Tecmo Knight arcade pcb repair

Game booted, but the red & blue color channels were clearly stuck on.  I went hunting for palette RAM and found it on the top board, it’s a slightly unusual type – AAA16K4P-35 with one chip per RGB color.  Swapping in replacements from a Ninja Gaiden parts board restored correct colors, and I could see the video board problems in more detail.

One tilemap layer had corruption – the game has 3 tilemap layers and each layer comes from a combination of the Tecmo 3 & 4 custom chips, graphics ROM plus a pair of RAM chips.  One of the RAM chips tested bad straight away, but with that replaced there were still problems.  A logic probe on the ROM showed one of the address lines was floating – but it had continuity back to the Tecmo-4 custom.  Unfortunately this suggested the line had died inside the custom chip.  Replacing this improved things but there was still a fault with tiles being ‘doubled up’.

A logic probe on the RAM chip showed the D0 data line was stuck low, and this traced back to the Tecmo-3 custom – so the doubling was because only even (0,2,4,6,etc) tiles could be selected.  This custom was also replaced with one from the Ninja Gaiden parts board and everything was fixed.