Question about Magnum Radio Communications

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Have a Magnum 257HP 10 meter radio. While connected to a 50 ohm dummy load, and a power meter, noticed that the USB and LSB would not modulate when a voice input is placed into the microphone. Also, on occasion, without any voice in the mic, the power would swing to around 10 to 20 watts in USB and LSB. AM mode is also over-modulated and the mic gain control does not appear operative.

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You may have a radio where the AM limiter has been defeated which can cause all kinds of issues. I would first check the mic is wired right and does not have a problem, the radio should have output with a tone to it on sideband--wattage up to over 20 watts on SSB is normal.

Posted on Dec 12, 2012


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Posted on Jan 02, 2017

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SOURCE: president mckinly ssb/am model with mb8719 pll chip.

It looks like you had some loose connections in that CB. you may want to also check for other cold solder joints throughout the units and touch them up with some solder. Might I suggest you use a desoldering tool and then just add new solder to the connection.

Posted on Sep 05, 2007

One Wire
  • 126 Answers

SOURCE: meter and St lights

The backlit nightwatch lights can not be repaired, they are fiber optic lights that are ran through the faceplate by the factory.

As for the squealing, the normal way talkback is installed on these radios is to solder a resistor, 68 oms up to 100 ohms, from the PA to the External speaker on the solder side of the PC board. The more resistance, the lower the talkback volume. Yourse is probably squealing because the resistor is breaking down, I just fixed a radio 2 days ago with the same problem. If you want to remove talkback, just open the case, flip it tothe solder side, and remove that resistor that was installed, or you can replace it with a new resistor and still have talkback.

The meter is fairly easy to replace, you just need to get a new meter, make sure it is one for the Cobra 29 (it will have the 3rd line to show SWR reading, which the Cobra 25 does not). You remove the case screws, the face plate screws, and then remove one screw and loosen the other that hold the meter inplace. You will have to solder the two wires to the new meter, observing which post they came from, this will determine the direction the meter swings. You just remove the light bulb and put it in the new meter casing with the rubber stay. This will be self explanatory when you open the radio.

Now for the dead key. It may have had a swing mod done, which drops the dead key down a couple watts. This is actually a good thing. If you have a lower dead key, this will allow you a larger range of power to swing. You can tell if it has had a swing mod by looking at JP36 to see if there is a jumper or a Capacitor installed. If there is a capacitor, the it has been done. To change back to factory, remove the resistor that was soldered to the solder side, across the capacitor, then remove the capacitor. Now just install a jumper in the place. Here is a link to the Modification Instructions, so you can get a better understanding:

If I left anything out, or you have more questions or need further clarification on anything, don't hesitate to email me

Dean PItts
Rollin CBs

Posted on Aug 31, 2008

  • 90 Answers

SOURCE: cobra ltd classic no modulation

Assuming that you've tried another mike, the problem may lie in the speech amplifier circuit within the radio, something that will require someone with technical knowledge to repair.

Posted on Sep 26, 2008

  • 39 Answers


Has anything changed with your antenna or your power supply ? The only real way to get an accurate reading is to check it with a dummy load and form a base line from there.Good Luck

Posted on May 23, 2009

  • 5 Answers

SOURCE: magnum s 9 sideband fault

sounds like tuning, have you been through the alignment procedure? take a look at for the info

Posted on Jun 25, 2009

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If this is a newly installed CB it's possible that the ariel is not connected properly. This can sometimes happen if the co-ax wire, that connects the ariel to the radio, has badly soldered joints or short circuits. These can be checked with a multimeter or continuity tester by checking that there is not a connection between the centre pin of the plug and the outer sleeve of the plug. There should be continuity between the centre pin of the co-ax at the radio end and the centre pin at the radio end. If the ariel end is sealed you may be able to check it against the ariel instead if it's metal.

If all these checks pass you can test that the CB is transmitting power by using an SWR meter and a 50 ohm dummy load. Connect the SWR meter to your CB (where the ariel normally connects) and connect a 50 ohm dummy load to the other end of the SWR meter using a short co-ax lead. When you transmit with the radio the dummy load acts as an ariel, though you won't be able to contact anyone with it. Set the SWR meter to POWER FWD and it will show if any power is being transmitted. If there's not then the fault is in the radio and it will take a technician to repair it. If the power is going forward then the fault is with the antenna or the connection to it. Dummy loads and SWR meters can be picked up fairly cheaply on ebay if you don't have one or if you know any radio amateurs (ham radio operators) they should have them and would probably lend you them.

I hope this helps you with your problem.

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Extndedmods done not problem tansmiting in am radio go back to 10 meter or they do not here my voice clear or a load nois lake a long roger bit or does not trasmit ad all i have to reset again no...

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