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Post by jacobs132 on Mar 13, 2010 8:46:11 GMT -5
can some one tell me how to get more than 40 percent modulation out of a cobra cam 88. everything else is working good on this radio, 4 watts transmit, swings to 8, frequencie is dead on, i have heard that these radios have good modulation, i have a amplified d 104 wired to it tug8 40 percent modulation is all i am getting, any help would be apreciated,,,,,,,,,,,, jim
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Sandbagger
Administrator/The Boss 
Posts: 6,250 
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Post by Sandbagger on Mar 13, 2010 18:33:43 GMT -5
can some one tell me how to get more than 40 percent modulation out of a cobra cam 88. everything else is working good on this radio, 4 watts transmit, swings to 8, frequencie is dead on, i have heard that these radios have good modulation, i have a amplified d 104 wired to it tug8 40 percent modulation is all i am getting, any help would be apreciated,,,,,,,,,,,, jim Those radios would just make 100% modulation with a 3 watts carrier. There just wasn't a lot of left over audio drive. That being said, 40% is way low. If the audio tubes are good, then one of the more likely parts to fail are the modulation clipper diodes. I've seen them become leaky and it will clamp the modulation too low. You can clip them out and if the modulation percentage jumps to 100% or greater, then the diodes are bad. You can leave them out but it's better to find good replacements to prevent overmodulation. There could also be a bad cathode bypass cap on the audio output tube, or leaky caps along the audio chain. |
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Post by jacobs132 on Mar 14, 2010 15:10:39 GMT -5
i put 2 new tubes in it, 12ax7 and a 12aq5, still 40 percent, and thats with a whistle, do you know by chance where the clipping diode is located?
thanks for your help
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Sandbagger
Administrator/The Boss
Posts: 6,250
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Post by Sandbagger on Mar 15, 2010 7:22:18 GMT -5
i put 2 new tubes in it, 12ax7 and a 12aq5, still 40 percent, and thats with a whistle, do you know by chance where the clipping diode is located? thanks for your help They (there are 2) are attached to the range boost switch. I forgot to ask if you tried the modulation with the range boost switch on and off. When the range boost switch is off, the clipper diodes are out of circuit, but are replaced by an resistor attenuator which will cut back on the modulation. But with enough microphone gain, if the clipper diodes are bad, you'll get more modulation with the range boost off. That's something to try. If it's still low, then I would look for an open cathode bypass cap, or some other part in the audio chain which has changed value. At that point it would be very helpful to have a scope to follow the modulation down the audio chain. |
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Post by jacobs132 on Mar 17, 2010 20:11:15 GMT -5
thank you for the information, we had flooding here in new jersey, as soon as i can i am going to open up the camm 88 and try the suggestions, , i will keep you posted as to what the outcome is. i really like this radio and the cam 98 that i have, both are mint condition but both share the same modulation problem,,,,,, jim
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