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Cobra 148 GTL Common Mods Question

Discussion in 'CB Radio Modifications' started by crash-harris, Dec 9, 2016.

  1. crash-harris

    crash-harris
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    New member here. I was on a CB forum once upon a time in the long ago, but can't for the life of me recall which one. I'm more of a mechanic and like having CB's in my pickups, but it's been awhile since I've installed one.

    A little info on the setup...

    A used Cobra 148 GTL that I got while helping friends clean their deceased uncle's place (along with 5 other various mid sized units) with 75-ohm cable and dual Firestik antennas mounted high on west coast mirror struts, 4-pin Diesel noise canceling mic to right angle adapter to 4/5 pin adapter. I haven't yet looked at the back of the CB to see where it was made, but it has the 5 pin mic jack on the left side. This is my first SSB unit (Uniden PC78XL in the retired truck on the same setup).

    Here's where my questions come in.

    First, it seemed to operate fine. No talkback, which after researching the unit I expected, but the needle in the meter was stuck and the RX/TX light would only light up a dim red when the mic was keyed (no green for receive). I ordered a new meter, meter bracket (old one was brittle and cracked in 2), and 3 pin dual color LED for the RX/TX light. After installing these, I put the radio back in the truck to tune the SWR and found I soldered the meter backwards. Yesterday I corrected my mistake and reinstalled the radio to tune it again. Got the SWR set and parked. Later in the day, the right angle adapter for the mic cord arrived, so I installed it, keyed up and got screeching feedback!



    It took me a minute to realize that the radio had suddenly developed talkback! I removed the right angle cord adapter to see if there was a change and there wasn't. When I was working on the radio, I looked to see if it had any extra added on boards for mods for talkback or a resistor bridging connections at the back for a universal talkback mod and found nothing of the sort. I did this because there had been holes of 2 different sizes drilled in the underside of the case for, I'm guessing, switches and pots.

    While still toying around with the radio in the truck attempting to get a radio check unsuccessfully, I noticed that the Brit/Dim switch didn't dim the lighting and that the Hi/Lo tone switch seemed to change something else, like channel? I listened to someone talking with it on Hi and when switching to Lo tone there was no longer anyone talking at all.

    So, before I pull it out of the truck again and start taking pictures of the inside of the radio, what are some common mods that use these switches that could have been done to this unit? If something is useful, I may keep it, but I'd like to restore the switches to original function if not, or at least know what they are doing!
     

  2. 711

    711
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    A switch can be repurposed to anything under the clouds. Sounds like you have added channels and a possible hi/LO power switch. Without a picture of where the wires run to , it's talking a shot in the dark. Talkback could be an incorrectly wired microphone, the 148/Grant Xl and other 5 pin mics have two ground wires . One for the TX/RX PTT and the other for a return path for the audio. The PTT ground also connects the negative Side of the speaker. Using a 4 pin adapter throws some of the mic audio back onto the speaker voila talkback.many different scenarios and I seven harder going behind other people work. Good luck
     
  3. crash-harris

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    I'll get some pics within the next couple of days.

    I did remove the bottom half of the case and disconnected the speaker last time I worked on it. Since the wires for it are both white, it's very possible that I reversed the leads when reconnecting them. I didn't think this would be and issue if they were reversed since there are no other speakers to be out of phase with. Could that have caused the talkback?
     
  4. LeapFrog

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    I have never seen talkback suddenly come about by simply installing the speaker wires in reverse.

    I suggest you try another microphone as you may have a wiring issue in the microphone and not the radio.
     
  5. crash-harris

    crash-harris
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    It would have to be in the right angle harness or 4 to 5 pin adapter, unless it damaged something in the mic. It didn't occur before the right able harness was installed, or on the Cobra 29LTD I had in the truck as a placeholder for this one previously. However, when I removed the right angle harness I still had talkback. I'll try another mic when I pull it out of the truck for pictures Sunday or Monday.
     
  6. johnkyrle

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    Talkback is only utilized when a resistor is applied between the plus side of the ext speaker jack to the plus side of the P.A jack.Simple really.
     
  7. johnkyrle

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    When you say a 148 GTL,do you mean the 40 channel AM/SSB version.I,E like a shrunken GTL-DX?
     
  8. crash-harris

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    I looked for one when I had it apart and didn't see one, but maybe I missed it?

    Yes. I'm guessing the GTL-DX is a base station unit? This is the mobile unit (only says Cobra 148 GTL on the faceplate) with the selector knob for LSB, AM and USB.
     
  9. Robb

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    No; the 148 GTL/DX isn't even the same radio as the 148 GTL. Its a misnomer used to equate a newer generation of SSB radios using the same PLL chip. It is a mobile radio - though.

    The talkback resistor is usually found on the trace side of the board; not on top.
     
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  10. LeapFrog

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    As Robb kind of pointed out the resister does not necessarily need to be installed on the solder side of the board, check around inside for something that looks out of place!?

    And it is a possibility that because of your right angle adaptor and or 5 to 4 pin mic conversion plug you have a problem, if it's not too difficult try a (known good) five pin microphone directly into the radio and see if the problem disappears.

    -LeapFrog
     
  11. 543_Dallas

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    Some continuity across the rx and common mic pin will do it too. This is how some mics like the rf limited 2018 add talkback to a radio.
     
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  12. crash-harris

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    As long as it's not damaging to the radio, I'd like to have talkback. I know that with my Uniden the pots are dirty and occasionally when I'd key up I couldn't hear myself. At that point I'd know to turn the mic gain knob just a bit to get everything making connection again. I still need to clean that radio.
     
  13. Tallman

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    W9WDX Amateur Radio Member, KW4YJ EXTRA class

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  14. crash-harris

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    I've got some of that downloaded haha, but thanks!

    Turns out the original Cobra 5 pin mic that was attached to this radio when I picked it up looks to have the exposed ground shield broken. I'll have to repair it tonight or tomorrow and test before I remove it to take pictures tomorrow.
     
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  15. crash-harris

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    Small update (doesn't look like I can edit my original past).

    Just checked the back of the radio and was made in Taiwan.
     

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