Unfortunately there are things on each of the 3 boards in the set that can cause issues like this. At least the A and B boards can be directly swapped with other sets with the same layout. The C board however can be restricted to often the one or two games (two in this case, Varth is the other)
First thing to do was try the A Board on another game. I tried it on my Mercs which has a faulty A board and it restored this game to 100% So that pretty much ruled out the A board. Worth noting though that some A Boards can work fine on one game and not on another.
It's always worth checking basic things like connections between boards, reseating ROMs and voltages as well as giving the board a general inspection for any obvious signs of damage or corrosion. There were some minor signs of oxidisation, but nothing very significant at first look, certaily nothing that looked fatal.
Next step was to burn a replacement GAL for the PAL at 1A on the B-board. This IC is responsible for the graphics, so a problem here could cause jailbars. Unfortunately the GAL didn't help.
Onto the graphics roms themselves. I couldn't see any obvious damage, so I went about verifying them against the MAME set in my rom programmer. The pinout for these mask roms matches a 27c400 EPROM so it's simply a matter of loading the correct rom binary from mame and selecting 'verify'.
Straight away rom 1M, first rom in the set at location 3A looked like this:
It wasn't even reading, let alone verifying. I booted the game without this rom and the display was much cleaner although with some tiles incomplete as you would expect with a missing rom. On closer inspection I could see the leg identified by the programmer was rusted right where it meets the die. (Apologies for the crappy phone pic, you can just about make it out)
Looks like we are onto a winner. I programmed a fresh 27c400 to replace the mask rom and hey presto...
Yay!
This game is harder than I remember.
Should buy myself an eprom burner / eraser also.
