That is essentially what this ENTIRE thread has been about. I think you finally understand!Also I am getting continuity between the scope ground and the metal chassis of the ATX PS. Isnt this dangerous?
--ETA: No, it isn't dangerous, since it is ground, Earth Ground, 0V, so if you are standing outside and wet, you won't get a shock from touching the enclosure.
Yes, I pulled 10k out of the air as a "Resistor that will limit current enough to not damage anything, yet show a value on his meter if there is a path to ground"
However, 5V to 12V should work fine for 7V if you use just the red wires, and call 5V "circuit ground" and 12V "+7V", That said, I'd still suggest an isolated supply. There is a reason they aren't as cheap as computer supplies, well, several reasons, but isolation is one of them.