The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

Hello I am new to the forums, and new to CB radio as well. I have some cheap equipment and I am a college student who is tight on cash so not many upgrades available. However I have dropped $60 on a Cobra 29 Ltd Classic and I have two 5' fiberglass whip antennas leftover from when my dad was an over the road truck driver. I do have a decent SRW meeter that
works and have a SRW of 1.2 on ch 19 and 1.5 on ch 1 and 40. Ok that is my info about my radio set up now here is my question.

I would like to mod my Cobra 29 ltd classic some to get better performance out of it. I have read everywhere to cut D11 or adjust VR 4 I would love to know more about these things before I do them. Mainly what does it change and which way to adjust it. I have already found them both on my board. Any help you are willing to give me is greatly appreciated.

the cutting of D11 and ajusting VR4 is for modulation. don't cut D11 just turn VR4 all the way clock wise and if yougot a good power meter you can tune L14,L13,and L12 for more RX power.but don"t try tune ing without a meter.

Thanks for the info. I do have a power meeter that works and is accurate. when tuning L13,L14 and L12 I am assuming I turn those clockwise until the power starts to decrease then go back up to my max power setting. I am also assuming this is something that must be done while the CB is on. If any of these assumptions are wrong please correct me.

Ok here we go.

1st. Speak into your mike while you are tuning the cans. Adjust them for the most forward power. I say Hell-o jell-o audio

Now, some of the coils are horizontal, and you can benefit by spreading them out.

There are good well written instructions on this site for getting those radios to slam. Those are some of the best AM boxes around.

If you can solder you're in the game.

Also, you can clip the diode if you want. Now, if you are going with the
"Super Swing" or super mod. idea, you just have to tune it so your carrier isn't so low that your audio is gubbered.

Here is a link that should keep you busy.
http://cbradiomagazine.com/March%202007 ... cation.htm
- - Adam

I really don't know how to tune my radio all I have is an SWR meter which includes a power meter in it. If you could be more specific about that process i would greatly appreciate it. Also those horizontal coils how much should I spread them? I have done most of the mods that involve the VR1-4 and L13.L14,L12 and i picked up the parts to solder in like that site suggested but I am going to brush up on my soldering before i do it. i also did do the talk back mod that was in one of the old archives on the site Thanks that site is really very helpfull

Ok, go back to that link. Now scroll down until you see the 2nd picture.
Now at the very top you will see the end of that coil. There is a ceramic cap to the left of it. That is the coil you want to spread open. stick a screw driver into it and pry it gently aprat. into like 4 sections. You will see the effect of this emmediately so just watch your power meter while you're wistleing.

If you do that cap and resistor mod, you will have a smile from ear to ear on you. That should get you motivated.

Also, if you're up to it, buy a 2SC1969 transistor and replace the final. the stock final is a 2SC2078. If you put a 1969 in that puppy. You'll be loud and proud.

Well, you'll be loud and proud without it, the final has absolutely nothing to do with the modualtion circuit.

Good news and Bad news... Today I did that "super swing" mod the problem is I tried all 4 resistors(100ohm 1/4 and 1/2 watt and 200 ohm 1/4 and 1/2 watt) and both capacitors (220 uf 16v and 220 uf 25v) that they suggested needless to say I spent a lot of time soldering here lately. Each time it lowered my effective range and wattage so what am I doing wrong am I missing some critical step? I know my power meter is correct I have double checked it against one that I borrowed from a friend. also expanding that coil did not help any and after removing the "super swing" mod and putting a jumper back in and adjusting L14 I am making 4.5 watts.

Oh the plus side I have learned a lot about my radio and the talk back mod I installed works like a dream after I switched from a 100ohm 1 watt resistor (like suggested) to a 100ohm 1/2 watt.

Again I would like to thank you for you help but it seems I am not doing something right and I just can not figure out what it is.

Ok well does your meter read PEP and average (RMS) or just average.
If there is no switch for PEP then it's an average meter. and your not going to see much a differenc on it. In that case after the mod is in (The cap%resistor) tune the cans for the most forward power on your meter, swing.

That mod works without doubt. Even with the stock final.

Well I tried it one more time with a 220ohm 1/4watt resistor and this time it works!!! When I transmit it reads 9-10 watts. I tried spreading that coil again and I find if I push it together I actually gain about a watt. So the mod is working like a champ this time... now what were you saying about clipping a diode? oh about my meter it just reads average it is an antique that my dad had laying around here is a pic of it.


it is a little fuzzy (my lens is dirty sorry)

Sounds like you had the cap in backwards. And that is nice looking meter. Probably better then the garbage people sell today.

Now is that 9-10 watts when you whistle or just dead key?

Hopefully when you whistle.

Clip D11 it's a small signal diode near the tx. red glass with a black band.
Also, trace the power input +12 volts through the tx (Well it's a choke) and then into a black diode. black with silver band. Jump it with a piece of wire. Across the solder side of the board. This will increase your PEP power quite a bit. That. .6- .7 volt drop across it really kills your headroom.

So jump it or pull it out completely and replace it with a jumper.

Yes the 9- 10 watts is when I whistle into it. Also I would like to say thank you for the information. I have heard bad things about cutting D11 and as I have said earlier I don't think I am going to cut it I might just remove it to see what happens but I don't want to ruin a good diode I have to keep things cheap you know

Would appreciate it if you could tell me a good spot to look for a new final to replace the stock one unfortunately where I live down here there is not a local CB shop for me to go to so I have been forced to Radio Shack for all of my parts to date this is something i am not happy about.

I am having a little trouble following what you mean here

Also, trace the power input +12 volts through the tx (Well it's a choke) and then into a black diode. black with silver band. Jump it with a piece of wire. Across the solder side of the board. This will increase your PEP power quite a bit. That. .6- .7 volt drop across it really kills your headroom.

I understand the idea that jumping will increase the power transfered but where am I jumping from (the TX?) and where am I jumping to, (the black diode?) I would like to think I am not an unusually dense person but you completely lost me with that one. So here is my thought on clearing it up instead of you trying to type out what you mean I am going to take a picture of the board and post it on my photo bucket account and if you are willing I think you can download it from there and draw on it with paint and show me exactly what you mean... it is an idea it might make things easier or it might not I don't know. Anyway my photo bucket account is
http://s164.photobucket.com/albums/u30/turkish1098/
I am going to put that up right now. If you don't feel like messing with it I don't blame you and I really appreciate all the help thus far. Man it is hard to find information about this stuff on-line.

Well I just looked on my 29ltd schematic. It's Diode 8

D8 now you can check by measuring the voltage to the input and output.
Also, looking into the parts side of the rig, front towards you. It will, and should me in the back right hand area.

D8

As for photo bucket. I can't do much with that because I don't know shit about computers. I can surf the web though.

RF is my gift, not puters.

Have you talked on your newly liberated 29. How do people say it sounds. avoid the hacks on 19 for feedback. talk to some people you know preferably. The stock mic on that rig should be fine really.
But there are a few more things you can do to it to make a bit better. A little deaper audio, more volume.

Well if you run into computer problems let me know I build them from time to time and I frequently fix them for friends. And if I don't have an answer I know some guys that will anyway D8 got it so I should just jump that from one side of it to the other then right.

Yes. It should look like a normal diode. Black w/silver band. This will increase your PEP power, so you may not see much a difference on your meter, but every little bit counts when your dealing with a low power set up.

For a guy who doesn't do computers, I know allot about their power supplies. Most computer techs are great with software. SMPS' are a real treat to trouble shoot.

About your signature. I always say "If at first you don't succeed. Apply more force"

Make sure this is not the polarity protection diode. But that should be up on the power plug receptacle.

I have an uncle who is a mechanical engineer his motto is "Beat it to fit. Paint it to match." But I always thought my sky diving one made damn good sense

oh yeah I jumped but I am not going to test it until tomorrow. If there is anything else you can think of off the top of your head to help let me know.

Let me know how that works for you.

It seems to have made a difference, but as you know my meter cant pick it up so I have to get a better meter before I can know for sure.

Where can i get a 1969 final for this radio?

MCM electronics. Eaby. Copper electronics, just google 2sc1969. This way you'll find it for less. CB dealers are going to rip you off.

I installed the 1969 final and not the radio does not work... i did not scorch the board and i hooked it up just the way the other one came out... when i switched back to the old final it does not work either? i am confused as all hell any ideas what is going on...

IS your rig on PA?

If not, it sounds like you lifted a trace somewhere. The fact that the old final doesn't work either is a good sign.

Are you sure your cap and resistor mod is still intact?

Nope not on PA it does not even have lights when i flip the switch... and yes the cap and resistor is still in place

Is your power supply producing voltage. Not on short circuit protect. Or bown fuse.

Check the DC resistance accros the hot and ground pin with the switch turned on.

Ok, I have steady current from my power supply. However when i connect it to the radio i end up with a pulsing current not a steady one and when i have the mic pluged in it makes a thump sound in the speaker that goes along with the pulse on of the power...

I should mention that the above situation is when the radio is ON if I turn the radio off I have power coming straight through no problem (like normal)... man am I confused by this one oh one more thing i noticed that there is a little piece of like fiberglass or something under each chip mounted on the side of the box... each chip that is except the final i just put back in and i can not for the life of me remember if there was one under it to begin with.

LMAO.

Those are insulators. And yes you need to use them. The back of the transistor is metal, and it's common with the center leg of the transistor.

Meaning it has to be insulated electrically from the chasis. If you did not use these when you changed the finals in and out, the finals no longer work, and that is why you are having problems, you blew them up.

I did that once as well when I was just learning, it happens.

lol so i need to order a new final and put the insulator in it then i will be good

By the way where can i pick up some of those insulator strips or what can i make them out of bc i certainly dont have the one to go behind that final anymore

You can order them when you buy your new finals.

You have to make sure your screw that holds the final to the heatsink is insulated too. It has a little bushing that goes on the shaft and seats on the head of the screw.

ok well I cant get them from the guy I am ordering my final from I cant find them... going back to me being cheap is there something I can make them out of? I can get my hands on about any matterial like fiberglass and stuff what are they made of and what exactaly are they called... sorry for the noobish questions but if I dont ask them I will never find out the real answers.

Send me your address and I'll mail you some. You have to use the real thing, because they are thermaly conductive.

They electrically isolate, and thermally couple. Without the right shit, your final will melt down.

Thanks i just sent it to you on your private messages if that does not work or if you have trouble accessing it i will post it on here.

I'll plop 'em in the mail tomorrow.

well it has been a while (sorry girlfriend left for London so i was busy for a few weeks getting her time in) ok well here is the thing i have installed the new Final using a semiconductor... i never received the ones you sent but i scrapped an old radio that i got at auction for a dollar and stole a few from that... but now when i key up i have no TX power the light comes on but the meter shows nada... any ideas did i blow something else by being an idiot?