Boots and passes self-test, but immediately coins itself up and starts playing itself(!), although not too well.
Well, it looks like some of the inputs are firing rapidly and randomly. Grounding one of the enable pins on one of the 74LS253s seems to make it settle down for a few seconds, so I will try playing with it.
[07/30/03] Update #1: OK, it looks like one of the outputs (za) is always low on the 74LS253, regardless of its inputs. Will try swapping it out.
Solution: None yet.