Question about Magnum 257 CB Radio

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Magnum 257 No Audio Being Transmitted

Hi Guys, could really do with a bit of help on this one, I have just bought a magnum 257, it Transmits fine (power wise) and receives no problem, but I can't get any audio output, Audio output IC seems fine, so I suspected mic/plug/wire, what I actually found was the mic socket has 13.8v on pin 3, which I thought was the Transmit pin, but it has no voltage on pin 6... which is where it should from what info I can find on the mic wiring of the 257 it should have voltage on pin 6..please correct me if I am wrong..unless magnums have a new mic wiring set up (the radio is the newest version) ...I know the microphones on these radios work like the old k40 speech processors where they charge a small capacitor, is it safe to assume the mic socket is wired incorrectly and put the voltage to pin 6? any help greatly appreciated.

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  • kevin Winkworth Jun 13, 2008

    I had almost the same problem being that the radio was still new I sent it back to magnum. Let me tell you I still haven't gotten the radio back it's been a month. Magnum sucks I wouldn't buy **** from them ever again. Spread the word Stay away from magnum products!



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Hello.I have a similar situation on a Magnum 257 HP operating on SSB (AM and FM works fine). If i Transmit it works fine, but when i release the Mic button (after TX), in the bottom side were the RX level should be, it appear somth like a signal level 3 to 5 (constant and continous), but i can't get any reception at all. Only if i turn the TX power level to MAXand i transmit, then it goes back to reception, but not every time. Somtime after TX i need to push the mic button 2-3 times to make it recive somthing, but this wors only if the power level is over 60-70% an i repeat, only on SSB.Can i get some help with this issue?

Posted on Jan 08, 2012

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You will need a full service manual to trace the pin layout, As you have detected 13,8 vdc+ it is not wise to use the pinout bridging method to find the RX pins..if you should choose to do this, use a 21watt auto lamp in series with the bridge. This will help save you from a "big smoker".If the set still has high value
i would take it to a raido tech. The comment from Winkster is valid.....some companies fall very short with after sales service.

Posted on Jun 27, 2008

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Theres a tiny and i mean tiny surface mount resistor on the back of the centre pin which blows causing this fault

Posted on Sep 12, 2013

  • kev bush Sep 12, 2013

    theres a tiny diode/resistor on the back of the cenntre pin which blows causing this fault

  • Anonymous May 07, 2017

    Yes I did find that out must be a diode so the current wont reverse to pin 3 or there is other components open, ill post as soon I get it fix.


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The magnum 257 has a power mike. pull it apart and you will see a pot to adjust.They are usually turned low from the factory and need setting to your specs. People who have done it have said all ok and very happy.

Posted on Jul 18, 2008

I have a 257, I found the mic that comes with it very poor audio output, so I opted for a power mic a astatic which works fine

Posted on Jul 01, 2008

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