Don't have to cut a diode to get distortion. It is a product of any transistor amp if it is driver hard enough. Even a stock mic can. You need to use a scope to see the final product, and you need to know what distortion looks like on a scope.
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Percentage of modulation
- Thread starter 2FB327
- Start date
But how do you ensure that the input level actually going into the radio is exactly the level outlined in the service manual? That is the whole point of using a calibrated signal generator. If the sig-gen is terminated in the proper impedance (either resistive or thru a transformer) it should accurately reflect any loading caused by the microphone.
Yes, my point exactly you are "guessing" at the input stage. (When you use computer speakers).
Stock mic, & over drive huh that's got to be a rare phenomenom, considering the FCC wouldn't like for radios to be sold (that when using stock equipment) will cause distortion or possible interference.
That's true. However, I found it easier/more effective and still come up with the proper results. IMO - it isn't as difficult to do this adjustment as manual specifies, and it still stays in the spirit of the end product. Like I said before, have done it way with consistent and clean results every time. Once again, tallman's method and device is doing the same thing I am. Both of us know what a distorted/clipped audio signal looks like; or a 30% modulated signal . . .
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All mics - except for condenser mics - will have a positive voltage output when driven with a sufficient db level audio source. It isn't guessing if you can see the percentage of modulation on the scope. Don't have concern yourself with an AF gen nor a transformer to match impedance when the mic is doing that already. All you have to do is make sure there isn't any back ground noise; also easy to do. That is also why tallman's device has a rubber gasket for the mic when placed close to its own audio source.
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If you don't have an Oscilloscope this is all pointless. The procedure listed by Cobra says to inject a 30 mv 1 kHz signal at the microphone. That means you open the mike and put your signal in at that point, I did not read the entire procedure but if I remember correctly. If you inject at the microphone plug you could end up under modulated.
Yes my description of the procedure may be incorrect that is why I have linked the factory service manual directly from Cobra.
Different manufacturers may use a different process, if I recall correctly the Galaxy service manuals mention injecting the af signal at the jack, I would have to read over the Cobra manual again.
I picture myself screaming into a stock coffin mic, just trying desperately to over-drive the mic amp transistors, using my voice alone. (I wonder how hard I'd have to scream before the radio causes interference, i'm saying it's not likely)
If 2FB327 has an oscilloscope, (as Robb has pointed out) he can monitor the final output as he tunes (w/ speaker method) and still get a satisfactory result.
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Gentlemen, my point was what the manual says to inject the tone at 30%. I think the Captain is onto my question. What good is all the equipment if not used right.
From the factory repair manual is shows AF generator, I am using a Leader LAG 127 audio generator, not a RF signal generator. Am I wrong here??
Perhaps I misunderstood the manual but I thought it meant to have a signal coming into the radio at 80%.
I understand there are different ways of doing things, I just wanted to learn how to do it by the book.
Yes I have a scope and a HP8656B signal generator.
So I ask again, is it as simple as using something to reduce the output of the audio generator?
From the factory repair manual is shows AF generator, I am using a Leader LAG 127 audio generator, not a RF signal generator. Am I wrong here??
Perhaps I misunderstood the manual but I thought it meant to have a signal coming into the radio at 80%.
I understand there are different ways of doing things, I just wanted to learn how to do it by the book.
Yes I have a scope and a HP8656B signal generator.
So I ask again, is it as simple as using something to reduce the output of the audio generator?
Now i'm the one guessing!
Yes the final modulation limit (80%-100%) is not the same as the modulation % of the AF signal you inject.
An RF signal gen is needed for receiver alignments and not the TX alignment, some units produce AF and RF and great to have on the bench.
Can you recognize 30% modulated signal on a scope? Can you recognize distortion on a scope?You see - you cannot get around using and reading a scope; that is the point. Tallman also pointed this out. If you can, then you see that both tallman's and my method are the same and will have the same results.
The scope has squares called graticule, these are reference points.
Learn how a symmetrical waveform should look @ 100% mod vs. just the carrier.
For a loose example:
If "two bars across" = carrier
Then @ 100% Modulation, you will have four bars across.
Double the size the signal takes on the graticule, try to set the volts per division so two bars in the middle is what you see when you tx a carrier, when you modulate the wave will rise beyond the two rows.
This may help you understand what i'm saying.
Guess what, I was guessing the whole darn time using the "speaker method"!
This Cobra 19DX is VERY CLOSE to producing distortion.
Try not to laugh please... I did & said many incorrect things but here you go:
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I used a lot of the jargon incorrectly, but you should get the drift.
This is to show you how to get 100%, ignore the fact that I used the speaker method that I talk down on earlier! Take that for what it is. I was probably overdriving the input @ mic, because I was not using the proper tool..
(You can see this in the wave-form as it loses the perfect sine shape, & "bends" slightly)
It takes a little while to figure out (trigger) settings on the 'scope (If you don't look it up or ask) when I filmed this I still hadn't got that down all the way yet.
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Ummm . . .yes.
But the other part is recognizing distortion too. Seems this will answer many of your own questions about modulation mods and adjustments once this well understood.
I suppose......not disagreeing with you or anything but was just curious. Really you are ball parking it but since there are a couple variables to deal with such as mike gain and internal adjustments then ball parking is close enough since the user operator will be able to adjust it anyway. The important thing is to set the max modulation limit properly and ensure that it cannot be exceeded by advancing the mike gain on the front panel too far.
I edited my post, to include a video. You can see my waveform become distorted towards the end.
I also mention (sort-of in the text) how the waveform must be perfectly symmetrical, otherwise it is distorted.
I was over driving the microphone amp stage because I was not using a proper instrument. (was using the speaker method.) This is apparent in the video, you can see when I put the microphone right on the tablet speaker it pushes the signal beyond 100% modulation, that was with a factory microphone now if I talk like a human being the radio will not exceed 100%, but add an amplified "power" microphone and all bets are off!
My Cobra 19 DX has no microphone gain adjustment, and I still used the speaker method. (dirty imho)
Trust me when I say I cannot wait to properly adjust this radio, I mean it.
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