This happened to me once. It turned out to be the cable. I was clumsy when inserting the cable into the input and broke off some connecting pins.
Seems they are very fragile and must be lined up carefully.
Or... as simple as it sounds, just make sure the cable is plugged in securely. I did this once too - I had pulled out my VCR for some reason, and when I did the S out cable came loose a bit - instant black & white. It only took me a minute or two to figure out what was up, but next time I'll know right away! ;)
Yep, its either a broken pin or the cable came lose, both occurances happened to me before.
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