I meant that you should connect a standard yellow composite video cable to the green AV2 input of your OSSC and test if you get stable sync on that input. If you get a stable sync that way then the issue is with the RGB cable and otherwise there is something wrong with the mod.
If you get no/unstable sync or no noisy black&white picture with the composite video cable then the modder might have connected unattenuated CSYNC to the composite video pin. You should send the console back in that case.
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