Famicom AV mod: "Jail bars"?

[Threedog]

I A/V modded my Famicom by replacing the Japanese RF board/box with the box from an American Toaster model NES, leaving all the metal shielding on, and the system now displays very faint jailbars. Not nearly as pronounced as on the US toploader NES, but they're there nonetheless. The only game I have to test with is SMB 3, and I understand that particular game displays this problem pretty prominently on the toploader system. Does anyone else's A/V modded Famicom have this issue? Does Mario 3 always show those bars, no matter what system?

Thanks!