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I plugged in wrong voltage supply and the dreamcast got burnt. How can I repair it ?

Hi, I would try opening it up and check around the area where the power supply is located. This guide covers the opening process: Replacing Sega Dreamcast Power Block (Power Supply) You will most likely find a burned component in the power supply board. If you own or have access to a soldering iron, you could try replacing the damaged components. Alternatively, could buy a new US Dreamcast power supply on eBay for around 20 dollars and replace it with the faulty one.