Big, Loud, Cobra 29 LTD Radios

I realize that statement came from op, wasnt trying to imply that you made it, and could detect hint of sarcasm when you posed your question.

 

Wolfy, you are correct up to a point, you have to remember that anything up above 100% modulation introduces distortion. You saw that in the first video. Now granted the human ear probably won’t hear that distortion , but you could be bleeding over on the side adjacent channels, which is not good. That is also lost power so therefore you really won’t transmit farther like most people think.

just to crank the AMC pot clear up like the OP said is not good, even though it sounds loud. Also with 100% modulation @ 16 watts PEP, the 2SC2678 transistor is fine. But when you crank up the modulation, say to 150% or more, and maybe put out 22 W PEP or more, then you start stressing out the transistor, which could cause the transistor to fail sooner than it should. Just something else to keep in mind.

 

I dont know for a fact, guess only way is to have a scope and a Spectrum analyzer, but if AMC ( vr4 on cobra 29) is turned up to max shouldnt that be 100% modulation and only way to exceed that would be to remove or bypass the limiter circuit which is what many cb shops do. Bells cb, as an example ( not implying he is right or wrong) even states on his website that he turns modulation up to 130 % and allows operator to control with mic gain.

 

I have on occasion with a Cobra 29 turned vr4 all the way ccw and then back it off just a bit and quite honestly a trusted receiving station reported no significant improvement over OTB stock modulation which was set at 75 % , however if perform a transmitter alignment per Cobra service manual it does recommend adjusting modulation to 100%.

 

I know a while ago there was a very lengthy and informative video that was posted on this forum ( I believe by CraigSez, could be someone else) that the individual displayed on a spectrum analyzer , with a Uniden 78, that just by turning up the AMC he was able to intrduce harmonics and distortion ( assuming nothing else had been done to radio) so by this demonstration it may be true that AMC adjusted to max with limiter intact will in deed exceed 100% modulation level. Any members with 1st hand knowledge please feel free to chime in . So I guess the question I am asking is, if amc is adjusted to max with limiter intact is that 100% or greater?

 

IIRC it’s been said elsewhere turning the AMC wide open has the same effect as clipping the limiter, was said it bypasses. However I’m not 100% certain in the truth of that. Anyone confirm?

 

I did a finger check on the transistor in my earlier post, it’s actually a 2SC2078E... just FYI.

It’s actually best for people who want to play with the pots for the first time, that they use a felt black marker and put a mark on the side of the pot. So when they adjust the AMC pot and it doesn’t work right, they can adjust it back pretty close to the factory settings and no harm done. Most radios come out of the factory around 80% modulation so it doesn’t take much to turn it up to 100%. I was told years ago the main reason for 80% is because most people would use a power
mic on the radio (because it was quicker & cheaper than going to a tech) and therefore it gave it some headroom to not overdrive the radio with a power mic.

Usually when I adjust my radios modulation to 100%, it’s probably 3/4 of a turn from the lowest setting (or something like that), so my pots are never fully wide open for AMC. Yes I can turn the radio up over 100 % modulation just by adjusting the AMC pot, and not clipping the limiter diode. Hope that answers some your questions. I use a 1kz 30 mv signal injected into the mics audio pin, per service manual.

You can use a tone generator app on your cell phone, and just hold the mic over the phone with the 1 kHz signal. It might get you in the ballpark for 100% modulation. But it won’t be as accurate as a real signal generator.

 

I don’t technically agree with screwdrivers theory on 130% modulation. I think the reason he does that, is because he knows everyone’s voices are different. some are weaker than others, that gives the operator a chance to adjust their mic gain to suit their voice. The problem with that is, they are relying on someone else’s ears. Not the best way to go in my opinion. To me it’s like a glass of water. If you fill it completely full, it’s 100%. You keep filling it and it keeps spilling over, and it just causes a mess. It’s the same with an audio signal, at least in my mind.

I’m pretty sure screwdriver doesn’t clip the limiter (he doesn’t seem like the hack type to me), so the 130% should be attainable just by turning the VR AMC pot. I guess I never have paid attention to how high of a modulation percentage I can get just by turning up the AMC pot. I’ve always stopped at about 100%.

 

Yes, that’s basically correct.

 

It was more along the sarcastic line but still makes you scratch your head..Its like haveing a louder stereo but you go to the bathroom and close the door tou suddenly hear nothing..