Question about Cobra 29 LTD CB Radio

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B;owing fuses, wont power up, just smokes

My Cobra 29 LTD keeps blowing fuses, and it smelled like something was burning. I noticed the power leads got hot, and some light smoke. Any suggestions on repair, or IF it can be repaired.

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  • Jim Jun 08, 2008

    Thanks for the response. Is the rectifier diode hard to change myself? If not, where can I get one? Also, where do I find it in the radio, to change it?



    My Cobra 29 LTD has the side mic, if that helps



    Any help would be appreciated



    Jim

  • Jim Jun 10, 2008

    CHANGED THE RECTIFIER DIODE, THIS DID THE TRICK!!!



    THANKS FOR THE ASSISTANCE



    JIM

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  • Master
  • 6,966 Answers

Hi,

Try checking the reverse protection diode. This diode would normally be connected right after the power connector and would be the biggest diode inside. One way to verify if this is defective and causing the fuse to blow is even with the power switch at the OFF position, the fuse will still blow.

If on the other hand, the reverse protection diode is good and fuse will not blow when power switch is at OFF, then the short/defective component is inside. There would only be two (2) major components that can cause the fuse to blow with the power switch to ON, the final transistor and/or the audio IC.

Hope this be of initial help/idea. Pls post back how things turned up or should you need additional information.

Good luck and kind regards.

Thank you for using FixYa.

Posted on Jun 06, 2008

  • Louie  Role
    Louie Role Jun 08, 2008

    Hi again,

    Is the rectifier diode hard to change myself? No, two (2) terminals to be soldered;

    where can I get one? all electronics parts store carry this diode or its equivalent (1N5400)

    where do I find it in the radio? inside the CB radio very very near the power connector on the PCB marked D23. It would be the biggest diode.


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Hello MyPete,

You have some form of short circuit in your CB, most likley I would suspect the Power supply or the RF Power Output Amplifier. Here are some basic instruction on how to tell wher the smoke is coming from.

1) disconnect and remove your CB from your vehicle and put it on a workbench
2) remove the cover from your CB and expose the circuit board to view
3) connect your CB to an external 12 volt power source and antenna or a dummy load
4) power up the CB and determine where the smoke is coming from then turn off the power supply to the CB
5) At this point you will need a schematic of your CB (get one from www.servicemanuals.com)
6) find the point on the schematic that relates to the point where the smoke is coming from
7) using a ohmmeter determine which component and or components are shorted
8) replace the shorted and or malfunctioning components.
9) test the system (power on, transmit and receive)
10) close the unit back up
11) reinstall into vehicle
12) run tests again.


This should help

Thank you
Shutle832

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Help


This may be a dumb question, but have you tried replacing the fuse in the power wire? That fuse blowing might have been the pop you heard. It is possible that you did short something out, though I have never had that happen by messing with the wires for the PA.

If replacing the fuse does not help, and you know that there is still good power and ground coming out of the power plug, you may want to try replacing the protection diode.

That is where I would begin, the fuse, power and ground coming into the radio, and then the protection diode. If nothing helps, please let me know.

Dean Pitts
Rollin CBs

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