I have Comcast cable TV and am using an RF combiner to combine it with the TV2 modulated output form my Dish Network 622 receiver. Previously I was feeding the cable signal from a passive 2-way splitter into the combiner and everything worked ok. I got decent signals from both cable channels and the modulated channel 80. I needed to change my setup a little bit to accomodate another TV, so I added a PCT 4 way amp to my incoming cable signal, which then feeds into the combiner. Now the cable channels look great, but the TV2 modulated channel 80 has a rather snowy picture. Can anyone explain why this is happening? Is the newly amplified cable signal so strong that it is interfering with the modulated channel? If so, could I remedy the problem by strengthening the 622 RF output with an inline amplifier or a 1 port drop amplifier?

I was also beating my head on that one until I talked with a friend who's a Time Warner tech in their NOC.
Guess what starts at CATV channel 80? Yup, the digital channel lineup that TW and Comcast send. What you are seeing is bleed-though of the digital noise on your combined network.

The way out of it is to get a Channel Plus low pass filter that takes out everything above 64 (I think it's the LPF-480) and that should take care of your problem. The way to test it is to leave everything alone and just disconnect (for a second) the input cable from Comcast. If channel 80 magically clears up then that's what it is.

What you've done is amplified the entire cable spectrum with the PCT amp (which BTW is a great amp) which then causes the digital channels to scream onto the cable. Another way to test this is to set your modulator to the lowest or highest possible setting and see if you can clear the digital channels.

The Channel Plus LPF-480 is a nice filter because it blocks everything above 64 but it allows 96-99 as cable providers put special programming on their like the Big 10 network, etc.

That's because 96-99 are lower frequencies (somewhere around ch14), It's also why the modulator on DISH receivers don't offer those channels.

Duhh,,,, you're right of course. I had a brain cramp.

91-115mhz for those channels. Thanks.

I looked at that, but won't it also filter out my digital locals? I would like to be able to pass both the analog cable channels as well as the in-the-clear digital channels. How well do the notch filters work? I have read some reviews about the filtering bleeding into channels outside the notch range, which could also cause me problems.

It depends where in the network you put the filter. Are your digital locals coming from an OTA Antenna or Comcast?
If OTA then don't worry because they are on UHF frequencies. You'd put the filter on the Comcast entry point.

If they are being sent via Comcast then you'd have to find out just what channels Comcast is sending them on. But considering that before you amplified everything was working fine, I think you'd be Ok.

Are you combining UHF OTA with the CATV cable? Your original post didn't say anything about how you feed the OTA free-to-air digital locals.

Channel 80 CATV is at 559.2500-563.7500 mhz. The closest UHF OTA are channels 28 and 29 (not sure of your local digital mapdown) at 555.2500-565.7500 mhz (spans the channel 80 CATV signal. If no one is broadcasting on those 2 UHF channels, you should again be ok.

The digital locals are coming from Comcast. I'm not sure where all of them are located, but I do know there are some on 81, 116, and 117. I think the Channel Plus NF-471 would do the trick, but I'm afraid that is is going to bleed over into channels 74 and 81, which could cause me to lose the Big Ten Network on 74 and my digital channels on 81.

You probably considered this, but did you just think about changing the channel 80 modulated channel to something like 76 or 77?

Yeah, I tried some other channels and still get an equally snowy picture.

Sorry, I should have asked this before.

Would you consider the picture 'snowy' or a 'moving diagonal crosshatch pattern'.

If the 1st 'snowy' then a small amp in the 80 line might help or equalize the other channels (lower them a bit with a 3 or 6db attenuator before you combine and then amplify everything). Just make sure that you don't overboost the 80 line.

Snowy generally means low signal. It could also be something simple like either a lose connector, RG59 instead of RG6 from your downlink (80) line or a bad barrel or wall connector causing a 3db drop. Remember, the rule of thumb is that for every 2.5db you either double or half the signal depending of you are amplifying or splitting. Also, please, please don't tell me that you use any 'push-on' connectors anywhere in your coax network and all connected but un-plugged jacks or coax is terminated by a 75 ohm terminating plug.

If a moving crosshatch pattern that generally is what I'm referring to. If a moving crosshatch then there is something else competing on the frequency.

In my network I have a 10db ACT amp but to equalize it's output it inserted a 3db attenuator and a DC block (passive) after the amp to get about a 6.5 db boost . I was lucky to have the cable techs give me a baseline of all my inputs, so I marked them and then use labels at each amp or splitter to tabulate expected gain or drop. I.E. I put my normalized combined signals at +6db into a 15db cable vision amp (21db). That goes into a 3 way splitter (5db each leg = 16db/leg). 2 legs go to 2, 4-way splitters (7db loss each leg) for 9db to the 1st floor feeds.

The 3rd leg of the 3way splitter then goes to another 15 db amp (for 30db) and then feeds 2 more 4 way splitters for 21db to feed all my very long runs to the upper floors, porch, etc. I can then put splitters at the terminating ends without worrying about loss of signal. If it's only 1 connection, I have a cache of 3,6,10,15,20 and 30db attenuators from Worthington Distribution, that I use to tweak the final signal delivered to the TV's, slingboxes or etc.

Just a warning though, if you are doing mega-boosting as I am, please use quad shield RG-6 and high quality compression fittings to prevent bleed out and becoming your own TV station! The cable techs offered to even give me 50 feet of 'mainline' but I just couldn't figure out how to run the darn thing or even bend it!