It seems that my little scope can't reliable display anything lower than 500mV, anything higher and it'll just draw a line at the very top of the display, even when I set the baseline at the bottom of the display. I can set it interval to 1uS, however, and get some readings... Sadly, I can't freeze frame at that speed.
Now, just for the sake of comparing things apples to apples, I removed all the tantalums from the Cell (both top and bottom), but left all the tantalums from the rsx in place. With that done, I compared scope readings between the Cell and the RSX, and they were interesting. See the video below
The first beep you hear is when I'm testing the RSX (which, again, has all the caps in). All I get with it is... a flat line. I mean, you can see it go up, and then down, but there's no visible wave.
Right after the RSX test, I test the Cell (remember, no caps) and you get some very jagged patterns fly by. So, here's my theory -- my cap installation is perfect, and my problem is elsewhere, not cap related at all. It's very likely that my lack of sufficient resolution is hiding any kind of "good patterns" out of the RSX reading... But I did a few readings on the Cell early on, when I removed the first 2 caps, and the change on the wave was immediately noticeable (it's too bad I didn't save those, but I'll record them when I put the caps back on).
So now I'm really curious about why the RSX reading is flat. Is it really just my lack of resolution, or something else? I'm in a bit of a denial, I don't necessarily think it's a BGA issue (for a variety of reasons).
I'm also really tempted to put all the Cell caps back on and then enable the tristate, just to see if it boots. I don't know that it will, but hey, science! If it does (big if), then it might prove that the problem is in the southbridge somewhere (since the tristate acts as a disconnect between syscon and southbridge -- I might be getting this slightly wrong), or maybe the firmware is corrupted (which is something that the psxdev wiki hinted at). Anyone around here experienced with hardware flashing can speak to this idea?