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Percentage of modulation

2FB327

Active Member
Oct 31, 2012
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Another tuning question: In the factory manual for the cobra 29 ltd it states when tuning transmitter:
TX mode/ 80% modulation
My question is, I am using an audio generator to send the 1 kHz tone to the mic jack. How do you reduce the percentage of modulation?
 

Another tuning question: In the factory manual for the cobra 29 ltd it states when tuning transmitter:
TX mode/ 80% modulation
My question is, I am using an audio generator to send the 1 kHz tone to the mic jack. How do you reduce the percentage of modulation?
How are you putting the 1 kHz signal into the radio? I have a CB tester and it has a microphone pad that you put the mike in. It is using a speaker.
http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/index.htm
http://www.cbtricks.com/radios/cobr...st/graphics/29ltd-st_29wx-st_sm_pg12_pg34.pdf
 
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Another tuning question: In the factory manual for the cobra 29 ltd it states when tuning transmitter:
TX mode/ 80% modulation
My question is, I am using an audio generator to send the 1 kHz tone to the mic jack. How do you reduce the percentage of modulation?
The same way that you increase it, by changing the mod pot, the mic gain, or the power mic setting. All of them can increase/decrease modulation.

But that is judged entirely by the amplitude/height of the waveform on the scope compared to the dead key/carrier - first. Remember, that mod % is relative to the unmodulated carrier. If the TX coils aren't adjusted properly and don't permit at least 90%/or more, then you cannot get 80% either. That 80% modulated 1khz waveform would also need to intact/undistorted, just like the same signal shape/waveform that was injected in.
 
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Another tuning question: In the factory manual for the cobra 29 ltd it states when tuning transmitter:
TX mode/ 80% modulation
My question is, I am using an audio generator to send the 1 kHz tone to the mic jack. How do you reduce the percentage of modulation?
Edit: The CBtricks FSM and the one from Cobra do not match..
Just like it says though, the one you want is VR4. ;) (Or maybe my PDF is backwords)
33w9tg6.png

"Mike Gain" to maximum, set your signal generator to output a 1 kHz sine wave (30mV), inject at pin 2 of mic socket or use a regular microphone & cradle as desribed by Tallman.
In a pinch, I have seen a 1 kHz sine wave youtube video used with a microphone put near the computer speakers {Not accurate, nor recommended}!


The F.S.M. states what to do for your knob settings & configuration of test bench equipment.

Straight from the horse's mouth.
 
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Edit: The CBtricks FSM and the one from Cobra do not match..
Just like it says though, the one you want is VR4. ;) (Or maybe my PDF is backwords)
33w9tg6.png

"Mike Gain" to maximum, set your signal generator to output a 1 kHz sine wave (30mV), inject at pin 2 of mic socket or use a regular microphone & cradle as desribed by Tallman.
In a pinch, I have seen a 1 kHz sine wave youtube video used with a microphone put near the computer speakers {Not accurate, nor recommended}!


The F.S.M. states what to do for your knob settings & configuration of test bench equipment.

Straight from the horse's mouth.
No; not exactly.
I use a D-104 lollipop wit the keydown slider engaged within 1 ft of a speaker supplying 1khz audio tone. Mic gain inside the D104 is set about 30% open. So long as the signal is not tainted with room noise (TV set, radio receiving, etc...), it works just fine. And no; the Mic Gain on the front of the radio should be set to where the internal mic amp isn't introducing distortion; usually around half way open - so as to not feed a distorted waveform in at that stage. At this point, i can adjust the mod pot on the board according to spec.

Not wide open; not even close. Not much different than tallman's setup and achieves exactly the same end.
 
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No; not exactly.
I use a D-104 lollipop wit the keydown slider engaged within 1 ft of a speaker supplying 1khz audio tone. Mic gain inside the D104 is set about 30% open. So long as the signal is not tainted with room noise (TV set, radio receiving, etc...), it works just fine. And no; the Mic Gain on the front of the radio should be set to where the internal mic amp isn't introducing distortion; usually around half way open - so as to not feed a distorted waveform in at that stage. At this point, i can adjust the mod pot on the board according to spec.

Not wide open; not even close. Not much different than tallman's setup and achieves exactly the same end.
I am glad that trick is working for you, I have done it in the past.
In regards to the mic gain level: Why would the waveform distort if VR5 is adjusted correctly, that's the point of an "audio limiter" & AMC.
I would never do this with a high-z mic or anything besides what will be ran w/ the radio, definitely not adjusting the modulation limit with a power microphone..

EDIT: Nevermind, I see it has a "chance for distortion" (over-driving mic amp stages) because you are not using a proper signal gen, i'm not exactly wrong now am I?

With the right gear
, you take the guess work out of it all..
Then you can run the mic gain full on open, & set the modulation limit "by the book".
Otherwise down the road, someone else will turn that gain knob all the way up; and then bye bye clean signal..

Not wide open; not even close.
The FSM, requests that you use the proper tools & follow the procedure as outlined...

For setting just your own radios, as a hobby; I see nothing wrong with what Robb has said. That method will "get you by" while a proper signal gen. can get pricey.
 
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I am glad that trick is working for you, I have done it in the past.
In regards to the mic gain level: Why would the waveform distort if VR5 is adjusted correctly, that's the point of an "audio limiter" & AMC.
I would never do this with a high-z mic or anything besides what will be ran w/ the radio, definitely not adjusting the modulation limit with a power microphone..

EDIT: Nevermind, I see it has a "chance for distortion" (over-driving mic amp stages) because you are not using a proper signal gen, i'm not exactly wrong now am I?

With the right gear
, you take the guess work out of it all.. then you can run that puppy (mic gain) full on open, & set the modulation limit "by the book". Otherwise down the road, someone else will turn that gain knob all the way up; and then bye bye clean signal..

For setting just your own radios, as a hobby; I see nothing wrong with what Robb has said. That method will "get you by" while a proper signal gen. can get pricey.
Think you might be missing the bigger picture here. First off, what and why I do it this way, is that it is a practical way of making it work when a mic is finally used with the radio by the operator. He knows where to set the Mic Gain control on the front, and it also lets him know how high to turn up his power mic. The impedance of the mic also interacts with the preamp as it will after adjusted. Tallman's way is also doing the same thing - as well.

These adjustments are best made AFTER the TX coils have been peaked and tested for distortion.
You can use a digital sig gen in this case, your computer.
Keep in mind when that SM was written - before PC's.
I've done it this way - well - all and every time . . .
 
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Think you might be missing the bigger picture here. First off, what and why I do it this way, is that it is a practical way of making it work when a mic is finally used with the radio by the operator. He knows where to set the Mic Gain control on the front, and it also lets him know how high to turn up his power mic. The impedance of the mic also interacts with the preamp as it will after adjusted. Tallman's way is also doing the same thing - as well.

ZThese adjustments are best made AFTER the TX coils have been peaked and tested for distortion.

I've done it this way - well - all of the time . . .
Like I said in my post after I edited it there's nothing wrong with the method you described. (for tuning a personal radio).
What I'm saying is that, while the wing-it-method can "get you by" it does not coincide with the factory service manual in any way.

I'm not sure if 2FB327 intends (just) to fix his own personal radios, or other people's radios & that is why I'm trying to be specific about the "proper method".
Doing it the other way works but it's not specific nor is it repeatable to any degree, that is a unique tune for a personal set-up, it is not something repeatable when you pass that same radio off to somebody else.

But, if adjusted to factory specifications there's a good chance he could then pass that radio off to somebody else & it would not inadvertently become a "splatter box" just because a different microphone was used, or the new operator decided to turn the mic gain up. ;)

The most exact, efficient, and repeatable method requires the proper tools.
We are all trying to help each other here, if I came across as being offensive in any way, I apologize.

73
 
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You can use a digital sig gen in this case, your computer.
Show me how to plug my sound card output directly into the audio path, then we can talk about repeatability (once you have a quantifiable input signal) and are not just guessing.

Also, these FSM are in PDF format, (scanned I know) if the engineers had something to add in this digital age, they would have by now.

You can use a PC as a sig gen sure, just not to an exact degree.. (Unless you have a progam and buffer module I don't know about)
 
Show me how to plug my sound card output directly into the audio path, then we can talk about repeatability (once you have a quantifiable input signal) and are not just guessing.
No; that is a trap.
You might have ground loop issues that way; run the risk of frying something.
Wrong tack.

Use this to let your PC speaker to produce a very accurate sine wave at 1khz.
I don't have to set it up to make a video and show you; I know it works just fine this way.
http://www.freesound.org/people/klangfabrik/sounds/28636/

Like I said, and tallman already pointed out using his gear, that a mic/speaker link works fine, and the radio's input impedance with a real mic is more in line with the final product for the end user.
 
No; that is a trap.
You might have ground loop issues that way; run the risk of frying something.
Wrong tack.

Use this to let your PC speaker to produce a very accurate sine wave at 1khz.
I don't have to set it up to make a video and show you; I know it works just fine this way.
http://www.freesound.org/people/klangfabrik/sounds/28636/

Like I said, and tallman already pointed out using his gear, that a mic/speaker link works fine, and the radio's input impedance with a real mic is more in line with the final product for the end user.
Okay just to be clear I agree with you, but that is still not a quantifiable input signal as you do not know what drive level (voltage) you are putting through that mic amp stage.

That said, heck yeah (for the end-user) that method is absolutely fine.
To be honest I have wondered how many truck stops even bother with this step, as they usually "clip the limiter" so it doesn't matter anyway.

While not perfect or exact, the computer speaker method works because you can observe the final output, and just adjust the mic gain & amc until the signal looks clean.
 
Okay just to be clear I agree with you, but that is still not a quantifiable input signal as you do not know what drive level (voltage) you are putting through that mic amp stage.

That said, heck yeah (for the end-user) that method is absolutely fine.
Neither does tallman's method; as will ultimately depend on the power mic's setting (if used) and the front panel's Mic Gain setting - anyway - regardless of what source it is.
 
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I like my way better, because you can throw almost any microphone at the radio and turn that mic gain all the way up, (long after the tuning is done) and still see clean output.

The other way leaves a lot of room for error, if the knobs are adjusted after the tune.
 
I like my way better, because you can throw almost any microphone at the radio and turn that mic gain all the way up, (long after the tuning is done) and still see clean output.

The other way leaves a lot of room for error, if the knobs are adjusted after the tune.
If you run it up all of the way open, distortion is guaranteed. Not better. You will have added distortion to your sine wave. Of course, you will see it on a scope. Providing you are using a scope at all . . .
 
If you run it up all of the way open, distortion is guaranteed. Not better. You will have added distortion to your sine wave. Of course, you will see it on a scope. Providing you are using a scope at all . . .
wth, how? Did he cut D-11?
Oh, I see we are talking about computer speakers here again and not the proper equipment...
 

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