Cobra 25 Noise Blanker function

[smalltiredart]

Cobra 25LTD Classic date of production Aug. 05

The noise blanker on my Cobra 25 sucks. When you flip it on you barely notice a difference. On my other Cobras it makes a substantial difference.

Is there any component value that can be changed to make the N/B more effective ??? (i.e. resistor, cap, etc.)

 

[Cheech]

Dont do a thing in my 25 either but the sound tracker does and everyone says its crap well it works when you push the button. Sound tracker does more than the NB how crazy is that?

 

[Switch Kit]

very interesting ? ....I personally run older 25s for a reason !! as far as IM concerned they have the best noise blankers in any radio as far as IM concerned concerning my car that I own now .....I run a 1983 Cobra 25 LTD .......I've ran a Grant XL / Cobra 29 LTD Classic / Uniden 76 XL / Uniden 68XL / Uniden 66a and 66/Realistic TRC-427 that did not have as good of anls/nb's concerning my car (on my Grant and the 29 and 76 they would act more like a semi-rf-gain control yes it would knock out the noise but it would also make people sound un-true or even harder to hear from miles away) ......also a few of the 25 LTD Classics of the mid 90s in which they also had great noise blankers as well !! .....The anl is built in on those radios for the most part unless you had a 1979 model 25 GTL which had those on switches on the front , the noise blankers do work on those radios !!!! you only think they don't ! )-: IM really not up on the newer 25s of today ? IM just kind of old school more so then not ......trust me ...your nb's work on those radios guys !! it just depends on where your at at any given time ......I can understand you thinking that they don't work .....but they do. This paticular board model has not changed that much over 25 years or so .....it must have been one of Uniden's finest AM only boards that is stilled used in some veriasion today. Your question was, can you better the noise anl/nb's in the radio ? what kind of vehical do you drive /antenna / grounding ? they do have filters you can buy but as far as the radio goes ....I don't know ? maybe DTB will chime in on this ? There's also noise that comes from alternators , fuel pumps , spark plug wires , electric fans , and the list goes on , they can be capped off to some degree , but as I said before , that's personally why I run a 25LTD because it truly does have a good anl/nb system in the radio to help from those kinds of engine / antenna TVI erratents. Good luck to you.

 

[DTB Radio]

There is a coil in the 25 (and all NB-equipped radios) for tuning the NB circuitry. You need a generator capable of a 100-hz, 1us-wide pulse at a few volts amplitude to be able to adjust it. The pulse generator will also tell you VERY quickly if the NB is working.

As for specific parts to change to make the NB more effective, I'm not aware if any specific mods for that. I'm sure it could be improved, though.

 

[Switch Kit]

as far as bleedover from other channels .....I understand theres a way that you can add to the 455 filters in the radios that will help the problems somewhat.

 

[DTB Radio]

Yes, you can add a couple extra in series, with some slight signal drop. If you add more than a couple, you'll probably want to also add a small helper amp in between someplace. Nothing fancy, just a simple VHF-UHF bipolar transistor arrangement.

 

[smalltiredart]

A 2SC2999 fixed the problem.

 

[Van Lifeson]

Yes, you can add a couple extra in series, with some slight signal drop. If you add more than a couple, you'll probably want to also add a small helper amp in between someplace. Nothing fancy, just a simple VHF-UHF bipolar transistor arrangement.

What is the number of the NB coil?

 

[Squints]

The noise blanket does not work at all in my 25, any repair tips please ?

 

[Handy Andy]

You got a spare 29 or 25 anywhere from circa 1990?

Then the coil from it should be able to drop in fit.

Now to ask why the NB is different in today's Cobras - you can see numbers on the side of coil can - yes, but the frequency response to noises like impulse and buzzing - relied more on high frequency Rolloff at the band in question - in the older days.

They purposely used coils that were not HF types, but IF types retuned to another intermediate frequency in-between - 23MHz to tune for noises and RF "images" and apply that at the last stage of the 10.7MHz IF amp.

What Cobra did is use a coil set up for 26MHz or less, (Peaks out around 23MHz to as high as 26MHz or it's tuned frequency of 12 to 13MHz) and took in the RF from the RF amp - directly from the RF amp (1st Stage) - and then passed this thru into the NB filter - just to get a sampling of noises from AROUND the 27MHz - just below it - near the 12M band where the thinking was at the time, if you can get it on AM on 19 - you can then hear something like it on a lower frequency. We have to use a coil that can shunt or tune out other frequencies so we don't get noises from their bands - also that we can tune out the 17MHz IF that being mixed to generate the 10.7MHz IF inside the radio - that way -the 23MHz to 26MHz typical 3MHz wide bandwidth of the 27MHz coil RF Amp would pass this noise (along with any RF) into the NB - but by the NB working to INVERT the impulses and buzzing - the shunting would lessen the noise but not generate a birdie to mix into the IF accidentally.

​

So, to bring you to the present - the issue is now the quality of the coils they are using is not the same as of the older days - they have a higher Q-factor - meaning they are really tight tuning so to obtain a noise in the general band area, means the coils coupling to too close to obtain a BROADBANDED signal and its noise level to work effectively as the older days.

So sub in the old leaky lossy coil to get that 23MHz bandwidth (or 16MHz bandwidth) they both worked (they are also images of each other) to get a noise into the NB strip - amplified, Rectified, detected and averaged - to obtain a level of pulse that is the DIFFERENCE signal - and apply it to the FET before the 455kHz downmix mixer.

 

[bologna_sammiges4me@yahoo]

I have never been a fan of the soundtrackers as they do more than affect the receive but also affect the audio to by just changing the sound of the audio slightly tho in most cases will affect the performance of some power mics.
Is soundtracker technology suppose to do all that ?.
All modern cobras are in my opinion very poor quality especially the 29 classics or 29 chrome everything about them is crap.

 

[Handy Andy]

Soundtracker is a lot like the older dBx systems - where Compression of Audio to transmit across to another dBx listener that can Expand that compressed audio back into it's dynamic waveform - is pretty much how it is supposed to work.

Like Dolby - get above the noise level - just below distortion - then compress, then de-emphasis of the compressed spectrum to restore the dynamics.

Either system, uses a form of compression to send signal - then decompress it - once received, to remove the noise and it's artifacts.

So when you discuss ST units versus the China-clones versus the Original Dynascan and Uniden types of chassis - that is a realm of Apples and Oranges.

And to compare those two - they're both Fruit and it stops there.

But to reference the quality, it's pretty much lost on the new people. For the older discrete technology - being what it was, was better in some ways than now, only now you have sensitivity and noise factors, while the older systems designed their Signal to Noise levels much differently and were a factor or performance than that of cost effectiveness.

Which; when you look at that word in context, "effectiveness" that really is a misnomer, for it makes it more of a throwaway device than that of a high performance piece of radio equipment they once had the status of being.