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Cobra 29 LTD Classic Problems : Weak Receive, No Transmit

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  #1  
Old 06-08-2009, 03:46 PM
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Default Cobra 29 LTD Classic Problems : Weak Receive, No Transmit


Hello,

I'm pretty not new to CB radios, but I am new to fixing them.

I got a Cobra 29 LTD Classic off of a friend of mine. It powered up okay, the display only works on bright, but I didn't think it was a huge problem.

I tested it a spare antenna and with my existing car rig (Cobra 19 + Wilson Antenna). The Cobra 29 receives very poorly. Sound is very weak and full of static when I key. Even with squelch all the way off, and gain all the way up, I still can't get the speaker to produce any regular static.

Transmit is another story. When I key, I hear the pop of the key activating and my Cobra 19 goes silent like someone is transmitting, but it won't transmit any of my voice.

Help?

Thanks,
Richard


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Old 06-09-2009, 09:33 AM
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Sounds like it could be the audio chip. Can you test the PA function?

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Old 06-09-2009, 03:22 PM
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Quote:
Originally Posted by moleculo View Post
Sounds like it could be the audio chip. Can you test the PA function?
Just tested it, no output at all.

Bad audio chip then?

"NPC 9222
D2816C" - the audio chip?

Also, would it interfere with anything if I removed the internal speaker and replaced it with a small 12V-switched fan? I wanna peak&tune this unit to get a bit more "umph" out of it. I've been into computers for a while, and I know heat is the bane of electronics.

It's going to go in my 90' Audi 80 "Wintertastic Rallybeater" with a Wilson-1000 probably.

-Richard-

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Old 06-09-2009, 03:36 PM
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One other thing, the onboard meter does not work at all. Not sure if that affects anything?

-Richard-

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Old 06-09-2009, 05:27 PM
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Quote:
Originally Posted by office888 View Post
Just tested it, no output at all.

Bad audio chip then?

"NPC 9222
D2816C" - the audio chip?

Also, would it interfere with anything if I removed the internal speaker and replaced it with a small 12V-switched fan? I wanna peak&tune this unit to get a bit more "umph" out of it. I've been into computers for a while, and I know heat is the bane of electronics.

It's going to go in my 90' Audi 80 "Wintertastic Rallybeater" with a Wilson-1000 probably.

-Richard-
No, it's IC4 - TA7222AP or something similar. You don't need the fan anyway if you're going to peak it out - even if you do a more extreme mod like a 50 watt FET. The final transistor is mounted to what is effectively a big heat sink. If you really want to add a fan, you could wire it to 12v and mount it on the back/top, but run the power leads through a small ferrite torroid a few times. If you don't you'll get nasty motor whine in the TX/RX.

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Old 06-09-2009, 05:29 PM
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Quote:
Originally Posted by office888 View Post
One other thing, the onboard meter does not work at all. Not sure if that affects anything?

-Richard-
Fix the audio amp first, then try the meter. It could be out or just stuck. If it's stuck, carefully lift up the tape on the top of the meter, apply a tiny drop of oil on the screw and turn it carefully a little bit. If it still doesn't work, you'll need a new meter - they aren't expensive.

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Old 06-09-2009, 07:00 PM
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Quote:
Originally Posted by moleculo View Post
No, it's IC4 - TA7222AP or something similar. You don't need the fan anyway if you're going to peak it out - even if you do a more extreme mod like a 50 watt FET. The final transistor is mounted to what is effectively a big heat sink. If you really want to add a fan, you could wire it to 12v and mount it on the back/top, but run the power leads through a small ferrite torroid a few times. If you don't you'll get nasty motor whine in the TX/RX.
That took a bit of searching, weird hiding spot on the side...

Surrounding the TA7222AP is a bunch of white powdery crap. Solder joints are more "hilly" than the rest. Looks like it might have been replaced before?

Regardless, $5.25 part, going to go ahead and replace it anyways.

No fan it is then!

Thanks for the help, we'll see if that's the issue.

-Richard-

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