, this is where you can obtain your reference voltage for the +5 at the grey wires. you can also sample the grey-yellow and grey-white (regulated and unregulated +12) and grey-green (-5) too. the ground is the black wires.
the Williams guide in my signature is usually somewhat helpful, but your game looks completely dead. please take pictures of the actual power supply showing how the wires are connected to the lugs, and take pictures in sections of the MPU board, so I can see if you're missing any chips.
also make sure the monitor has a video signal coming into it, they'll be purple wires, and they connect to the middle big plug on the MPU. and last, from a diagnostic standpoint, the rom board will show all 4 LEDs because the MPU is probably not working at all.