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Reflect and audio.?????

Discussion in 'General CB Services Discussion' started by 1iwilly, Sep 4, 2010.

  1. 1iwilly

    1iwilly Active Member

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    i know whe you check reflect or swr that there is no audio applied but i like to check it with audio to see what is doing here is my scenario.cobra 29 classic
    5 watts of reflect on 600 dead key when i applied audio it goes up to 8 watts. now with my 148 gtl same reflect and dead key when i add audio on a 25 watt slug it goes up to 20 to 25 watts. i check with another 148 gtl it goes up too.not as much but to 15 watts.



    2nd question my theory is if it swings up does that mean my signal is going forward if it goes down so should the signal no.?? but here is the thing even though the reflect swings down when you modulate the power is actually going forward.seen it on the bird meter.
    which is the right scenario.????:confused::confused:
     

  2. GnG8d

    GnG8d Well-Known Member

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    What you really need to do is to buy an antenna analyzer. Set your antenna, and THEN if you want to make your SWR meter happy with coax length, you can do so knowing that your antenna is resonant, no matter what your amp is doing.
     
  3. Beetle

    Beetle Sr. Member

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    The SWR is NOT changing. The reflected power is changing, but the RATIO of forward to reflected power stays the same.

    It's a RATIO.

    359 gave sound advice.
     
  4. Shockwave

    Shockwave Sr. Member

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    It seems impossible that an increase in forward power could cause a decrease in reflected power. Until you consider it's a solid state CB amp. Since VSWR is a ratio between forward and reflected power, more forward has to equal more reflected. This is true except when the amplifier is not running stable especially at low drive. When the amp is not perfectly stable and run at low drive, this allows more opportunity for undesired signals to be generated within the amp. This can be seen on the spectrum analyzer. Reduce the drive and more spurs will pop up. Increase the drive and they go away (for the most part).

    Transistor amps don't have the benefit the isolation of a grounded grid offers. This means it's easier for changes on one side of the amp to be seen on the other. For example changing the radio or drive level changes the amps output VSWR. You'll see it's not just audio that changes the reflect. You can use no audio and vary the carrier through it's range and see the same thing. Most common causes for this are, missing negative feedback, missing tuned input circuit, and poor grounds on the board particularly around the transistors emitter connections.

    Your problem is not as severe as some cases I've seen where the reflect goes right off the scale as the power is reduced below a certain point. Knowing who built your amp I would suspect he's got the grounds and negative feedback correct. However, just like every other amp I've seen him build he never installs a tuned input circuit. I spoke with him years ago about this problem and he got an attitude with me saying the problem was the variable carrier on the radios and not his amps. Wrong. Look at the bright side Jose, at least you didn't get the special transistors where he covers over the non matching numbers with black magic marker.
     
  5. 1iwilly

    1iwilly Active Member

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    so you are saying that if i add a 464 trimmer to the input it should fix half of the problems lok at the picture and let me know what need to be change i'll fix it my self as long as i know what needs to be change. here is pictures of the driver section. the input cap to ground is 270 puff. the input transformer has 2 turns rap and the output trans has 2 turns rap. i was told the input should have 3 or 4 turns.?? on the output trans cap going to ground is 180 puff. the tunning section has 2 10ohm rresistors and a 120 cap going to ground. i also mincludeda picture of the 8 pill output section the door nob i beleive is 80 puff any help or advice and guidance would be appreciated.:D:D
     

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  6. W5LZ

    W5LZ Crotchety Old Bastard

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    Beetle - Sssshhhhhh, don't tell'em that!
    They're talking about actually trying to make those "no-tune" input/output tunable, or closer to 50 ohms. What'a concept!
    Of course, using a varying level audio input to do an SWR check isn't going to work without using a SWR meter that will automatically compensate for that varying level, but worry about that later. Let'em make that a tunable amplifier first! They'll show all them (us) old farts how it ought'a be done.
    - 'Doc
     
  7. Shockwave

    Shockwave Sr. Member

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    We have to remember he is not using an SWR meter that would require calibration according to forward power. He is using the Bird to measure actual reflected power. The ratio of reflected power to forward power is not staying the same. In this case as the amp is driven harder the forward power goes up as the reflected power and VSWR go down. I know that sounds unusual but it's not so uncommon with transistorized CB amps.

    Willy, changing a fixed value cap to a variable does not make a tuned circuit. It just gives you the ability to adjust the padding. By adding a resonant circuit with a coil we can match the impedance while providing attenuation of unwanted frequencies and adding some isolation between stages. One simple tuned circuit that works well here is the PI network.

    Remove the existing brown cap across the input. Break the coax from the red input wire. Feed the drive from the coax trough a coil made from 4 or 5 turns around the Crazy Glue bottle coil form. Connect the other end of the coil to the red input wire. Add two 464 variable caps. One on each side of the coil to ground. Adjust the caps and compression of the coil to provide the perfect input VSWR. This should help reduce the effects that different drive conditions have on the output reflected power.
     
  8. 1iwilly

    1iwilly Active Member

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    what size wire for the coil.?? also you know they are doubting you on these replies. what about the number of raps on the input trans for the 2 pill section? remind you this is a 2 x 8 amp.:bdh:
     
  9. Shockwave

    Shockwave Sr. Member

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    The size of the wire is based on the power it will have to handle and the Q you want the tuned circuit to have. Both power capacity and Q are increased with wire size. Q relates to the bandwidth the circuit will pass. The higher the Q, the lower the bandwidth. In most cases we would use a thin wire perhaps #16 or #18 enameled. This is due to the fact we only have perhaps 20 watts of drive and most bands are wider then 11 meters so more bandwidth on an input circuit would be desirable (lower Q).

    The problem is we are looking to knock out the likelihood of undesired frequency content that may be the result of oscillations within the amp at low drive. In this case we want a higher Q. Even though it only has to handle low power, a # 12 or #14 wire would be beneficial to increase the Q. I've had to repair this issue with his amps before and that circuit worked. If the problem is what I suspect, you can turn the carrier down lower and find spots where even more reflected power is shown on the output. If it gets real bad the relay may even hang.

    With respect to peoples doubts, there was a time when people doubted our ability to go to the moon. Once it was done the doubts stopped. Let enough time go by without people being able to see it in front of their eyes and the doubts begin to surface again. It's human nature to want to see something to believe it. Why do you think it was so hard to keep customers out of the back room all the time and so hard to make a living doing it? The truth is I could be wrong. This may not work for you. It's simply an educated diagnosis based on the symptoms from 2000 miles away.

    It's also been too long since I've wrapped these transformers to remember the specific turns ratio for a given stage. I do remember that 4 turns worked well in many cases with 2SC2879's in the finals. This is dependant on several things. Including the impedance of the transistor used, the value of the cap across the transformers, the voltage being used, and the number of transistors in the stage. Two "pills" use fewer wraps then 8. Again, knowing who built it I can say he probably got the turns ratio right so that it runs fairly efficient. Stability is another issue.
     
    #9 Shockwave, Sep 4, 2010
    Last edited: Sep 5, 2010
  10. 1iwilly

    1iwilly Active Member

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    we i have to buy 2 more 2879 hopfully this will fix the problem cause i know a 2 x 8 on 14.50 volts suppose to do more that 600 watts max. maybe i need to get your buddy pustie malaka to fix it .i have never seen anybody like him
    takes your antenna tune to your vehicle. somebodies else coax. a half ass amp and lock channel 6 down like he does. i garrantee more tell you.
    thanks for the replies.:bdh::bdh:
     
  11. GnG8d

    GnG8d Well-Known Member

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    I'm pretty sure that I've said that more than once and on more than one forum.

    Buy an antenna analyzer like the 259b, set your antenna and THEN work on system swr or feedback.

    Beyond that, you are fighting two separate battles at the same time.
     
  12. 1iwilly

    1iwilly Active Member

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    thanks for the advice and replies but i can assure you it is not an antenna issue you can ask shockwave when i was up in ct i caught him out there while i was tunning and he seem pretty amase on the rejection on my astro van. running 2 antennas even with the amp on it had good rejection and we were like 20 to 25 miles away.i trust more my bird before i trust the mfj all is good to me for is to see how close to 50 ohms i am.
    with 5 watts of reflect on 600 to 800 forward is vswr of 1.2 mismatch of 0.04db.
    return loss is -20.8. reflection coefficient of 0.09
     

: cb setup, swr

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