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Remove lid

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Remove shielding. The extra stuff near the voltage regulator is an s-video buffer circuit, pics of which are also available at this site.

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See the "NTSC/PAL" spots right next to the NEC chip?

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Flip over, desolder connections, and cut trace to center connection.

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Locate a good spot for the switch. Nothing used the SMS expansion port, so that's a good area.

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Mark, drill, solder wires to switch, and test fit. Ensure the shielding doesn't contact the switch.

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Solder the wires to the right spots, and secure the cable.

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Put 'er back together.