Question about Cobra 25 LTD 40-Channels Base CB Radio

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Cobra 25 no receive & no modulation

I have a Cobra 25 that doesn't have any receive nor any modulation. It will dead 5 watts. No swing. Could some one help on this. Thanks

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  • forty niner Nov 03, 2008

    Hello mechanic. I wasn't real clear when I posted a problem with my Cobra 25. The radio doesn't have any swing on the meter or any sound from the speaker. It will key up 5 watts but will not modulate any at all. Thanks 49'er

  • 93mustanglx Mar 24, 2009

    same model same problem. it dead keys 5watts and it receives and shows signal but no sound.i ran my fingers over the sound chip as described and suddenly i get sound for about 30 seconds and then is fades out to nothing. how much is this chip?i can replace it if i know where to get it if its cost effective, but wonder what the cause of this problem really is or if theres other factors. i have the cobra 25 NW LTD CLASSIC



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Hello 49er, When you say "no receive", do you mean no sound, or no signal on the meter at all??I'm just grabbing at straws here so bear with me as there can be different problems doing the same thing.If this radio has been tightened up in a bracket, sometimes that will pull the solder legs loose on the audio chip. That could take away the mod. and RX volume. Is the Microphone ok? If the radio has been in storage, the switches and knobs could have dust and dirt in them. How much troubleshooting are you comfortable with?? If your not, then you might need to take it to a good CB Radio Repair shop to have it serviced. That's what I used to do for over 10 years,,, Let me know how your coming along,,, Mechanic

Posted on Oct 31, 2008

  • Mechanic688
    Mechanic688 Nov 05, 2008

    49er,, Ok the modulation chip and area right around there controls the incoming audio( meter swing) and also the moduation during keyup. The chip can blow (or other parts) when the radio has been hooked up backwards. The mod chip is located (take the top lid off) on the left edge two thirds of the way back. It has about 9 legs that are soldered to the solder side of the board. The chip is located along the edge and is heat sinked to the metal side rail. Sometimes you can take your finger and touch around on the board with the radio lit up and hooked to an antenna. If everything is ok you should hear some squealing/screeching going on. Once in a while I have found a bad componant doing the finger check thing because of heat.I'll come across a hot spot. There is quite a few different things that could cause what your seeing, It's tough on my end not being able to have it in my hands, it would only take me 15/20 min's to diagnose it. All Well,,, Mike



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Yes it can, but it requires the addition of some components. It isn't really a straight forward install. It needs some reworking. In my opinion, not worth the trouble.

Sounds like you want to run a linear amplifier. If you really want to run an amp, get a better radio to do it. There really is no other reason to lower the dead key that low. Also, 1 watt might be too low. May not drive the amp well.

If you just want to make the radio swing, 1 watt is way way too low. You will have barely any carrier and the radio will sound like ****. That radio stock dead keys 2.5 watts. That's perfect for swing. Now you just need to have more modulation. Find DM1 and unsolder 1 leg and pull it out of the board. Now you will have full modulation. You will deadkey 2.5 and swing to about 15 watts.

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The radio needs to be tuned for the box. 3 watts sounds too high for a 2x8.

I would say it would be more like 1-1/5 to 2 watts. That may be why your not getting that much swing.

You should set the radio so the box dead keys around 300 to 400 watts.

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did you make sure the caps stripe is facing the back of the radio? If not, its the wrong way.

Also, don't use a 1/4 watt resistor because it will get too hot for it to last. use 1/8th.

changing the cap size will not change output, however it will change the tone. Bigger the cap, the deeper the tone of audio.

to be honest with you, high output mods are worthless in these radios. they are fine with out them. mosfets are unstable and are very dirty transmit finals. As well if you are planning to use a linear you may hit it too hard unless its a high drive, in which case you will need more then 35 watts swing to drive it fully. A stock cobra 29 that has been peaked and tuned, then the swing mod put in, 220uf e cap and 100 ohm resistor, which should give you about 2 watts, should swing to about 27 watts, and kick an amp in the pants.

I run a cobra 29 in my blazer with a dave made 5 pill, and I key on 6 pills and I near keyed on a 2x8. very close. All that I have done to the radio is peak and tune, (of course I have a few tricks with that up my sleave which I don't usually share with just any one.) and variable power, wich will control the dead key, which is basically a varible swing kit. I run it just under 2 watts, just enoug to get a 125 watt dead key out of the amp. The amps swings over 920 watts.

any more swing from the radio will beat up the pills in the amp.

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There are no knobs for AM power in the cobra 29 LTD. Soldering 2 parts inside the radio is needed to do this. If you are not comfortable with soldering inside of the radio, take it to a technician.If you feel you are up to it, here is how you to do it.

First make sure VR4 is all the way to the left. This will turn your modulation to full and will make your audio louder.

Next you will need a 220 uf 35 volt capacitor and a 1/2 watt 100 ohm resistor.

Now remove JP36. It is located right in front of the final transistor in the back right that is bolted to the heat sink.

put the capacitor in its place with the black tab facing the back of the radio.( this is most improtant because if you don't face it the right way you will blow your radio). Solder it in place.

Next on the trace side of the board, where all the solder joints are, solder the resistor to the spots where the cap was soldered. So the resistor is in between the 2 points of the capacitor.

Now your dead key will be at 2 watts. Now, you did say you wanted 1 watt. From my experience, 1 watt is not enough. you will under drive the box and blow the pills in it.

If in fact you do need only 1 watt, you have to play with what size resistor you put on it. if you increase the value of the resistor, you will get a lower dead key. If you lessen it, it will make the dead key higher. Get a multi pack of 1/2 watt resistors if your going to do this.

BUT** 2 watts should be what you need.

So you know. There are 2 types of wattage outputs in an AM radio. There is carrier signal or dead key, and then there is modulation or swing.

Your radio is going to do 2 watts of a carrier signal or dead key, and when you talk the meter will swing up when you talk if you have peak reading meter. it it only reads average, it will only show your dead key with only a swing of about 4-5 watts.

For low drive amplifiers, the dead key is usually all that needs to be set. The swing should be turned up (thats why you turn VR4 all the way to the left) witch will give you just over 20 watts swing. this will drive the box good. Any where between 20 and 35 watts will dive most boxes good.


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Um... What?? This is hooked to a switch? CB radio by FCC regulations is only supposed to have a max of 4 watts total output. If you can flip it to 7 watts dead key, you are doing really good. It won't do much more then that. It sounds like its set up to run on a linear amplifier, which cannot have more then 3 watts dead key transmitting into it.

Dead key, and modulation are different. You may have a meter that only reads dead key, and won't see the modulation. This radio is probably swinging close to 15-20 watts. you need a meter that can read PEP, Peak envelope power. Cheap meters only read AVG, or average power, witch will only show your dead key.

7 watts dead key is really good for that radio.especially since stock only has 4 watts. you cannot get any higher then that without changing finals.

at 3 watt dead key, or 7 watt dead key, your modulation wattage is still swinging much higher.

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On the meter you are using, does it mesure Average power or peak envelope power?

This is very important to know when you are measuring.

AVG measures basically your dead key. It usually will very little if at all.

PEP measures your peak output, or swing. Modulation.

So even if you have a radio that has been peaked, on a meter that reads AVG, it will still read around 3 to 4 watts. And, if it has a swing kit in it, it will read even less. A swing kit lowers the dead key it increase modulation. Higher PEP.

For example. I peaked my cobra 29 and it does some where around 22 -25 watts PEP. I have variable power in it as well, but usually run the dead key at 2 watts because I have a linear. The Radio alone, when mesureing on PEP with key up at 2 watts, and then when you talk, swings to about 22 -25 watts.

If I flip the meter to AVG, it stays at 2 watts even with talking.

If you have one of those cheap pyramid meters, etc, they usually only read AVG.

Get a meter from radioshack. They have a fairly decent radio for the money. It reads both PEP and AVG and has SWR as well.

If the radio has been peaked and it has a swing mod, it will most likely be swinging to some where in the 20 watt range.

On a side note. According to FCC regulations, a CB radio is only allowed to transmit with 4 watts of final power. Most times the manufacturer will have it a hair under so that they are leagal.

Basically it is illegal to use a CB that puts out more then 4 Watts.

But its a free for all these days. There is no more funding to the FCC for monitoring CB radio, so they do not want anything to do with it. As long as you don't over do it, have over 50000 watts and terrorize a small community with RF, you can do what you want.

Now a days most cb radio's bought have already been peaked if you got it from a cb shop, or off a website that does it.

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