I was hoping maybe your CRT had some kind of info button on the remote to show information about the incoming signal on the screen, or perhaps there’s something in the on-screen menus.
Next thing I’d check is the OSSC. With the Dreamcast on and connected to the OSSC via SCART, and input set to AV1-RGBS, is its LED green (has a sync signal), and what does the LCD say?
If the OSSC is fine, and you have a multimeter that can check continuity, you can check the cable to see if pins 14, 15, and 16 on the Dreamcast end are connected to pins 7, 11, and 15 respectively on the SCART head.
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