i have SEVERAL older VCRs and have successfully connected roku to old tv's with coax input only through a JVC HRJ-22OU, an emerson EWV401 and a panasonic PV-V4522 but the emerson EWV601b and the toshiba W-515 didn't work
also i directly connected both an N1000 and a Roku1 XD/s to the composite ports on a Durabrand TV DWT1304 without success.
i don't know what's wrong with yours specifically, but i know for a fact that several old stand alone VCR models will recognize all the generations of roku and work as an rf modulator
yeah, why do i have so many old VCRs? because people give them away and they don't give away rf modulators.
i should point out in case it helps that neither of the emersons nor the toshiba will recognize a ps2 or sega genesis.
N1000, roku1 XD/s, roku2 xs, roku3