Initially a completely dead board, with no clock signal at the CPU. Some probing revealed the crystal oscillator didn’t have any power going to it, and in fact nothing on the top board had +5V. There was a very unusual broken trace on the main +5V line from the edge connector, and a handful of capacitors that had pretty much came apart.
With that fixed the board booted to a lot of garbage graphics – no sprites and no road layer.
The road layer is handled by the top board – pretty much the custom chip, two RAM chips, a ROM and a PROM (the other two RAM chips in that corner are the palette RAM). In this case the 63s141N PROM had failed and had dead outputs. A PROM was taken from another board and the road layer fixed.
The title screen had some interesting sprite behaviour – the first entry in the sprite list seemed to be drawing somewhat correctly (The ‘CH’ of the Chase HQ logo). That made me wonder if some kind of counter had failed and it was only processing the very first address of sprite RAM over and over. There are a bunch of LS161 counter chips on the video board and there was a dead input on some of them. This traced back to a PAL chip which was replaced by one from another board and all sprites displayed, albeit with a lot of garbage and vertical line corruption.
The final part of the puzzle are the 16 sprite work RAM chips on the video board – if any of these fail it will tend to be vertical lines or other corruption throughout the screen. The sprite RAM seems to be in ‘screen space’ – so faults don’t follow the sprites but instead have constant positions on the screen. On this board 5 chips were confirmed as bad and when replaced the board was 100% again!