Capcom Gun.Smoke PCB repair

This is a bootleg PCB though a well made one with excellent image quality.  The problem is that the buildings have corrupt colours.  This is a rather strange fault as Gun.Smoke only has 1 layer of tiled graphics and all of the other tiles were fine.  This rules out almost all potential RAM or TTL faults (maybe with the exception of some of the palette circuitry), as those components are clearly working fine with the other tiles.  From the look of the graphics I suspected a couple of bit-planes for the tiles had disappeared – because each tile only has 4 colours per block, instead of 16.  MAME confirmed the tile ROMs were laid out as 4 pairs with each ROM contributing 2bpp and the corrupt buildings were within the first pair.

The missing 2bpp theory was confirmed when I ran the board with ROM ’13′ removed – the buildings completely disappeared – so this means that ’13′ was working and it’s partner ’9′ wasn’t contributing anything.  However all ROMS read ok in the reader and matched the MAME set exactly.  The logic probe lit up for all pins on ROM ’9′ so the socket seemed good.  Next theory was the chip enable (/CE) logic was faulty and ROM ’9′ was not being asked to output any data.  However examination of the traces showed /CE was linked for each pair of ROMs in the set (as they always output at once to give make 4bpp output) so could not be that.

I then burned a new eprom with ROM ’9′ data from MAME, placed it on the board and everything worked!  So the fault was definitely with the ROM even though it read fine on PC.  The only theory I have is that the original ROM ’9′ is more susceptible to low voltage than the others.  There is quite a bit of voltage drop from the +5V on the main board to the video board.