NTSC PSX Backups on PS2

[Geforce]

Right guys i need help/advice, I`v ejust purchased a UK PS2 with mod chip fitted.. Got it home and tried to play my psx backups, UK Backups worked fine but all NTSC Backups stopped dead and did nothing.. Is there a problem booting NTSC backups on UK Ps2 if so how do i combat this so i can play them.. Even Games that have the NTSC/PAL Switch won`t boot.. Any help u can give will help me a lot..

I`ve had a quick look for a ps2 boot disc but can`t find one so i presume there isn`t one yet?