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Cobra 29 modulation adjustment.

Robb said:
Another myth to explode - is the export radio. It is just as clean as a stock, untouched/unmodded CB radio. In fact, much of the design is exactly the same. Look at the schematics for yourself if you don't believe me. Some Hams gave them a bad rep, and undeservedly so.

The first harmonic for a export radio isn't at 54mhz; it is at 56mhz.
28.000mhz x 2 (1st harmonic) = 56mhz
Click to expand...

Harmonics are related to the operating frequency, not the design of the radio. If your transmitting on 28 mhz you are correct.

The design is not exactly the same. How many type accepted CB radios have the mixer circuitry that gives you 8+ bands of channels? Harmonics in these mixers can and do cause spurious emission. Not to be confused with harmonics on 54 mhz caused by the golden screwdriver. The worst part it a low pass filter will not help.

Ham or not wierd things happen with exports and large amplifiers. Issues that some are oblivious to or choose to ignore. I won't pay more for something that works worse. My cobra had to meet FCC standards, exports just need to be shiny and have echo so truckers will buy one.
 
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Robb said:
Examples of wierdness?
Click to expand...
I think it's wierdness happening in the rf spectrum(obsiously lol), dang I need a real S.A. arghh!

IMD? ..Back to the books for me.

Maybe the setup was talking/bouncing between 27 Mhz and 28 Mhz! A local runs a Galaxy 2517 and I here him jump frequency all the time, one second he's talking on 28, then 40, then 1, and back to channel 28! (probably a cold solder joint in that locals radio)
 
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Robb said:
Examples of wierdness?
Click to expand...

Little things like less average power out of the amp out with more reflect.

Increased RFI with vehicle electronics. In thses cases the radio was turned down to be used with an amplifier so it wasn't being used outside its rating. I still got less watts from the amps and more RFI.

Ive had some amplifiers oscillate with an export connected and have no issues with a cobra, granted the amplifier can be blamed for some of that.

Ive had issues with the input tune using some exports. Perfect match with a cobra but throw an export in line and not so much. To be expected from a company that crams a bunch of cheap mosfets in a radio to make more watts. Why not use a better transistor like the ameatur rigs do. More profit to be had with a 50 cent transistor I guess.

Burning up parasitic arrestors on tube amplifiers.

Even everyones pal motor mouth maul ditched his galaxy and started using a cobra 29 when he found out people were hearing him on 28mhz. Surely he would have been able to properly tune the galaxy and get rid of that. For some reason he kicked it to the curb.
 
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543_Dallas said:
Even everyones pal motor mouth maul ditched his galaxy and started using a cobra 29 when he found out people were hearing him on 28mhz. Surely he would have been able to properly tune the galaxy and get rid of that. For some reason he kicked it to the curb.
Click to expand...
Yes it's a much better idea to start with a type certified radio!
I think we have all seen the Asymod (mmm clone) produce infinite sideband harmonics w/ an export radio!
 
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543_Dallas said:
Thats more misuse of equipment. What I'm talking about happens with a stock rig.
Click to expand...
Oh yes he did a "super sweep" w/ the sig gen so that test does not reflect real life use, I forgot.:oops:

Okay, after watching the video you linked I am convinved a converted export with likely have issues with 28Mhz interference due to the mixing scheme, if that same radio was actually used on 28 Mhz would the problems still happen?[@29mHZ?] (Another thing to test lol)

eDIT: Going to finnish watch said YouTube flick
 
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Another thing I've saw with the mmm type boards is some type of oscillation or harmonics that cause distortion in the waveform. I'm sure that also made the signal wider than it has to be.

An rf choke in series with the b+ line from the board to the collectors stopped it. Something like a 100uf ferrite core choke from rat shack right at the radio pcb.

Anyway, im trying to be better about staying on topic so im going to stop here.
 
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LeapFrog said:
.... Okay, after watching the video you linked I am convinved a converted export with likely have issues with 28Mhz interference due to the mixing scheme, ....
Click to expand...

To the best of my knowledge that same mixing scheme is used in some CB radios, in a cb they're not intended to transmit on 28mhz so the filtering takes care of this spur. The problem with exports is they can't filter this spur out because 28mhz is within the radios intended use (while they are primarily hotrodded CB's they are sold as ham radios, you can't call it a ham radio unless it is capable of use on the ham bands).
 

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