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The CM7777 preamplifier can be over loaded from the strong signal strength Tv transmitters at the top of the channel list , over loaded amplifiers can be the cause of odd reception problems.
Use a amplifier that is more tolerant of of strong signal strengths , use a RCA TVPRAMP1R preamplifier.
The signal you are trying to receive is Weak signal strength and the antenna might be on a dead spot , try moving the antenna up and down and change location of antenna.
Aim the antenna at about 262 degree magnetic compass direction.
Here is how to aim antennas ,
www.kyes.com/antenna/pointing/pointing.html
Use a Real and Actual magnetic compass to aim antenna.
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Digital Tv tuners can develop -Digital Glitches - that are not cleared out with simple channel scans.
To clear tuner do Double Rescan.
www.wchstv.com/DoubleReScanAlert.pdf
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WBRC , Real UHF channel 50 , virtual number (6.1) FOX.
WVTM-DT , Real VHF channel 13 , virtual number (13.1) NBC.