My AM21 Odyssey - Problems and (finally) a solution.

Discussion in 'DIRECTV Installation/MDU Discussion' started by pogo, Apr 1, 2015.

  1. pogo

    pogo AllStar

    Oct 31, 2007
    From the first day I installed an AM21 the local CBS affiliate was flaky, seemed okay sometimes and then would break up so badly as to be unwatchable. I never really pursued the cause. A month ago I accidentally bought a second AM21 (Put a bottom fishing bid on it and won). It arrived and I set it up on the HR34. It became immediately clear that both the CBS and NBC affiliates would be unwatchable. Now I’m getting serious about figuring it out. Did a little reading and found the edit channels screen in the Antenna Setup menu. First box showed duplicate entries for CBS affiliate and all of its sub channels. Second box the same but it added duplicate entries for the the NBC affiliate.
    In both cases one of the channels was very strong and the other quite weak (around 20%)

    Head scratching time. Multipath? But I’m line of sight to the tower on a mountaintop 10 miles north of me. If you’ve got good eyes you can literally see them from my roof on a clear day.

    My setup includes a four channel ntsc modulator whose output is combined with the antenna at an eight port distribution amp. Although I was careful to make sure the modulator used vacant channels, I first decided it must be causing the problem. Disconnected the first AM21 from the distribution amp and plugged the antenna directly into it. Reran Antenna setup and …. Lo and behold now the first box had duplicate listing for both networks! Progress of a sort since I like it that both boxes behave the same.

    More head scratching. Finally asked myself what happened to the “translator” stations these guys used to run before they went digital? A visit to antenna confirmed that not only are they still active but they also went digital and carry the same digital channel number as the main station.

    So now I know my topography is the channels I want to receive are atop a mountain a 9 degrees 10 miles. The translators are atop another mountain at 257 degrees 16 miles. The antenna is low VHF/uhf combo. (The local ABC affiliate chose to stay on RF Channel 9 when they went digital.) The side lobes of the VHF component were apparently strong enough to pick up channels 4 and 13, 16 miles west of here. What to do. Finally dug around in the shop and found a small bow tie with a screen type reflector. Hooke it up in the house to the the AM21 and aimed it roughly opposite 257 degrees. Ran setup again, and sure enough the duplicates disappeared from the list. Reconnected the boxes to the rooftop antenna and all is well with only one channel in the listing with max signal.

    I’m posting in the hopes that someday somewhere someone will find this and it will save them from tearing their hair out and throwing a perfectly good AM21 in the trash. Otherwise, at least I got to tell the story of how I finally got something right. :grin:

    IMPORTANT EDIT: As NR4P's reply pointed out It is important that you reset the OTA settings before you rerun antenna setup -- this will cause the channels to be rescanned.
    2 people like this.
  2. slice1900

    slice1900 Well-Known Member

    Feb 14, 2013
    Did you have the same problems when you plugged the antenna into your TV's RF port? The AM21 is a fairly old design as ATSC tuners go, so compared to a TV made in the last few years it is probably much worse at rejecting multipath (or in your case two identical signals coming from different directions)
  3. NR4P

    NR4P Dad

    Jan 15, 2007
    Sunny Florida
    You correctly identified that the modulator was a source of your problem. Looking at the specs on line it has tremendous output, was designed to support cable runs up to 1200' and has a 10db noise spec. Ouch! So you have this thing after your distribution amp with unknown noise coming out of that too.

    By removing the modulator temporarily you did the right thing. But at that point before connecting only the coax to the AM21 it would have been better to wipe the AM21 and HR DVR clean of all OTA readings. Reset all OTA settings and power cycling the AM21 would have helped eliminate duplicate channels.

    Granted the AM21 is not a great device but it does require clean signals, free of considerable multipath and noise introduced by amps and modulators.
  4. pogo

    pogo AllStar

    Oct 31, 2007
    Yes, see my OP. But to be clear this is not a multipath rejection issue. The tuner did it's job and scanned for and found the channels in the list. These two channels aren't just from two different directions but on different RF channels (different bands actually).
  5. pogo

    pogo AllStar

    Oct 31, 2007
    Good catch on the reset. When I said Reran Antenna setup that included first reseting OTA settings. Otherwise I don't think the tuner will rescan for channels.

    Beyond that I think you've confused my setup with someone else's and the problem ultimately had nothing to do with my modulator which does not have a temendous output and isn't inserted after the distribution amp.

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