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Whats up W/ New Cobra 29

longhaireddwb

longhaireddwb

W9WDX Amateur Radio Club Member
Oct 8, 2008
555
30
38
Colorado Springs, CO
I had someone bring me a cobra 29 (dated Oct 2009) because he wanted an echo board put in it. No problems with the radio as it works like it should. I noticed two things I'm hoping you all might be able to explain to me.

First, If you try to put the cap/resistor mod in to lower the DK you end up lowering the AVG and PEP as well.
What is so different that makes this happen?

And Second, I noticed that it no longer has the 10 pin audio chip that is screwed to the chassis. Where did it go or what is replacing it?

I didn't dig any deeper I just went ahead with the echo board and he's happy but what the hell man?

Thanks for any light you can shed on this as I got a feeling I'll be seeing more of these in the future.
73
 

im not sure why you are having issues doing a swing mod to the new 29LTD's, ive worked on quite a few that were newer than 09 and have not had an issue with this mod.

the new audio chip is a TDA2003 i think, and its just a more modern part than the old 10 pin ones. they are soon to be obsolete if not already.
LC
 
As far as the swing mod, I'm using a 1000uf cap with the ground to the rear of the radio and a 100 ohm resistor and installing them in the JP36 position. I've done a bunch of these on the older cobra 29's with great results but it seems that the newer one just don't act as well.
Let me know if maybe I should try a different resistor and cap on this swing mod. Maybe that's the problem?

As far as the new audio chip, I hope your saying that the new chip is going away. I liked the older chip (TA7222) at least I'm used to it.
 
I tried to find a schematic to see what they did to tone this new audio chip down but could not find one. The 10 watt TDA2003 is a notable improvement over the 5 watt TA-7222AP. I suspect they have done a small modification to limit the current through the chip because it can operate into a much lower impedance.

I'll bet there is something in the DC supply line limiting the current or the modulation transformer is not loading the chip down enough to get any real power out of it. The 100 ohm swing kit resistor sounds way too high a value to me also. You'll probably have better luck with around 25 ohms.
 
I'll give it a shot. I got another radio here on the bench. Ill try a 25 ohm resistor and see what happens. I'll post up the results tonight. This time I'm looking at a 29 LTD Chrome and it also has the new style audio chip. 5 pins instead of 10.
 
I am having good luck with the following: a 470uf cap and a 68ohm resistor 1/2watt type. Dk is right at 2 watts, with good forward swing.
5y9a4e7y.jpg
 
Here's what happened.

With a 1000 uf cap or a 470uf cap made no difference in the output level.

As far as trying different resistors,
With a 100 ohm resistor it gave me a DK of 1.5 and avg of 4 and swing 16
With a 25 ohm resistor it gave me 3.5dk and 5 avg. and swing 20.

With the Jumper in I'm seeing 4 dk and 5.5 avg with swing 22.

I just don't see much help from this mod.
Lower the DK and lower everything else too.
I guess if you wanted to run an amp with it you could lower everything and be easier on the amp.
Not what I was hoping to see and not the same as the older cobras reaction to the mod.

Anyone else seeing this or got any ideas of whats going on?
 
the old audio chip is not coming back.
its an obsolete part, meaning its no longer being manufactured.

one thing to remember is that the "swing mod" does not increase the modulated output of the radio one iota.
not one bit!
niet, nunca, nani mo.

all this mod does is lower the deadkey of the radio while maintaining the modulated output that the radio already had.

without replacing the final transistor, or doing any other mods to try to increase the output, 20 watts of swing is about what i expect from 29LTD's.

some will do a bit more, but not too much more for sure.

when doing this mod, be sure to give the TX section a full alignment/ peaking before you do the mod.
tune the TX strip for max modulated wattage, turn the AMC all the way up, then do the mod.

also, the cap value matters very little. you could use 100uf with no problems.
i use 220uf because ive got lots of them.

im sure there are things that could be done to the new audio chip to increase its output, but ive not looked into it at all.

maybe Shockwave will look at a schematic and provide some tips/ ideas! :D

these days, most people that want me to increase the output of a 29LTD are asking me to put RFX75's in them.
that way i can give them a radio that will key at anywhere from 2-10 watts, and swing upwards of 60+ watts. (i usually set them for a 2 watt deadkey and 50 watts of swing. yes, its dirty, but they have competition amps and thats what they want)
LC
 
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The value of the cap will determine the cutoff frequency of the audio passing through it. Since the driver audio does not pass through the cap, the changes in frequency response are not as noticeable. However, if you listen closely with a good mic on the radio you will hear the a 100 uf cap has less bass than a 470 uf or 1000 uf cap.

If anyone has a schematic of the 29 using the TDA-2003 audio chip and not the TA-7222AP chip, please post it. I'd like to have a crack at it. I would check to see if they jammed any extra resistance inline with pin 5 or 6. If not, they just aren't matching it's 2 ohm output impedance through the modulation transformer.

I think the transmitter winding is just a center tapped auto transformer. That matched the 4 ohm output of the old chip but I think the feed from the new chip should be closer to about 25% of the overall winding. You could probably poke around with a good digital ohm meter on the ends of the windings and find a better match.
 
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Thank you Sonoma. I found this new schematic on that site for the 29LTD: https://downloads.cobra.com/CB/Models/29-SERIES/New Version/29LTD-New_Schem.pdf I'm not able to spot any apparent weak link in the circuit using this new audio chip. It's fed directly off the 12 volt line and the output goes through a 1000 uf cap directly to the modulation transformer.

If this chip does not produce the same audio output as the old chip, the particular radio may have other issues such as the AMC or final stage alignment. I say this because it should be working just as good as the old chip if it's using the same modulation transformer since that will determine the load impedance and how much power is drawn from the chip.

I'm confident more audio power can be had by retapping the modulation transformer to provide a lower input impedance to the chip. This chip can put out much more current than the old one and is being wasted by not matching the higher impedance of the transformer to the lower chips impedance.
 

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