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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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.

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

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

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

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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.

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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.

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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.

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