Looks like on the forums other people have this kind of issue with their originals. Can't figure out what is is.

There's a programmable latch ('74 gate) that seems to stop a keyboard interrupt getting through, that's not passing the interrupt. Also played with reset & /reset with no luck. The tune seems very ordered. Maybe an AY thing? It stops once a game with sound's loaded etc.

Built a second, doesn't play a tune. First build now doesn't play a tune on boot screen, but does in Chuckie Egg highscores, working on that.