Also you must move the PAL "monitor" from DEVS:Monitors to Storage/Monitors and do the contrary with the NTSC icon.
---------- Post added at 17:37 ---------- Previous post was at 17:33 ----------
Isn't it something on the mobo (can't remember exactly what!) that selects the default video output?
Yes, a zero Ohm resistor hooked in the Alice pin41 which you must remove/insert to change the machine from PAL to NTSC. But the encoder is setted to PAL, RF modulator will still be PAL, mainboard crystal still will be wrong frequency.