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I thought I would pass this experience along in hopes it would help someone else solve a problem with an unlikely cause. My wife was complaining that certain channels were black on her TV in the bedroom but were working on other TV's in the house. I checked and all the bad channels were on the same QAM namely 537 Mhz. Some channels were breaking up while others were just black. About 8 channels were effected. The signal strength was about the same on the bad QAM as on the other good channels, that is about 1 dbmv but the signal to noise ratio was significantly lower. 35 db on a good QAM and about 25 db on all the channels on the bad QAM. At first I thought my wiring could not possibly affect one QAM so I looked at swapping the STB and splitters. No good. The problem was always on the bedroom outlet even after swapping the splitter feeding that outlet and trying a box from a good bedroom. I then ran a length of cable from the splitter directly to the bedroom TV bypassing the in-wall wiring. Problem solved. S/N on the "Bad" QAM was now 35 and the 8 channels were coming in fine. I dreaded replacing the in-wall wiring but that seemed to be the only solution. Then lady luck smiled on me... The problem was caused by the short length of cable running from the wall outlet to the STB. Somehow this piece of wire was acting like a noise injector reducing the S/N on the 537 Mhz QAM 10 db. A new cable here was the cure.
 

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^^^^
Transmission lines can do stuff like that. A shorted quarter wavelength line appears to be an open circuit and an open line, a short. The wavelength of 537 MHz is about 0.53 metres, but the cable will also have a velocity factor of about 0.8, so a quarter wavelength in that cable will be about 0.42 metre. So there's likely a fault in that short cable that resonates at 537 MHz. Your cable is likely around 0.42 M or some multiple of it. (assuming the fault is at a connector)
 
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