I have an older Cobra 29 LTD. I had it "tuned" years ago, and now I have to set the dynamike down to about half way because it sounds too burbly and distorted at any higher level. I know how it sounds because I have listened to it from the receiving end. When the dynamike is set to about halfway, it sounds great. Above this, and it's "loud" and somewhat distorted. Is this normal? And is the transmit as good with the dynamike set so low? The RF meter on the cb doesn't swing to the right much when the dynamike is set low. I know that having a "loud" mic is important, but I want a clear one too. Any help would be appreciated, thanks.
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Question about dynamike on Cobra 29
- Thread starter Steve H
- Start date
Are you running a "peaked and tuned" radio?
Clipped limiters cause overmodulation, spurious emissions, dirty outputs, and an overall bad / distorted output.
Modulation should not exceed 105% with you screaming into the microphone.
I dial mine to be at around 90-95% or so at casual speaking volume and average mic distance. It produces a fairly clean sounding output.
Also, mic quality has a lot to do with your output quality.
Clipped limiters cause overmodulation, spurious emissions, dirty outputs, and an overall bad / distorted output.
Modulation should not exceed 105% with you screaming into the microphone.
I dial mine to be at around 90-95% or so at casual speaking volume and average mic distance. It produces a fairly clean sounding output.
Also, mic quality has a lot to do with your output quality.
Open the Bottom of the radio, be careful of the speaker wires so you do not pull them loose from the board, look to see if D-11 is cut, or has been removed, also check to see if VR 4 is cranked wide open.
Located about midway up the board. right hand side....D-11 is right to the left of of VR 4, behind it.
If this is cut, the AMC is Disabled.
if it is just clipped, and spread apart, carefully re solder it...if it is gone replace it with a 1N4148 switching diode, you should be able to get these at radio shack......used to be able to get a Pack of 50 for about 2 bucks.
73
Jeff
Located about midway up the board. right hand side....D-11 is right to the left of of VR 4, behind it.
If this is cut, the AMC is Disabled.
if it is just clipped, and spread apart, carefully re solder it...if it is gone replace it with a 1N4148 switching diode, you should be able to get these at radio shack......used to be able to get a Pack of 50 for about 2 bucks.
73
Jeff
If you are running a power mic what do you expect????????? To run a power mic you'll have to turn the mic gain way back. If you were running a stock mic then it would sound fine with the mic gain wide open.
You asked the right question. They do it because some people like running more than 100% and then driving that into a linear amp. This is why some Hams AND CB operators get upset. The net result is if a station transmits with gear setup in this way; they will 'bleed over' many channels above and below the channel they are actually on. Not to mention, the second and third harmonics of that fundamental transmit frequency (~27mhz) can also be splattering on 54mhz (which is the 6m Ham band - BTW).
The worst part is trying to convince them that they are really just wasting power. Power meters are 'dumb'; they just read ALL of the power being put to it. It cannot tell if the frequency is 26mhz or even 28mhz. So, the person doing this is 'convinced' that they are putting out 500 watts on a given frequency. In reality, only a smaller portion is getting transmitted where they want it. They waste power and mess up frequency usage - the real net effect.
On the other hand, a Ham radio has very tight filtering and any adjacent frequency is unmolested when transmitting. When they are transmitting 100 watts of clean power on that particular frequency - you can be relatively certain that all of the power is going where it was intended.
The BEST way to run a CB radio is to be sure that it isn't 'bleeding over'. Keeping the modulation below 100% is one way. The other is to drive it with a power supply that doesn't budge from 13.8v when transmitting. This helps a little to keep internal distortion down; as running everything @ 15v can make for more IMD. Most importantly, is to NOT run a radio with a clipped out AM modulation limiter. That is a SURE way to make it splatter - and badly at that! Another is to run a low pass filter between the radio and a amp - or out to the antenna if no amp is used (this doesn't always make things right with your neighbors - BTW). If using an amp; make sure the voltage is constant when transmitting and use an amp that is Class AB biased. This will keep the amp running more efficiently and create less 'bleed over' as well. Classs C biased amps are notorious for wreaking havoc by splattering everywhere. Avoid them!
Not quite sure that I have all of my 'ducks in a row' over a couple of my statements. But I'm sure that someone will come along and correct the finer points I made.
If D11 has one lead clipped; then just solder it back together with a low-watt soldering iron. This will make VR4 active once again. Just make sure it isn't up all of the way; or you will have to turn it down.
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Only CB channels 1-3 second harmonic would fall into the 6M amateur band as the top band edge is 54 MHz.The third harmonic would be near 81 MHz, well above 6M.
Yeah, they sure aren't bashful about taking a man's cash to screw up his radio.
The D9 is clipped in my 25 ltd, i'm going to put it back together tonight.
Good info.
Next question, so I solder the D9 back together on my 25ltd, try some twisting on the VR5 today, the first part of a word I say is loud but I can tell the limiter kicks in and holds it at a certain max modulation, both audibly, on the radio rx/tx meter, and my watt/SWR meter. So, no discernable increase in modulation for me. Does the increase in modulation from turning VR5 wide open vary by radio or what?
If you find that the radio is still not achieving full Mod with VR 5 fully open, look over by the Mic socket of the radio for R-76. This is a 3.3K resistor, replace it with a 1K resistor, then retest the radio and adjust VR 5 for max Modulation....actually i back it down just a touch below max for a nice clean sound.
The adjustment points are the same but each radio can haves small differences.
Quick question, what mic are you running?
73
jeff
The adjustment points are the same but each radio can haves small differences.
Quick question, what mic are you running?
73
jeff
yes it will "sound" louder, but it disables the AMC, allowing he radio to over modulate, creating that crunchy distorted sound.
73
Jeff
73
Jeff