O, I have a scrounged AC transformer I was hoping to power this with. It has 16 (?!)  ~12v secondary windings, which I had chained all up to give me around 30V to rectify and regulate for powering the Raspi. However it seems the over-all current supplied by these many taps is just barely insufficient to power a bare Raspi 4. I am able to measure the transformer output current to about 1.5A, but this is not quite enough. I can see on the scope where it dips as the RASPI tries to boot, sucks too much juice, and then shuts down for a second in a lamentable waltz for watts. I have tried disabling various components on the RASPI with no luck... So I am going to have to order a new transformer to power this box.

I am pleased with the ability of the diodes and beefy 3905 to convert 30Vac to a clean 5Vdc. The ripply DC hitting the regulator IC is ~40V!

I suspect the plethora of taps is reducing the total current of this transformer; The wires in the secondary windings are tiny, which I believe is what limits the available current of a transformer. If I was using something better suited, such as a 1:5 transformer with a single beefy secondary winding, I probably would not be having this problem! Sadly, sometimes recycling old parts to drive vastly over-powered microcomputers does not work.