[ChrisW6ATV]

I forgot if Atari 2600s used TV channels 3 and 4 for their RF outputs, or if this was the device that used channels 2 and 3. In either case, a few good tests include:

-Check if you have sound but no video from the machine (by starting a game even though you cannot see the video).

-If you have no video or sound, change the Atari to its other channel while it is on and check if you see or hear any difference on the TV set. Then, check the same new channel on the TV set that you selected on the game box.

-If I remember right, the RF output cable from the 2600 has an RCA male connector inside the game machine as well as at the other end. And, those cables were relatively low quality. Try some other RCA-to-RCA cable from the modulator inside the machine.

-As Electronic M mentioned, try a direct RF connection to your TV set using an RCA-to-F adapter on the Atari's RF output cable into your TV set, and also try it into a second TV set if you did not do so already.

Going back to 1979 when I bought my 2600, that same year I bought my first new TV set, which was a 19-inch Sony that had 300-ohm and 75-ohm VHF inputs, with a switch to choose between them. So, it essentially had a TV/Game switch built in, once I bought that RCA-to-F adapter (since my antenna used 300-ohm cable). It had a significantly better picture that way compared to using the old silver box with the long slide switch. The real picture and sound improvements came when I modified my 2600 (and later, Intellivision, Colecovision, Atari 5200, and Atari 7800) for line-level audio and composite video outputs.