Question about Cobra 148 GTL 120-Channels Base CB Radio

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Cobrba 148 gtl TX

Receives AM and SSB, but does not transmit in any mode.
The PLL MB8719 is ok mb3756 the voltage regulator is also ok the amplifier TA7222AP is ok and the final transistors also are working.
Yes and the (PTT) microphone is working i tried on another transceiver.
Who can help me? What is the problem?
Thanks in advance

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  • tartaruga Feb 01, 2008

    The topic is - Cobra 148 GTL - TX problem

  • tartaruga Feb 01, 2008

    Hello! In the first thank you for the answer.

    The s/meter does not work when I press the ptt.
    I connect the watmetter in output and has no sign.
    I know that PLL is ok I listened in another cb on the same frequency and heard a small carrier when the press ptt.
    The PA active when press the PTT but not hear the audio of the microphone.

    Excuse me the bad English
    Thank you for your help

  • tartaruga Feb 02, 2008

    Ok now PA works, I hear the sound of the mic.
    Final transístor, driver and pre driver are ok.
    I am now to check the voltages of other transistors with the information obtained in factory service manual on pages 16-30.

    Thank you for your answers

    SHARCNET-USA May 11, 2010

    Good advice chiaflub67. The voltage for the RF metering function of the S/RF meter is produced by a detector diode that samples a portion of the output signal from a RF coupler circuit. I was also thinking a pre-driver / driver issue as well. As for the PLL, the modulation of the RF carrier occurrs "after" the PLL generation and is accomplished by the modulator circuit - not in the PLL circuit itself. let us know how its going.

    - Jim

  • Dean Dishman May 11, 2010

    Have you tried another mic?? You said you tried it on another reciever . It may also be the keying Diode I'll find a location # for you. Try using an external speaker plugged into the PA port in the rear put radio on PA make Sure the mic gain is turned up then try to key the mic and talk If you have sound then the audio amp is fine check and make sure the voltages are correct on all components go to under radio info you can download the factory service manual on pages 16-30 youll find a voltage chart check your voltages if any are off then go from there. If the final or the driver both check good also check the pre driver I hope this helps....Good luck Dean

    SHARCNET-USA May 11, 2010

    When you press the PTT do you see the meter deflect on the S/RF meter? Have you put a wattmeter on the output? If so, is there any defection on the wattmeter? I'm sure you probably tried transmitting on this radio whiile listening for it on another CB. Perhaps there is something wrong in the keying circuit. The keying circuit receives the command (usually a pull down to ground) from the microphone and operates a transistor that enables (turns on) a RF stage or RF stages, causing the transmitter to transmit. How did you determine the PLL & RF chain were ok? Just curious.

    The PLL will still use a crystal oscillator for a reference. Is there a LO oscillator?

    Please let me know if you find out anything. I'd like to help you all I can.

    Good luck!

    - Jim



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Thanks sharcnet i just going through the steps in my head trying to figure out what I would do if it was on my bench ..Between the two of us we'll figure it out!!!..... Dean

Posted on Feb 02, 2008


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