So ... continuing on this project - thanks for everyone's help so far
I've got the AC straight, the 5V and the 12V at the Jamma connector are good.
I connect a monitor, I turn on power and I get this screen. It flashes back and forth. I took 2 pics so you could see it, sorta.
I plugged another Jamma board in, and the video on it seemed OK. Any ideas?