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Mods for the Cobra 200GTL

DTB Radio

Well-Known Member
Sep 14, 2005
1,014
69
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Carlisle, PA. USA
www.dtbradio.com
These are above and beyond the conversion and talkback modification.

Echo mod:

R550, NOT R39 as previously posted, controls the delay of the cho (speed of the echo). By changing it to a lower value, the delay is decreased. Higher value (obviously) increases the delay. Values below 1k would be inadvisable. I've not experimented with a max value yet, but a couple hundred K would likely be the practical limit. If you want, you can put a 100K pot in place of R550, making sure to use a bit of hot glue to keep the wires from moving around too much. Keep a 1k fixed resistor in series with the POT.

Mic Preamp Mods: (I haven't actually tried these yet, but the theory is sound)

1. R181 is the feedback resistor on the 4558 mic preamp IC. It is 150K stock. Change it to anything from 470k to 1M. This will allow much more effective use of a good noise cancelling mic on the radio. As with the talkback mods and echo mods, use hot glue on the parts you install to make sure they do not flex. Values above 1M will be problematic from a noise standpoint, and 470K is likely going to give more than enough added gain for practical purposes.

2. C158 controls the speed at which the AMC circuit responds. To speed up this circuit (and thereby give more even modulation at max levels), change this to 10uf. If you go lower than 10uf, you start to get a choppy sound.

If you need a parts layout, visit cbtricks.com and look at the data for the Cobra 200 GTL


These mods are also listed on my website.
 

960

Member
Aug 8, 2005
33
0
16
West Coast
Also Q44 is the modulation limiter. I have not pulled this but have heard it will open up more power (SSB seems to govern around 100watts or so) and more modulation.
 

Thestonerguy

Active Member
Apr 3, 2005
338
1
26
Texas
www.cbgazette.com
960 said:
Also Q44 is the modulation limiter. I have not pulled this but have heard it will open up more power (SSB seems to govern around 100watts or so) and more modulation.

My Two Cents:
I've always found that it's best NOT to pull the limiter but to find another way to improve tx modulation. Less problems down the road, but it's up to every man to make his own decision about it.
 

riq165

Member
Apr 27, 2005
37
2
18
Receive Tone Modification:

Placing a 0.0047uF cap from R133 (the side opposite C26) to ground cleans up a lot of the white noise. I ran mine to the "non functional after modification +10kHz" switch so I could switch it in and out.

73

riq165
 
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DTB Radio

Well-Known Member
Sep 14, 2005
1,014
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Carlisle, PA. USA
www.dtbradio.com
Ok, I finally got around to testing the other two mods I posted. WHAT a DIFFERENCE!!! Between speeding up the AMC timing and boosting the mic preamp gain, There is a BIG increase in over-all perceived loudness. In my case, I used a 470K for the preamp feedback resistor (R181) mod, and a 4.7uf cap for the C158 mod. Going to as high as 1M for R181 would be ok. I think a good medium will probably be about 820K, after testing with the 470K value.

I also got some reviews from a couple of customers who bought faceplates for the 150 and 200, and they love them. The nightwatch panel actually makes the plates glow, especially in areas of lighter color, such as lightning strikes, etc. VERY cool effects - green-tinted lightning and green-tinted fire
 

960

Member
Aug 8, 2005
33
0
16
West Coast
DTB,

If the mod limiter is still intact, then these mods (R181 and C158) will bring the average level of modulation up, but these mods shouldnt induce any additonal splatter or such, right? I dont want to start tearing tv's up. I run this radio on high power and it is clean as a whistle.


I will do these and let you know..

Where are these faceplates for th 200gtldx from?
 

960

Member
Aug 8, 2005
33
0
16
West Coast
DTB,

Did the mods you described. (C158 to 4.7uf and R158 to 470k ohm). My Cobra 200GTL is bare stock. No mod limiter pulled, no pots have been touched or turned for power alignment.

I noticed the background noise (computer fans, etc..) in my room picked up (verifiying the mic preamp mod works).

It appears that when I talk the power stays up higher longer ie: if the radio was set on HI, it DK 40w and used to swing to 140-150 and back down fast (I figure this is the purpose of the AMC) and it would average talk around 70-80-90 watts pep if I was talking about 1-2 feet away.

Now it DK 40 still, the radio does not go as high as before (goes to about 120watts max (STOCK RADIO)) but the average talk watts it now around 100-120 and staying there unlike before.

The average "talk power" has risen and people say that my audio level is louder and bolder sounding (ie: more punch).

I do notice that the radio does not peak at a high power and come back down like before (example: on SSB it would peak very fast at around 120 and govern down to about 100 watts even. Now it goes straight to 100w pep) AM is the same. It appears that the 4.7uf cap has sped up the AMC circuit so far that it doesnt let the power go up and come back down like stock radio does and keeps the power at more of a consistant level.

Anymore cools mods or ideas, I will beta test them.
 

rachet_jaw

Member
Oct 8, 2005
58
1
16
mooresville, NC
Dag nab it!!!! You guys are gonna make be buy one of those 200 GTL's yet!!!!!

:wow

No.... seriously, great sounding radios, I would almost rank it up there with my 2970 DX 150, and I bet it does rank there with it with the DTB mod. Can't wait to hear one of them with the DTB Mod.
 

DTB Radio

Well-Known Member
Sep 14, 2005
1,014
69
58
52
Carlisle, PA. USA
www.dtbradio.com
I can't say this enough - NEVER remove the limiter from any radio!!!

That being said, no, these mods do not cause splatter. I found that after these mods were done, I could back down the AMC POT a bit to the point where the negative peaks just met, and the audio was absolutely beautiful. I could turn up the AMC a bit from there, but fidelity began to fall off a bit, although it wasn't horrible.

As for other mods, Thats pretty much all the radio needs. There might be some amp mods, but the amp in there is so clean I'd hate to even try to play with it. Maybe a couple 2879's and rewound coils, etc, but I won't try it, lol!

As for the faceplates, I get them from me. Thats one of my products for sale at the shop - custom-made plates for just about any radio, and quite a few amps (see the picture in my signature).
 

rachet_jaw

Member
Oct 8, 2005
58
1
16
mooresville, NC
King Mudduck said:
With all of the posts i have seen on the poor performance of the radios maybe the best mod to do would be to throw it out the door. :LOL:
I think a lot of the reports on poor performance is do to the fact a lot od"techs" out there do not like the SMT, do not know how to work with SMT, and just in general pretty incapable outside of the realm of Cobra 29.
Just MHO. :LOL:

You can look on here an see just as many negative reports on the Magnum radios, Galaxy, RCI, and so on.

Pretty much up to personal likes, expectations, and intended use. :roll:
 

King Mudduck

FEAR THE DUCK!
May 6, 2005
864
15
28
285 South Western Virgina waving!
True.I really think part of the problem with these radios is that most techs dont know how to work on them.

I have heard 5 of em so far.Of the 5 two were done buy the tech i use the other 3 were done by someone else.The ones done by the my tech had a good sound to them.Not loud like a Galaxy or Connex but not bad.The others sounded like total crap!
 

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