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

Driver_dave I don't mind at all typing it up. Shoot me an email at [email protected] 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 Driver_dave I don't mind at all typing it up. Shoot me an email at [email protected] 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

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.
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.