Cobra 29 modulation adjustment.

[Robb]

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.

So, what you are saying is that these radios create a number of harmonics. Which is odd, seeing that their use on 10m falls under FCC specs. Like any radio, these harmonics are usually a result of some golden screwdriver monkey getting both feet in there and yanking hard. Seen that too many times to say that it is more the norm - than the exception.

I don't have a SA yet to test them out, but I haven't seen any issues with these radios so far after a good alignment and returning them to stock. Not that I doubt you, because I don't use an amp still. But I have owned and used a number of them, and still do. For a parasitic filter to fail in a tube amp; something must have been faulty from the radio - is true.

It might be possible that the 2nd harmonic filter is no longer in tune for 11m use.
Plausible.

 

[543_Dallas]

So, what you are saying is that these radios create a number of harmonics. Which is odd, seeing that their use on 10m falls under FCC specs. Like any radio, these harmonics are usually a result of some golden screwdriver monkey getting both feet in there and yanking hard. Seen that too many times to say that it is more the norm - than the exception.

I don't have a SA yet to test them out, but I haven't seen any issues with these radios so far after a good alignment and returning them to stock. Not that I doubt you, because I don't use an amp still. But I have owned and used a number of them, and still do. For a parasitic filter to fail in a tube amp; something must have been faulty from the radio - is true.

It might be possible that the 2nd harmonic filter is no longer in tune for 11m use.
Plausible.

I mentioned the thing about spurs because of LeapFrog's SDR. No intention of getting into an export vs cb debate, theres been plenty of those. It can be worse than politics and religion. I've used a lot of different equipment and had far less issues with a certified CB.

 

[Robb]

I mentioned the thing about spurs because of LeapFrog's SDR. No intention of getting into an export vs cb debate, theres been plenty of those. It can be worse than politics and religion. I've used a lot of different equipment and had far less issues with a certified CB.

No debate intended. But you did make me think about the problem. When a trap filter is designed, it can only have a given width in which it was designed to operate - as it should. Since the space between 10 and 11m is greater than 1.5mhz, that surely is the reason that the parasitic filter on the amp would fail when using a 10m export on 11m. That trap can no longer be working! It is outside of its designated width.

IOW - to allow these export radios to work on 11m w/o a harmonics problem, the values of four parts in the trap filter circuit (2 disc caps and 2 choke coils) would need to be changed out to suitable values.

Thanks!

 

[Robb]

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).

Aww; you already figured it out. Didn't see your post until now. Oh well, at least I'm on the same page as you.

 

[StrangeBrew]

Honestly I didn't figure anything out myself. When I first heard about the mixer issue I thought it was strange that no CB's seemed to have this problem because to the best of my knowledge the basic design of most exports was ripped off from an old CB, with a little research I found the explanation I gave above.

.... to allow these export radios to work on 11m w/o a harmonics problem, the values of four parts in the trap filter circuit (2 disc caps and 2 choke coils) would need to be changed out to suitable values.

Is this a known mod?

 

[LeapFrog]

I don't believe anyone here is arguing or fighting.
I believe this discussion deserves it's own thread!
We need: (A peer reviewed study published in a scientific journal)? no, this problem/question along with the L12 shenanigans should not be dificult to demonstrate by someone with a proper spectrum analyzer. I don't know why I thought a 20$ Amazon device would cut it..

My cheap RTL-SDR, with an internal up-converter and no low pass filter has issues with it's chip scaling scheme going that low and picking up spikes/interference from inside of itself, when a real spectrum analyzer would not. Also the blasted things are very prone to interefernce from commercial broadcast transmitters without additional filtering and amplification for HF range.

I'm finding out this cheap tool is really of use to me for seeing how wide the audio transmitted is, and to sample the audio it's great, for RF spectrum work it's limited but I do use it (perhaps erroneously) to see if i'm on frequency!