If you can do another test, to check whether the 12V supply's startup is a factor (difference between starting up the 12V supply while connected, versus pre-starting the 12V supply and then plugging it into the 5V regulator) that info would really help me to know whether I should focus more testing effort on the 5V stuff. I do have some LM1117 chips here, but they're all 3.3V, not 5V. I can get the 5V one in a few days, but maybe it's really that 12V 2.4A supply I need?
i'll hook up the 12V supply when i get back from work. what i can say is that the board does boot up properly when i hot-plug the 5V regulator (i.e., with pre-started 12V, only then plugging the 5V regulator), so yes, it doesn't seem to be the LM1117 per se that's causing problems. that said, so far i've tried with three different makes of 12V supply, it happens with all of them. fwiw, the one used in the picture above comes with this; it's in fairly common use with synth people, i'd assume.