There is a circuit within the VCR called an RF Converter. This is the circuit that converts the audio and video signals into the RF channel 3 or 4. It sounds like the vidoe is not being processed or the video is not being sent to the converter. Without tracing the video inside the VCR, I can't tell which is the problem here. You can try to play the tape in "review" mode (hit play then FF) and see if you get any video signal. If you do, then there is a possibility that the heads are clogged and need to be cleaned.

Please try that and update this with the results.

Thanks,
Dan

Have you tried cleaning the video head with Servisol Video 40?