A small update. It seems that after some cool-down time, the OSSC can display picture from CPS2 and Neo Geo [haven’t tested anything else], but it only works for about 10 minutes or so. After that, the red diode will start blinking and lose the signal, until there is no picture at all, and “no sync” appears on the LCD display.
I’ve been using the OSSC every day since I got it, and this never, ever happened with any of my boards. Not even the F3, once it locked onto the signal. I honestly have no idea what could be at fault.
As for the TV prongs, my current TV setup hasn’t changed for years. I use arcade PCBs with it directly via SCART, but the TV’s scaling leaves a lot to be desired. That’s why I got the OSSC. Arcade boards get their power from an ATX PC power supply. I don’t know how this setup could have damaged the OSSC after working perfectly for so many years, without any incidents with all of my other equipment.