GOAT80: I think his point is whether the composite signal coming out of the DIN jack could work even though the RCA (RF) doesn't.
The answer is yes it could, but there's no way to know for sure whether without further investigation. The composite signal could also be broken before it even enters the RF modulator, in which case the DIN signal won't work.
However, I personally think the problem is the RF adapter is your problem. In the 80s there were different ways of doing RF modulation. One way was to modulate the signal externally. If you've gotten ahold of that kind of adapter, it won't work with C64 as it already has an internal RF modulator.
Then there were Nintedo's RF adapters which, if I've understood the matter correctly, modulated the signal internally in the control deck, (Yes! That's what they used to call the NES!) but had a filter inside the box so that the signal wouldn't interfere with other people in the same house connected to the same antenna. Those are great, but they're not tuned to same frequency as the C64 is using, so they will effectively filter out the C64's RF signal.
Yet others just had a switch that you had to had to set to "TV" or "game". To prevent that kind of interference.
I suggest finding a regular antenna cable and trying to shrink the shielding on the right side of the cable, so the plug fits snugly into the RF jack. That should work.
