update: i tried to manually apply pressure (like shimming) to the chip while turning on the machine, but it had no effect: nothing on the screen, but the computer was booting nicely (i coud hear the hard drive working and even the silent hissing sound when the sound came on (typically this is somewhere at the end of the boot sequence).

i'm more and more convinced that this has nothing to do with the vreg chip. i've examined it with a magnifying glass, and the soldering seems to be perfectly intact. i'll try to make a picture if you're interested, maybe you can spot something i missed.

next, i'll try pressing down on the video card itself.