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Cobra 29 Ltd no transmit

Discussion in 'General CB Services Discussion' started by Rooster74, May 18, 2019 at 4:24 AM.

  1. Rooster74

    Rooster74 New Member

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    Hey guys, sorry to bring up yet another cb no transmit problem, I know this has been covered many times, I know because I've read many threads on the same problem but nothing really seems to match my symptoms or cover specific diagnostic steps, and my diagnostic skills are weak at best, but hopefully you guys can help get me in the right direction.

    The problem radio is an older model Cobra 29 Ltd.
    The RX seems nice and clear, and PA works well. So far I've tried two known working mics and checked the board for cold solders. There is evidence that work has been done in this thing but I'm not sure what, it looks like a different final at least has been installed, which I've pulled and tested with a multimeter. Based on all the data I found online it tested sat so I reinstalled it.



    When keyed the Rx/Tx light turns red but I get no deflection on the S-meter. There is an audible "bump" noise that comes from the speaker, then from there zero transmit, I can't even hear a change in the static on an adjacent radio on the same channel.

    Based on the block diagram and advice gleamed from other threads it seems like IC3 is the first most likely cause but I'm unsure on how to test it. The only diagnostic equipment I have is a multimeter. Any help would be greatly appreciated.
     

  2. kaos513

    kaos513 Well-Known Member

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    Could be a shorted cap if the radio is old and never been recapped.
     
    Shadetree Mechanic likes this.
  3. nomadradio

    nomadradio Analog Retentive

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    If you have another working radio to use as a 'sniffer', it's useful to find out if the radio is producing a transmit signal at all.

    If not, it's a waste of time to look at the final and driver.

    Yet.

    Plug a spare coax jumper into the monitor radio and back the shell of the plug at the far end of the jumper down the coax. This exposes the center pin of the plug, making it a half-inch long "sniffing" antenna.

    Put both radios on the same channel and see if the monitor radio can hear a carrier coming from the broken radio when you key it. Hold your sniffing antenna near the driver transistor. If there is any transmit signal at all being generated, you should hear it this way.

    If so, next check the receiving radio's S-meter to see if the slugs in L21, L20 and L17 are all set for peak reading. If they have been diddled, this alone can put the transmitter off the air.

    If they all show a proper peak on the receiver's S-meter, checking the driver and final transistors would be next.

    73
     
    Gypsy707 likes this.

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