Ways to do it:
- Connect your old RGB monitor (or an A520 hooked on a TV set). When the A3k loads the WB, change the screenmodes to NTSC (you know how to change it?*);
- Connect your old VGA monitor on the FFed output and do the same;
You could also download an old app called "Amiga2NTSC" (Amiga2PAL). Findable on Aminet, IIRC. Just for CLI use, of course (put it in your shell-startup).
* Open Storage/Monitors and move the NTSC monitor to DEVS/Monitors. Move PAL to Storage/Monitors, BTW.
