Pulled the other two boards out of storage and man do I have a fun family of weird. The board that I was trying, which is the original board I bought over 10 years ago and had a friend do a battery conversion on it, comes up with multicolored snow. Of the two from storage, one comes up and plays great, except the audio seems like it's scratchy/overdriven. The last board comes up, sounds great, looks great, but the image on the screen is upside down, even if I flip dip #2. I may try some mix and match to see if I can get one good working board (probably swap out the graphics board/12 volt plane on the one with upside down video to see if that'll do it, as it works great otherwise), but at least this gives me a nice range of issues to experiment with and hopefully find a fix for your situation.
One thing I did just for giggles was throw the pos/neg sync dip switch on the one with the upside-down video, and the screen changed over to a fast, vertically scrolling signal which had a lot of red & white in it. When I had the dip in a position to match the sync on the monitor, it was just black for about 30 seconds until my monitor warmed up/the signal fully came in/whatever and then it came up. From the sounds of it, you've had it on the black screeen for a bit to check if it plays blind, but may want to give it a couple minutes. If the startup noise "TIME KILLEEERRRRSSS" keeps playing over and over, even though the green LED may be blinking it's not getting quite enough voltage. The one thing I noticed with all of the boards is that they seem thirsty as hell. I stepped the voltage up slowly until they were working, and I had to have the voltage up near 5.5v as measured at the harness w/o it plugged in for the board to be working. When I measured the voltage off the contacts where the harness connects to the board with the board connected, the read voltage was down near 5.2 or 5.3, when the board was unplugged it was showing a bit above 5.6. Board hasn't been exploding at that voltage, though, so I don't think it's above what it can take.
And yeah, the other two boards have nothing at U27, so that's just a random empty IC normally on the board, goo to have it confirmed, thanks for checking.
Let me know which of the list of things I suggested you've tried and any changes that happened, if any did so we can continue isolating the issue. Most of what I know, and the stock pcb troubleshooting steps comes from reading these forums the last couple years, part of why I love this place.
