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DTB "IF Filter Mod" for RCI 29xx radios?

Discussion in 'CB and Export Equipment and Accessories' started by buickid, May 19, 2013.

  1. Mudfoot

    Mudfoot Elmer

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    Exactly, which is why no shops are posting videos. They just stroke your back with fancy descriptions.


     
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  2. The Mad Scientist

    The Mad Scientist W9WDX Amateur Radio Club Member

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    M42duster it isn't just that they are just pulling your chain...That modification was intended to cure an entirely DIFFERENT problem that no one including the manufacturer could reproduce.

    I have to say again as I have said before Image rejection and a lower noise floor are two completely different things.

    Someone should post a complete explanation on a sticky and nail it to the top of this section so people can educate themselves as this seems to come up every so often.....:bdh:

    Nevertheless, I am curious as to the perception of whatever when the radio comes back.
     
  3. buickid

    buickid Active Member

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    Mad Scientist: So what is this modification intended to resolve? It must do something, no?
     
  4. The Mad Scientist

    The Mad Scientist W9WDX Amateur Radio Club Member

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    :blink::blink::unsure::sad:

    I think you should go back and re-read this thread. It has been mentioned more than several times.

    *******IMAGE REJECTION********
     
  5. buickid

    buickid Active Member

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    Sure, I read that, no need to get aggressive. Hell I started this thread, I have every motivation to read the posts. But what does that mean to me? I know what white noise is, but how exactly does image rejection affect me? Sorry that I'm not an RF engineer. I tried to do some reading about image rejection, and understand what it "does" (passes only the secondary frequency that we want to create the heterodyning, since another frequency a similar distance away from the primary will also create the heterodyne on that frequency, to put it in a very non technical manner). Thats all quite plain to see, I have a pretty fair understanding of undergraduate level physics. But I don't quite grasp what that means in RF design, as an end result of going through all the rest of the radio's circuitry, coming out of the speaker. Surely it must have some noticeable effect on the receive, since its got a supposed solution (this mod). I understand its not the noise that it affects, but perhaps something else, like tone?

    Thanks for taking the time to explain,
    Trevor
     
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  6. AudioShockwav

    AudioShockwav Extraterrestrial Admin
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    It was intended to cure a "problem" that popped up during a test of the radio. I would guess the the open traces under the filter were capacitive coupling and allowing some unwanted RF to get past the filter.
    Ranger could not reproduce the same problem.


    The Full review can be read here:
    Review: RCI 2950DX Dual Band Amateur Radio

    73
    Jeff
     
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  7. buickid

    buickid Active Member

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    So as I understand it, this only becomes a problem if there are very strong signals on adjacent frequencies.
     
  8. The Mad Scientist

    The Mad Scientist W9WDX Amateur Radio Club Member

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    Yes you are correct, But acording to most the problem cannot be reproduced. I tried this modification almost ten years ago with no decernable difference. Thanks for posting a link to that thread Jeff, It explains it better than I could.

    I must apoligize as I didn't mean to sound agressive, I didn't know that you tried to look into it. I guess I got so used to most people wanting to be spoonfed information rather than looking into it themselves. And I hate to see you waste your hard earned money on something that wont benefit you at all.

    73's and good luck

    Eric
     
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  9. buickid

    buickid Active Member

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    No worries Eric, I understand the frustration (I work part-time at a RadioShack... some people shouldn't be allowed near technology). :headbang I am a very inquisitive person, and try to read and understand as much as I can, and seek outside help only when I've done everything reasonably possible. Thanks for you help, I appreciate it.

    Anyway, plot twist, Dave has offered to do the modification at no charge, so I'm only out the cost of shipping. No biggy, I'll do it in the name of science for WWDX! :D I'll be making that video when I get a chance, and get it sent out.
     
  10. loosecannon

    loosecannon Sr. Member

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    this is an interesting thread.
    not sure how i missed it the first time around.

    Mad Scientist has it right.
    this IF mod is not about the noise floor, and wont make any difference in the sound of the receive with no signals present.

    bascially, its like having two small antennas hanging off the bottom of the IF filter, causing some of the trash that "may" be present to be coupled through.

    if you had someone close by, or with a really strong DX signal on a harmonically related freq. you might get some additional noise from them.

    maybe a ham close by on 30 meters? :)

    as for the noise floor issue, after discussing it with a couple of people who know way more about it than i ever will; the conclusion is that its because of a crappily designed local oscillator in the PLL section, and the only fix would be redesign the whole thing.
    IE: not going to happen.

    there is a way to make it less noticeable, and anyone who tunes up a galaxy/rci/ whatever style radio should know this.

    its called the "antenna coil" and in the 2970DX its L8.
    in other radios it may be a different number.

    if this coil is tuned for max like you would any other coil during a receive alignment, you will have a high noise floor.

    the fix is to do the RX alignment, peak L8 like normal, then remove the sig gen, and anything attached to the antenna jack on the radio.

    now give L8 a bit of a turn clockwise and the noise should drop right off.
    turn it just enough to drop the noise a bit, and you will notice that you have only lost a tiny bit of sensitivity. (not enough to make a difference IMO)

    thats how i do it, and it was shown to me by a Galaxy service tech.
    LC
     
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  11. The Mad Scientist

    The Mad Scientist W9WDX Amateur Radio Club Member

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    Thanks Loosecannon, This particular modification has come up alot on multible forums over the years.

    I would also like to add this to your perfect explanation,

    Before you look to adjust the antenna coil or L8 as listed on some radios, Please check BOTH VCO voltages Where the RCI2950DX is concerned. They are supposed to be set at 1v + or - .1

    I have seen so many shops jack these voltages way up as a cheap and quick means of increasing the RX and causes an extreme amount of "white noise" as a result. Some shops still practice this leading some to beleive it is just part of the radio design. The RCI2950DX does have a small amount of white noise and is due to the higher noise rating of the SMD components used in manufacture and in my experence isn't worth writing home about.

    I hope this helps as well.
     
  12. loosecannon

    loosecannon Sr. Member

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    mad sci,

    i saw that you mentioned that previously, and i forgot to comment on it.

    ive not heard about this before, but it does make sense.

    ive never run into that before, but i just set them the way the service manual tells me to.
    im just funny that way i guess.

    thanks for the info.
    LC
     
  13. Mudfoot

    Mudfoot Elmer

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    Any updates?
     
  14. buickid

    buickid Active Member

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    It's on my list, little time, little money.
     
  15. Sarcasm

    Sarcasm New Member

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    So you bought a very nice expensive radio and it was never opened for the mods. I thought all his new radios came with an alignment in this price range. And being in Hawaii that's got to be killer for shipping. After reading the experts opinions did you end up sending it out yet? Or refund?
     

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