Solved! 1970 rca colortrak plus

Solution
Any old VCR can be tasked as a cable tuner for the set. Use the audio-outs from the VCR to supply the surround sound. You change channels with the VCR remote. Double-solved.
Any old VCR can be tasked as a cable tuner for the set. Use the audio-outs from the VCR to supply the surround sound. You change channels with the VCR remote. Double-solved.
 
Solution