Cobra 2000 GTL Question

[Sickpuppy22]

nice thanks Mike, i'm currently performing the mod that LC linked, that Moleculo re wrote.

almost done with it, just need to go back to the electronic store for another 2n6488 for the power supply, .2 mica because they did not have any monolythic's left in stock.

My hat is off to all the people who do work on their own radio's, why did cobra have to hide so many identifing marks for the resistors and capacitors lol.

 

[Klondike Mike]

>>... performing the mod that LC linked, that Moleculo re wrote.

I studied that post as well. I'm at a stage now with my 2000 that I can start experimenting with Moleculo's and LC's findings.

 

[Sickpuppy22]

Well after a hole lot of pm's trouble shooting and changing some parts my cobra 2000 GTL is back on the air.

I did the mod that was linked by loosecannon the steps i did were

2. Change R114 to a 10K, and C46 to a .2 monolithic, this will widen up the audio on AM.

5. R180 - Jumper it. This brings up the gain of the TX mixer.

6. IC TDA6130 - Pin 12 to ground through 100 ohm resistor. Upper left, pin 14 = ground.

7. Remove the AM regulator, (usually a 2SC1419) and replace it with a 2N6487.

8. C174 - remove, add 1000uf electrolytic. connect POS to base of yellow wire next to it. If the yellow wire is too far away, it might be easier to connect the POS leg to the leg of the AM regulator closes to the back of the chassis via the bottom of the radio.

9. For 148F, change C18 to a 1000uf electrolytic, 16V

i also discovered a broken leg on tr24 just at the plastic case, all that is left to do is re align the tx section and adjust final bias for ssb, and then i will take some messurements and i will let you know the final resuslt's.


Loosecannon many thanks to you and all you're help the beer will be cold if you're ever up here in montreal!

 

[Klondike Mike]

>> ... i will let you know the final resuslt's.

I look forward to your results. Of course you will get the on-air feedback, I would also be interested in a 1khz audio tone waveform measurement on the scope.

 

[Sickpuppy22]

Mike on the air im getting very good reports, tested the out put with a dosey so take it with a grain of salt.

Barefoot dead key 2-2.3 watts swing to 18 watts pep at 90% modulation.

Amp inline on low deadk key 40 swing to 100

medium 80 swing to 150-160

high 100 swing to 250

box is ab1 palormar 450

if everything comes through my friend will be the owner of a heithkit scope/ so that will be the next step in getting it 100% working order and clean.


so far with my mixer mic combo into the mic chain everyone i've talk to tonight said it was very nice and clear not overly loud and sounded smoother not super punchy, basically sounded more natural.

 

[Klondike Mike]

>> ...Mike on the air im getting very good reports,

Well, that's good to hear! With all the research on the net yielding all kinds of mods; NPC-RC, clip this, remove that, I am pleased that you got something. Thank you and the members of this site for digging up the information.

I'll start getting the needed parts together.

 

[Sickpuppy22]

ive got conditions hear in montreal and im monitoring ch 1 give me a call

Sickpuppy225 the north east corner montreal!

 

[Toll_Free]

Mike on the air im getting very good reports, tested the out put with a dosey so take it with a grain of salt.

Barefoot dead key 2-2.3 watts swing to 18 watts pep at 90% modulation.

100 percent modulation on a 2 watt carrier would be 8 watts PEP.

I think you need a real mod meter, or need to understand how you use the Dosy. You need to calibrate it each time you change carrier power.

And forget about the SSB "mod"... It's like tits on a boar-hog.

--Toll_Free

 

[Sickpuppy22]

No offence Toll free but notice how i said take it with a grain of salt it is after all a dosey and not the most accurate of meter on the market, so before you insult someone please re read carefully,

yes i know that 100% modulation of a 2 watt carrier equals 8 watts.

With that said when i had it worked on before i put it back together the "tech" which i will never go back to again had played with coil L39 and l36 thus explaining why the 18 pep reading on the dosey.

I recently found a dealer for Icom and kenwood that does cb repairs aswell and asked them to do a full alignment and to re adjust l39 for lowest interference on 54mhz, i even provided them with the 58 page sevice manual that klondike mike linked just incase they need specific test point frequencies.

Now i appologize if i offened you in anyway I did what i could with limited tools, and I had fun learning and that is what counts is it not?

 

[Toll_Free]

My point still stands and is still valid. 90 percent modulation with a 9 to 1 carrier to pep ratio is NOT 90 percent modulation.

Even if it is 'just a Dosy'... They may not be everyones most accurate meter, but they do read RELATIVELY accurately... IE, if it's off 2 watts at 2 watts, it will be off the same 50 PERCENT, not two watts, at 20 watts..... So, you STILL have a 9 to 1 carrier to PEP ratio.

ALTHOUGH you are somewhat correct in that messing with the TVI traps can cause meters to read a bit higher, in 22 years of working on radios I haven't seen it make more than a 1 to 2 watt difference on peaks. At least, on the Cobra 8719 chassis..

If your Dosy won't full scale the mod meter in the calibrate position, then you have no idea what the mod percentage is.... AND keep in mind, those don't differentiate between positive and negative modulation percentages...

I wasn't intending to come across condescending, but it probably did.... I was attempting to show that the numbers didn't match up mathematically... And even if it is just a Dosy, they are relatively accurate, if not accurate to the particular number being shown:

Good luck... That radio should be able to key 2-2.5 and PEP 18-22 and still maintain 100 percent negative modulation. The key is in the modulator circuit AND LEAVING THE LIMITER IN!!!!

--Toll_Free


increasing modulation percentages to about 130 percent POSITIVE has a great effect.... LOUD as hell. BUT, after about 130-135 percent, most detectors in most receivers start to distort. Add to this the distortion created in the ANL and NB circuits, and you can start to distort to the average CB receiver.

Increasing mod percentages to more than 100 percent NEGATIVE (or baselining) causes IMD (what people refer to as bleedover). It ALSO gives that growly loudness sound so many people equate to being "loud" on the radio.

 

[Sickpuppy22]

It's all good Toll Free, i understand and agree with what you are saying and as for the dosey, well lets just say it is old and has been bounced around quite a bit being in my friends mobile.

The limiter is not cliped or bypassed and i dont plan on doing that anytime soon.

when i did on air test i had set my mike gain to 12 oclock and adjusted my mixer so that under normal talking conditions i would just peak 100% which would give me roughly 10-12 pep, where as an ahhhh would just reach near 18.

I also did a hifedelity TX and RX mod to it, as this is my goal to have a clean sounding station with minimal interfernce and good sound.

I will let you know what the final results are when it comes back properly aligned and hopefully TVI free.