My HD Local channels ABC, NBC and CBS have a very weak signal, especially NBC and ABC, where I occasionally lose the signal completely. This is frustrating because these are the three major networks and the ones I most like to watch. I have 80+ channels and have no problems with the rest of the channel lineup. I don't use a box and receive my signal directly through the Qam tuner. Zip code is 21117
Turns out it was the inside wiring. The signal coming in from the outside was indeed strong and after perusing the channels more thoroughly, I noticed it wasn't just the local HD versions of the major networks that were weak, although they were the only ones that seemed to go out for some reason.
The cable goes through 4 splitters before it reaches my TV. I was able to eliminate one of them and replace another, which was a 3-way, with a 2-way and now the signal is much stronger.
Yes, only certain channels can be affected by a wiring fault, poor connector, long or short center conductor, water in the connector or coax. You would be amazed what you can see across the complete band of channels with a spectrum analyzer or hi-level SLM, and the resulting signal variations.
It could be as simple as a shorted braid touching a center conductor, any f connector issues, squirrel chewing on the drop, or other reason. All it takes is one very little issue, bad jumper cable or splitter to cause it, and it does happen in only certain parts of the band of frequencies.
If you have limited basic, or another tier that has a trap on the drop, the trap could be causing it also.
If you want to attempt any do-it-yourself troubleshooting, you can try and eliminate any inside wiring issues, by bypassing the inside wiring at the ground block, or call and have a tech find the exact issue with his test equipment. Check you inside f connectors, jumpers, splitters for any visual issues.
Bad wiring would not explain why only the HD locals are affected. At least I don't think. Are the signals delivered from local major networks notoriously weaker than the rest of the channels? All of the channels show 2 or 3 bars out of three. NBC, CBS and ABC show none and go out frequently.
Since your don't have a Comcast supplied set top box, it would be harder to troubleshoot.
Basically, you will need to determine if the signal issues are outside and Comcast's issue, which they will fix for free, or inside wiring issues which Comcast will charge for repairing, or you can attempt to fix it yourself.
If you want to attempt troubleshooting yourself, then, check all your F connectors, for looseness, poor attachment to the coax. Make sure all F connnectors are snug at each device and splitters. Check for damage on all sections of coax. Best thing for a do-it-yourselfer is to acquire a 100' roll of RG6 coax, connect it to the ground block on the side of your house, and connect directly to the TV. This will eliminate any inside wiring issues. If you sitll have trouble, then call Comcast for a service call.
Otherwise, call Comcast and the tech will determine if it is inside or outside, and he fix any issues, but if it is inside, they might charge you.