As you've already suggested, garbage on screen like that usually does indicate a RAM issue, particularly since you've already identified a faulty voltage regulator and RAM is very easy to damage when the power goes bad.

Before testing anything else, I'd definitely recommend replacing & adding sockets for the RAM. I'm not familiar with those VTech systems, so there could be other issues as well of course.

That sounds about right, thanks. Just a little update since I don't have the SRAM chips yet, I fiddled around not really knowing what to do. Tested the ram chip input voltages according to Hitachi datasheets and they seem about right at 4.99V, so the board appears not to have any more problems, so leaving the motherboard as is for the time being, especially since I now get some output and it doesn't overheat anymore. Figured out that the shorted thing is inside the little box over the D780 cpu, so I unsoldered it from the board and pried it open. It's autonomous, no traces for it on the motherboard, it's drawing power directly from the 7805 with cables, and its output connects via another cable to the floating PAL board, and its chassis grounds with the modulator housing. After inspecting with the multimeter, I discovered that the shorted component is a NA31XJ transistor, which appears to be unobtainium by now. Didn't bother to seek an equivalent, as I actually came across the technical reference document online at vz200.org, and by looking at the schematics, all this thing does is to provide the PAL board with 12 volts. Tomorrow I'll try to skip that altogether and connect a newer 5V-to-12V DC booster circuit and see if I get this corrupted picture through RF, to test the PAL board/modulator combo. If all is well, then I'll unsolder the ram chips and fit sockets, then sit back and relax till the new chips arrive 😀