I've buy a nes a couple of weeks ago and the gyu have tested it front of me and every thing was write but when i've get homed and try super morio bros./ Duck hunt on my plasma tv but when come a new screen, the screen turn darker ( i can always see the game) and the sound run out for 1 or 2 seconds before going back. but when i reach the level 1-2 the music insted of going (i warned you it need a litle bit of imagination) pada pada pada, pada pada pada, pado pado pado, pado pado pado, pado pad pad pad pad pad pa putiputiputputputputput, it's going like pad,-------pad-------pod-------pod------------pad pad pad pad pad pa putiputiputputputputput the dark screen bug appears in some other games but i've try many cartridge of the same games and always do the same the sound bug happened sometimes in other games but not this far (sometimes a music note is remplaced when a other sound hapen for example if i get a coin) not total music who's missing lot of notes I've also tryed others cable and it work but only whit the one that you need to put your tv to channel 3 or 4 not whit av cables
That sounds like the NES has a faulty RF module. Mine does that if I use RF. AV cables work, just make sure you have the TV on the correct input option, and make sure you have Audio > Audio and Video > Video. Hopefully your AV ports aren't broken, otherwise I don't know why AV wouldn't work for you.
finaly I fingk it's maybe a problem whit my tv because it do the same dark screen thing but whit a sorter periode whit my nintendo 64