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All coax is suspect of being defective.
All connectors on the ends of of coax are suspect of being not put on correctly.
All couplers are suspect of being defective.
All splitters are suspect of being defective.
All devices that are connected to the antenna system are suspect of being defective.
All etc. is suspect of being defective.
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Digital tuners can develop -Digital Glitches- that are not cleared out with simple channel scans.
To clear tuner do Double Rescan.
http://www.wchstv.com/DoubleReScanAlert.pdf