Here's a high-level AC wiring diagram from Bob Roberts. You want to disconnect the AC wires going to the power supply marked with a red X. Be careful when working with AC power. Unplug the cabinet from the wall first. Make sure to carefully disconnect, isolate and wrap each wire with electrical tape so they don't short together or to ground (or to you).
You might want to spend some time searching and reading threads over at BYOAC. They get a lot more traffic on this kind of stuff... http://forum.arcadecontrols.com/
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Re: Mame PC in to Jamma cab
