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Stock Cobra 2000GTL "air noise" on TX

Discussion in 'General CB Services Discussion' started by Crazy_D, Sep 26, 2007.

  1. Crazy_D

    Crazy_D Member

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    Got a stock Cobra 2000gtl. When deadkeying, (and the mic gain all the way down, or at any setting for that matter) I get a air noise or whooshing noise that makes me sound like I am in the mobile or something. I tried 3 different mics (and with the Mic Gain all the way off), and concluded that the problem is either in the input mic preamp circuitry, balance modulator area, or something into the AF audio chip area.

    Also, accompaning this whoosh noise (kinda sounds like light white noise).... the deadkey wiggles up and down alittle, (it is set at about 4watts). Also, I see a small movement on the mod meter when the mic gain is all the way down.

    My thoughts is either:

    A) Bad cap somewhere (probably a disc cap)
    B) something needs to be realigned or such
    C) Maybe audio chip needs replacing or some other transistor


    Any thought please let me know, and I will be willing to try to fix this nuisance.



    Thanks
     
    #1
  2. bushwacker

    bushwacker Active Member

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    Hows the power supply?
    Got any voltage fluctuations?
    60 cycle hum?

    If I were to guess, without any more background info,... I would say the problem could be in the power supply.

    You have what kinda sounds like a 60 cycle humm coming into the radio. That and you also have deadkey fluctuations which could again be related to fluctuations in voltage from the power supply.

    If you have another external power supply around that you know is good... power up the mainboard/radio with that and see if you have any noise.

    Rule out the power supply first so at least you know you are not dealing with a bad power supply . It might not be the power supply... but it definatly could be so you have to rule it out.

    Take care

    60 cycle humm with voltage fluctuations is common for older power supplies
     
    #2
  3. Crazy_D

    Crazy_D Member

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    Hi,

    I checked the internal power supply, and also, I connected the 2000gtl to a Astron VS-70m and powered the radio by that and the noise was still there.

    I can rule out a problem with the internal power supply.

    Thanks!
     
    #3
  4. bushwacker

    bushwacker Active Member

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    OK cool..... now rule out the audio IC and the mic

    Get the radio to TX without a mic and check if its still there.

    Remove the power to the audio IC and check the same.
     
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  5. Crazy_D

    Crazy_D Member

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    Hi,

    I have the radio powered off the Astron VS-70M for testing purposes.

    I now key the radio with a 5pin mic plug jumpered to make it key when I stick it in the mic plug.

    I still have the noise in this configuation.
     
    #5
  6. bushwacker

    bushwacker Active Member

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    OK.... you will have 1 pin on the audio ic that is the audio in... .... unsolder that leg of the audio IC... key the mic and see if you have any noise... if you do the noise is coming in through AFTER the audio IC

    Do you have this noise when recieving in AM? or is it just transmit audio?

    If its just transmit...check the input and output of the audio IC.
    If you can recieve good without this noise, I would think the problem is bettween the mic input and audio ic OR audio ic and rf amp.

    I assume you checked for cold solder joints and loose connections... check them all in the audio input /output of that IC.

    I wish I could hear what it sounds like.

    Oh well... its hard to help people over the net.... my advice is just work through it in stages.... once you find what area or stage the noise is coming from you can concentrate on it.
     
    #6
  7. Crazy_D

    Crazy_D Member

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    Hi,

    I will try this tonight. (lifting pin1 of audio IC chip to turn it off)

    This noise is ONLY on AM transmit only. SSB is clean. Also RX is clean, so this noise has got to be in the audio chip area, as looking at the schematics, the TA7222 chip is only for AM audio, and the SSB audio is diverted into the AN612 balanced moduator chip via TR21 (2sc945) AM audio switch.

    I know what you are wanting me to do, is turning off the audio chip circuit by removing the 13.8v going into it. Once thats turned off, and if the noise goes away, then we know its either the Audio chip or associated circuitry. If this "air" noise still persists then we would need to look towards the back end of the radio. But since it only happens in AM mode, and the "mode switch" is before the final output stage, it MUST be in the AM audio chip area.... because if the noise was also on SSB, then we could rule in the output section....

    Will let you know more later tonight.....

    I think we are onto something here....
     
    #7
  8. TonyV225

    TonyV225 Supporting Member

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    If not the audio IC if it not that look at the Capacitors ithat general area I would say a leaky Cap in the audio section.
     
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  9. Crazy_D

    Crazy_D Member

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    Will do. I will change out all those caps, as well as all the other 10v caps and anything else I can inside the radio to newer and better caps.
     
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  10. bushwacker

    bushwacker Active Member

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    The audio IC should be OK.... that radio uses the same audio IC for recieve audio that it does for AM audio. So if your getting good RX audio... the line from the rx'er to the audio IC to the speaker is good and has no noise problem. This means the problem is JUST when the audio IC is being used for TX audio. This is why you need to check which side of the audio IC the noise is. I would think that if the noise was in the rf amp it would also be there with ssb transmissions... but since its not we have to look at components that are used JUST for TX audio on AM.

    Yea your on the right path... on alot of radios there are usually 2 caps at the output.... usually its something like a 470uf and 1000uf. It depends on how the output of the audio IC is on your radio ( I can't remember and don't have time to look).
    I'm not saying the problem is there... but it could be. Your on the right path anyways..... If its a bad cap it would most likly be an electrolitic or tantalum .... if there is any tantalums in that area I would look at those..... I've had alot of problems with tantalums in the past.

    I would first try removing just the audio input from the IC.... If you remove the "audio in" right at the audio IC, you will be able to check if the noise is still there or not.
    If its still there, then the noise is coming in after the audio IC. If the noise stops , then the noise is between the audio IC and mic.
    You just need to figure out IF the noise is in this section ( and I kinda think it is since the noise is only there in AM mode)... once you do you need target which side of the audio IC the crap is coming in on..... the input or output side.... then you have a smaller area to track down the problem.

    I think you know what your doing though.... I would remove the audio in/out pins on the ic insted of removing the voltage to it. Try it either way if you want...whatever is good for you.

    Take care.
     
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  11. Crazy_D

    Crazy_D Member

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    I know what I am doing, I am a completely competent technician, I just dont work much on CB gear.

    Ok, lifted pin 1. Noise stopped!

    So we know that its either Audio chip area, or beforehand mic preamp circuit or such.

    OK NEW DEVELOPMENTS!

    Removed C110 and C46 (both caps carry mic audio to the audio chip input), and the noise is gone. (so is my audio obviously).
    The problem is now isolated in the mic preamp circuit.

    I am isolating and checking that now....
     
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  12. Crazy_D

    Crazy_D Member

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    OK Isolated More and more preamp circuits....

    and I found the problem!

    I isolated TR22 and circuit, no noise.... ok moved toward the front....

    so isolated the next circuit, which is TR23 and parts (by lifting R124 out of the circuit path).... and voila ! Noise!

    Saw that TR23 is a 2SC458PGC part....

    Looked at a Cobra 148gtl schematic (which is same) and saw they use a common 2SC945AQ there for TR23.

    I replaced TR23 with a spare 2SC945AQ I had laying around....(and then reconnected R124).... and NO MORE NOISE.

    NOTHING BUT CLEAN AUDIO, NO MORE DEAD KEY FLUCTUATING OR THAT AIR NOISE IN THE SIGNAL...CLEAN...CLEAN...CLEAN.

    So if anyone ever has this problem in their Cobra 2000GTL, replace TR23 with a 2SC945AQ and your good to go! (and as well as change all the other caps in the radio too)
     
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  13. Crazy_D

    Crazy_D Member

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    Thanks Bushwacker for the extra motivation today, as I was going to not do this for awhile but now I am screaming on the air!
     
    #13
  14. bushwacker

    bushwacker Active Member

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    Cool.....945 transistor.... yea theres lots of those little guys used in radios. Dam little switching transistor giving you grief LOL.

    I didn't do to much to help you out.....you did all the work and tracking.
    Take care
     
    #14

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