Question about Cobra 25 LTD 40-Channels Base CB Radio

1 Answer

Sound problem i have a cobra 25ltd i got it hookes up but there is no sound the mic works on other radios and so does the antennas and coax i have hooked up and external still no sound when i watch the gauge i can see when other ppl key up and i think they can her me just no sound whats so ever please help

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  • driver_dav Feb 02, 2008

    i have done both those and its not new a buddy gave it to me i think that might be why glad i didnt pay but when i have done the second suggestion i can see other ppls get stronger but still no sound

  • driver_dav Feb 02, 2008

    i have done that before to still no sound i can try checking the solders but if that would bother you to post that, it would be great thanks

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First, is the radio new? As long as the mic, antenna, and coax work properly with another cb radio, I would start with two areas:

1. make usre the PA/CB Switch is set to CB. I have seen many people accidentally bump the switch to PA and not know it. I have done it myself.

2. Turn you Squelch all the way counter-clockwise and your RF Gain clockwise.

If these make no difference, please reply with a comment so we can further assist you.

Posted on Feb 02, 2008

  • 1 more comment 
  • Dean Pitts
    Dean Pitts Feb 02, 2008

    Next thing to try is hooking up the external speaker to the PA spot, putting the cb on PA, and see if you get sound from the speaker. Please do that and let me know.



    Next question is, how in depth do you want to troubleshoot this. It could be as simple as a cold solder, a broken foil trace on the pc board, or a broken wire. Or, it could be more in depth. If you want me to, I can type up the troubleshooting section from my text book pertaining to this exact problem, titled:



    "No sound from radio, but S/RF meter moves over normally during RX, indicating signals are present. Mike checks ok."



    You let me know if you want me to type it up, and I will when I get in from work tomorrow. I don't mind at all.



    Dean

    Rollin CBs

  • Dean Pitts
    Dean Pitts Feb 03, 2008

    Driver_dave I don't mind at all typing it up. Shoot me an email at rollincbmods@yahoo.com and put in the subject "Audio problem, please send troubleshooting info". I will have it typed up and saved in a Word document. There is one page with a picture to help kinda of locate the individual parts they are talking about. I will scann that page and send it also.



    Dean

    Rollin CBs

  • Dean Pitts
    Dean Pitts Feb 03, 2008

    Well, I will post it here, but I would like to email it to you also, because there are 2 figures that go with it, and You can't post a full size picture here. I have tried before and it wont work. If you want those figures, they will have to be emailed to you.



    No Sound from Radio, but S/RF meter moves over normally during RX (receive), indicating signals are present. Mike checks ok.





    Other possible symptoms: May hear very weakly from speaker with volume turned all the way up, but no modulation on TX (transmit). Audio transformer may get very hot.





    Make sure the problem is inside the CB first. Often the speaker just burns out. Or the wires to the speaker or the EXT SP/PA SP jack(s) break off. Confirm a bad CB speaker by plugging in an external speaker. If the internal speaker appears to be bad, confirm by unsoldering either of its two wires and touching your ohmmeter to both terminals on the Rx1 scale. A good speaker will make the ohmmeter move most of the way over, and you will also hear a tiny crackling noise when you touch its connections with an ohmmeter.



    If the problem is not in the speaker, the most common cause will be a blown audio output transistor or IC. This shows up as either no sound at all, even though the S/RF meter is moving, or a very weak sound with volume turned wide open. Also you will not hear anything on “PA” position either with an external speaker. And in any case, you will not get transmit modulation; i.e. a “dead carrier”. If the transistors/IC are shorted rather than open (less common) it will cause the large audio transformer to get very hot. (This will be the largest transformer on the PCB (Printed Circuit Board. NOT to be confused with the AC transformer on base CBs. See Figure 25.)



    You can also confirm using the modulation light/dummy load. It will probably light brightly when the CB is keyed, but fail to get brighter when you talk. (Dead carrier” with no modulation.)



    This problem is extremely common, and is exactly like the blown final (final transistor) in the TX section discussed later. Only in this case, just think of it as the audio final amplifier. Since these parts are common to both receive and transmit modes, the symptoms will show up in both modes. And since we are discussing power transistors, remember that they are most likely devices to fail.



    Almost all modern CB radios use either a matched pair of identical audio transistors in the TO-220 case, or an integrated circuit (IC). A few older models, such as the Pace 2300, use a single TO-66 “flying saucer” type transistor, which runs very warm normally. Other older sets will use a matched pair of TO-66 type transistors. Refer back to Figure 22 for pictures of what all these devises look like.



    You can always spot these parts very easily. The matched TO-220 pair will either be on the same piece of aluminum heat sink, or occasionally one side of the main metal case. An IC (which is the equivalent of several transistors inside one physical package) will also be bolted to its own piece of aluminum or a side of the case. The single or double TO-66 types will be bolted directly on one side of the back of the CB case itself. And in all cases, they will be located very close to the audio transformer, which is very large and can’t be missed. (Refer again to Figure 25.)



    Probably 80% of the CB sets made in the last few years, including all 40-channel sets, use the TO-220 type output transistors. (they are cheaper than the TO-66 types; remember how manufacturers think.) The rest of the current breed us IC outputs. Typical TO-220 type transistors in use now will have numbers like D235, C1173, C1096, C1014. Typical ICs are the TA7205P, such as used in many Midlands, Kracos, Hy-Gains, and others. Sharp has another type, the TBA810S-H. (Both these ICs are notorious for blowing out). All of these transistors and ICs are black-colored with white lettering on them. RF finals and drivers are always individual transistors rather than ICs. However audio outputs, voltage regulators, etc. may be either type; in fact, a device in the plastic TO-220 package is often an IC and not a transistor.



    As a footnote, American transistors usually begin with the prefix 2N, while Japanese counterparts start with the prefix 2SA, 2SB, 2SC, or 2SD. HOWEVER, many times the Japanese prefix is not printed on the case. So when we talk about a “C1096”, we really mean a 2SC1096. This is what you would buy.



    If the audio problem happens to be the case of the weak signal with volume turned all the way up (but the s/RF meter registers a strong signal) then only one of the two transistors is burned out. But since they are designed to operate as a matched pair, you should replace BOTH; they’re cheap, and the bad transistor may have weakened the good one. Review the discussion about power transistor replacement before attempting these repairs. You might forget something. And that also applies to the next section.

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You say you are getting no sound, do you mean that you do not even get static?

Here is a couple of things you can do:

1. Make sure the CB/PA switch is in CB
2. Switch the NB/ANL switch to off
3. Turn the squelch all the way counter clockwise
4. Turn the RF Gain all the way clockwise

With all of the settings like this, you should at least get static if the antenna, coax, and coax connector into the radio pc board is good.

You can make sure the antenna is not grounding out to the antenna bracket. Make sure the plastic ferule (usually white or black) is mounted on the top of the bracket, under the coupler with the lip facing down. This is used to insulate the antenna from the bracket. If the antenna itself is grounded out to the vehicle, it will be like you have no antenna at all.

If you look at the picture I attached, you will notice a small white piece on top of the mounting bracket.


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3_6_2012_5_41_29_am.jpg3_6_2012_5_41_55_am.jpg

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i know checks sound basic but they could save you a lot of headaches later

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You probably have got a bad mic, or a break in a wire in the mic cord. With out a properly functioning mic, the radio will not go into receive mode. Thus you will have no sound.

I wouldn't waste money on a new mic for that radio. Instead save a little more money and get a cobra 29. its a much better radio. Also, if you haven't, when you get the cobra 29, read the part about SWR in the manual.

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