Question about Cobra 29 LTD CB Radio

6 Answers

Squelch making scratchy noise

When volume is all the way down and i turn squelch up it makes a scratchy noise only at the thresh hold. if i go any further with the squelch the noise goes away. so as soon as the squelch starts to mute. the noise is there and my recive is gone when volume is returned to normal loudness. just the scratchy noise (not static). if i turn the sqelch up the least bit scratchy noise is gone but it takes a very strong signal to brake the squelch. if i turn the squelch down my recive returns with no squelch. i replaced the knob and no change

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  • 13 more comments 
  • a12voltguy May 08, 2009

    i replaced the switch with a brand new one and problem still exists

  • a12voltguy May 08, 2009

    i replaced power/squelch assembley once with a used part and then again with a brand new one from cobra the problem has not changed

  • a12voltguy May 08, 2009

    i am checking to see if you got my last posting about changing power/squelch assembly with first a used one then a new one from cobra.

  • a12voltguy May 08, 2009

    i have changed it twice. do you think all three are bad?

  • a12voltguy May 09, 2009

    i replaced tr 11,12,13 also 33uf cap. no change. before i changed them i noticed that with the squelch set ether at thresh hold or max setting the receive is not understandable it seams the squelch is eating part of the signal and the problem still exists

  • a12voltguy May 09, 2009

    so far you are the only one who knows what i am talking about. if possible i would like for you to assist me till we figure whats going on.

  • a12voltguy May 09, 2009

    i have also noticed no mater how much signal is induced there is no receive as long as squelch is any ware past the thresh hold. just faint sound of signal induced and full receive with squelch off

  • a12voltguy May 10, 2009

    recieve works fine as long as the squelch is any ware form the thresh hold or less. when squelch is at thresh hold or higher no signal comes through no matter how strong. just very faint voices and normal static comes through with full volume. with volume all the way down and squelch at thresh hold a fuzzy noise (not static) is present. if i go past thresh hold no recieve and fuzzy noise is gone.

  • a12voltguy May 10, 2009

    not sure about end code and it dose have delta tune i also am aware of the functions and how they work. so when i turn the squelch just high enough to close the squelch there is a fuzzy noise with the volume all the way down or up does not change volume. turning up the squelch just the slightest bit the noise goes away and so dose my receive if there is a signal coming in no matter how strong the volume has to be all the way up to bearly hear them

  • a12voltguy May 10, 2009

    so when i turn the squelch just high enough to close the squelch there is a fuzzy noise with the volume all the way down or up does not change volume. turning up the squelch just the slightest bit the noise goes away and so dose my receive. if there is a signal coming in no matter how strong the volume has to be all the way up to bearly hear them

  • a12voltguy May 10, 2009

    i an using a dummy load, mic gain all the way down, astron 25 amp power supply and all my problem seems to be in the squelch. are you able to see my previous postings? they will explain every thing

  • a12voltguy May 10, 2009

    i replaced all the parts you listed no change . any other suggestions?

  • a12voltguy May 11, 2009

    tryed it with a battery no change

  • a12voltguy May 11, 2009

    i put the radio on my antenna and that's when i figured no matter how much signal that comes in it wont brake the squelch. even when set at lowest position. but i am able to slightly hear the incoming signal with full volume and receive is back to normal with squelch all the way off and still scratchy at thresh hold. the scratchiness is at the same volume no matter if volume is all the way up or down

  • a12voltguy May 11, 2009

    i changed r 31 47k no change i also checked voltage on tr 11,12,13 and found different readings then the chart you sent here are my results



    tr 11 squelch at max tx e = 0 c = 4.2 b = 0.2 rx e = 0 c = 4.2 b = 0.05



    tr 11 squelch at min tx e = 0 c = 0 b = 0 rx e = 0 c = 0 b = 0.7





    tr 12 squelch at max tx e = 0 c = 0.2 b = 0.07 rx e = 0.66 c = 0.05 b = 0



    tr12 squelch at min tx e = 0 c = 0 b = 0 rx e = 0 c = 0.7 b = 0





    tr13 tx e = 0 c = 0 b = 0.6 rx e = 0 c = 0 b = 0







    i have checked and rechecked there are no errors in my documentation





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6 Answers

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  • Master
  • 5,807 Answers

What antenna are you using? same with the mic and power supply also the coaxial cable this is not the solution yet i just want to know so that i can ***** what is causing your problem.state are you using power mic, what is your power supply capability how many amperes can it load , what type of antenna are you using? a directional antenna or an omnidirectional , is it golden penetrator or an ordinary quarter wave? the coax is it rg-8?

Posted on May 10, 2009

  • Rommel Cana
    Rommel Cana May 11, 2009

    have you tried not using dummy load?

  • Rommel Cana
    Rommel Cana Jun 10, 2009

    if problem still persist maybe the board needs replacement

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  • Master
  • 6,966 Answers

Hi and welcome to Fixya,

It would appear to be electronic noise introduced into the receive audio line after the volume control. Primary suspect would be the actual muting transistor, TR13 (2SC945) which provides the shunting to ground of the RX audio before it enters the audio amp (IC4, TA7222AP). Triggering TR13 is achieved by the DC bias from the TR11 & 12 pair which would likewise have to be checked. Most likely fault of TR13 would be a leak. It is also possible for the 33 uF capacitor connected to its base to likewise have a leak. Since the 2SC945s are relatively inexpensive general purpose transistors, you might as well just replace them with known to be good (without leak)

Pls reference the schematic available here. Would appreciate a postback for updates on any developments.

Good luck and thank you for using FixYa.

Posted on May 09, 2009

  • 1 more comment 
  • Louie  Role
    Louie Role May 11, 2009

    Apologies for the delay in response, different time zone.

    Appreciate your postback and additional information.

    Additional checks:
    check / replace R31 (47K);
    voltage checks on TRs 11, 12 & 13.




  • Louie  Role
    Louie Role May 12, 2009

    Here is the Service Manual while here is a very clear schematic diagram (should you need copies).

    Looks like a 'shotgun approach' required, i.e. check all the components of the squelch line from D6, R125, R44, C30, C147. You could also try removing (temporarily) R35 and note any changes. Pls verify that D6 has no leak. Since the 3 transistors have been replaced and confirmed to be good, resistors and capacitors then to follow.






  • Louie  Role
    Louie Role May 12, 2009

    Oopps, schematics here.

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  • Cobra Master
  • 76,649 Answers

THE SQUELCH CONTROL RHEOSTAT IS DEFECTIVE.

Posted on May 08, 2009

  • yadayada
    yadayada May 10, 2009

    try the rheostat solution, u may be surprised when it fixes it.

  • yadayada
    yadayada May 11, 2009

    REP[LACE THE SQUELCH CONTROL RHEOSTAT AND YOU WILL FIX THE PROBLEM.

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  • Master
  • 413 Answers

Actually the first problem is the knob and second is the switch that is damaged internally so you may replace the switch unit......................if not possible by yourself kindly contact local technician as even the minor hardware injury infiltrates your mood

Posted on May 08, 2009

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  • Master
  • 8,220 Answers

Try it without the power supply attached.
I would be willing to bet that cheap *** mexitronic amp is what the problem is.

Posted on May 10, 2009

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  • Master
  • 11,896 Answers

Sounds like a capacitor is slightly damaged or going out. Though it could be another part of the powersupply system. Also It could be the DC-in board part of the logicboard.

If what you are describing is actually the squelch tail, on repeaters it is a function of the repeater controller which is set at a particular length intentionally. On Amateur Radio repeaters it is common to have fairly long squelch tails to encourage users to allow time between transmissions so that someone with urgent traffic can break into the conversation to use the repeater. On such repeaters you are encouraged to wait for the squelch tail to drop before transmitting. This assures a short delay between transmissions.

Posted on May 10, 2009

  • Nate Stansfield
    Nate Stansfield May 10, 2009

    You shouldn't be hearing any squelch tail on trunked systems.

    do you have "end code" on ignore?
    Do you by chance have Delta Tune?

    CB Controls Technical Service Information

    NOTE: Not all models have the following controls.

    SQUELCH: Squelch controls the audio of the radio, turning it on or
    off. If the squelch is turned down and you are not receiving a signal,
    you will hear background static. This is normal, but can be annoying.
    To avoid hearing the background static, slowly turn up the squelch just
    until the noise stops. This is the normal operating position for the
    squelch. If your radio receives a transmission, the squelch will "open
    up" and you will hear the transmission. When the transmission goes
    away, the squelch will "close" and you will not hear the background
    static. Do not turn the squelch any higher than necessary, otherwise
    your receive range will be reduced.


    RF GAIN: RF Gain controls the receiver sensitivity. Normally you
    would have the RF Gain turned up all the way for maximum sensitivity
    (and maximum range). If the other person is nearby and you are
    receiving a strong signal, you can turn the RF Gain down. This will
    provide very quiet reception.

    DYNAMIKE: This control amplifies your voice to make you sound
    louder. It is like a built-in power mike. Normally you would have this
    control turned all the way up. If you are using a power mike and you
    experience squealing or distortion, try turning down the Dynamike.

    DELTA TUNE: If the other person's CB is a little off-frequency,
    their signal may sound a little distorted. If so, you can adjust the
    Delta Tune to clarify their voice. Delta Tune only affects received
    signals, it has no effect on transmit. Normally you would leave the
    Delta Tune at the 12:00 position.

    S/RF, SWR, CAL SWITCH: The normal operating position is the S/RF
    position. In the S/RF position the meter will show transmit power and
    receive signal strength. The CAL and SWR positions are only for check
    the SWR of the antenna.

    SWR: This is a measure of antenna performance. Some radios have a
    built-in SWR meter to measure the SWR. It tells you how well the
    antenna system is tuned to the CB band. If you want to adjust your
    antenna, you need an SWR meter. A separate document is available about
    SWR.

    AUTOMATIC NOISE LIMITER (ANL): This is a noise filter, to help
    eliminate engine noise. All mobile CBs have an ANL. Most mobile CB
    radios have a full-time ANL, while others have a switch to turn it on
    and off. In almost all vehicles it will be necessary to have it turned
    on.

    NOISE BLANKER (NB): This is an additional noise filter, to provide
    more filtering. If there is no NB switch, then the CB does not have a
    noise blanker. Don't use the noise blanker unless you need to, because
    it slightly lowers the receiver sensitivity.

    ST SWITCH: SoundTracker (ST) is a Cobra exclusive noise reduction
    circuit, to provide quieter reception and stronger transmission. If
    there is a lot of noise, however, the SoundTracker circuit can become
    overwhelmed and will not provide any noise reduction. SoundTracker
    works best with moderate to low noise levels. Additionally, maximum
    benefit is obtained if both radios have SoundTracker. Using
    SoundTracker is optional. Try it on and off, and choose which way you
    like best.

    PA SWITCH: To use PA (Public Address), you need to connect a
    loudspeaker to the PA jack on the rear of the CB. When you talk into
    the microphone, you will be heard over the loudspeaker. Typically you
    would install a weatherproof speaker under the hood. On models with a
    Dynamike control, the Dynamike will control PA volume. On models
    without a Dynamike, PA volume is adjusted with the volume control.
    Please note that on some models you will hear CB reception through the
    PA speaker. This is a feature to allow you to monitor CB traffic while
    out of your vehicle. If you do not want to hear the CB reception, just
    turn the volume control down.


    CH9, CH19 SWITCH: Channel 9 is reserved for emergency use only.
    Channel 19 is the most popular channel, where you can get traffic
    reports, etc. This switch provides instant access to channel 9 or
    channel 19. In order to select other channels, this switch must be in
    the NORMAL position.

    USB/AM/LSB SWITCH: This switch is used on radios equipped with
    single sideband (SSB), which is a special mode of transmission. Use the
    AM position to communicate to regular CBs. Use the USB and LSB
    positions to communicate to other single sideband radios.

    VOICELOCK: This control is only used on single sideband radios. When
    you are switched to USB or LSB, received signals may sound distorted.
    Adjust the Voicelock control to clarify the signal. Voicelock only
    affects received signals, it has no effect on transmit.




  • Nate Stansfield
    Nate Stansfield May 11, 2009

    I am not sure what else to tell you about this situation. You can go to this site for more information;
    http://www.kc2rlm.info/soundcardpacket/5...


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