I have an LG DVD recorder/video cassette recorder Model RC 68227. It will paly audio cds, but will not play any type of DVD. The error message I get is 'unknown disc please check disc again'.
I have the exact same problem on my rc6821w lg recorder. I think it happened after a power blackout / power surge. When did yours happen? Seems like it's not worth repairing - probably will break down again! Had it repaired for another breakdown with DVD recording 2 months ago under warranty. Now it's out of warranty of course! No more LG equipment for me!
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Try fixfault.com to fix dvd players.