Sega SG-1000 & Mark III: R/F doesn't work!

[PingvinBlueJeans]

The Mark III works fine through its AV connection, but nothing at all happens when I try to plug in the RF switch. Same with the SG-1000. They both power on, but don't display anything through RF. When I say nothing happens, I mean NOTHING. The RF is fine with the PCE, but when used with the Mark III or SG-1000 the TV doesn't even flicker. I opened up the SG-1000 and it looks fine inside.

Does anyone have any ideas what the problem is with the RF connection on both systems?

The RF output on Japanese systems was designed for Japanese TVs. If it works at all, it will work on channels 97/98 (not 3/4). Give the channels in the upper 90s a try. If you're in the U.S. you'll need to use a North American switchbox or RF-coax adapter as well (the one that came with the system probably won't work).

EDIT: It looks like you're in Japan? So the issue is probably from the fact that you're using a newer (automatic) RF switch. You need to use an older (manual) switch.