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Nintendo Mario Bros arcade pcb repair #3

Very strange fault on this board – two misplaced tiles on the title screen – but absolutely no fault anywhere else.

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That kind of fault is very strange as usually a fault with video hardware like this would affect all sprites, or at least groups of related sprites.  Indeed when the CPU board was matched with a known working video board the fault remained – so it was on the CPU board rather than in graphics generation.  As the program EPROMs verified correct the main suspect would be the Z80-DMA chip which copies data from memory to the video board – however I found the fault with some luck by swapping the two main CPU RAM chips.  The fault went away with different RAM, and came back with the original RAM replaced.

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New RAM installed to properly fix the problem but still a very strange fault – usually when RAM fails an entire an address row or the data bus is taken out – which usually means the CPU program will fail quickly.  This RAM must just have a handful of bad bits that happened to correspond to work area for the title screen.

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