Mercs Repair Logs
Repairer: channelmaniac
Symptom: Missing background graphics
Cleaned pins on the 0224B PROM and tested.
Symptom: Corrupt foreground and background graphics
Resoldered the C board chip and tested
Repairer: Apocalypse
Source: Mercs - Capcom 1990 (repair log)
The game was almost 100% running ok. But here were bars across the screen visible only when background was black. Nothing noticeable in game.
I swapped the A and C boards and narrowed the fault to the B board. Then I swapped the ROMs and PALs and found the B board itself was faulty. I probed the few ICs on the board but didn't find any suspicious one. It's the third time I face this exact same issue on CPS1, two previous times it was simply a cut trace. This time made no exception. I found only one broken trace and patched it with the core of kynar wire.
Game fixed.