Konami X-Men arcade pcb repair #2
Game played and passed all self tests (including mask ROM tests) but some background elements had incorrect colors or appeared completely black. Colors ultimately come from the palette RAM but that was most likely good as the self test had passed. A group of 3 LS157 chips arbitrate access to the palette RAM address lines – these can be controlled either by the main CPU, or by the 053251 priority mixer which combines all of the layers (sprites and tilemaps) to send a final output pixel to the palette lookup.
All address lines seemed good with a logic probe so the problem was most likely the 053251 itself or the tilemap custom that fed it. As other tilemap elements were fine, the 053251 was replaced with one from a scrap board and this fixed the problem.