Because mame's emulation is rarely that perfect/low level. I think the boards are clearing something out but I don't know what. There are little things like that that mame sometimes just bypasses. A protection could get skipped over instead of emulated correctly and certain functions could be flat out not supported yet. I don't know why certain revs do things differently and if sound is dropping out you might actually have a fault on your board.
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Re: Strange graphics when switching on a specific arcade board
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