Cobra 29LTD Chrome need Schematic.

[dss56]

Another 29 bites the dust.

Customer brought in a Cobra 29LTD Chrome.
Radio does transmit and receive.
The receive is very noisy lots of static when people are talking.
When channel is silent from talking there are pulses of static in the audio.
Also the rf gain does not fully work
it will work a tad but when turned totally off the radio is still making static noise in receive.
I put a regular 29LTD classic beside it and receive is nice and steady and rf gain will turn receive
almost off to silent.
The radios are basically the same but there is a difference.
That is why I'm looking for a chrome 29LTD schamatic.

Any suggestions on the static riding in on the receive very erratic like a loose antenna connection.
I checked the board for cold solder joints. All look good.
Mic is ok /output ok/audio ok/ radio has builtin Talk back thats ok/

 

[kaos513]

cb tricks has the schematics

 

[sonoma]

if you do not find it on cbtricks here is cobras web site for all the 29's they have posted

http://downloads.cobra.com/CB/Models/29-SERIES/

 

[nomadradio]

You don't hear this noise in the transmit signal, do you?

If you do, this points to a resource shared by both transmit and receive, like maybe the PLL or the audio chip.

If the transmit is quiet, this narrows us down to a signal stage in the receiver.

Does the S-meter kick about in step with this noise?

73

 

[dss56]

The output transmit is quiet its just on the receive mode noisy and static. Radio might of been in a shop before as some solder joints dont look factory. But I checked them all and seem ok


And again the rf gain in front will not control the signal coming in. It will just a bit but will not quiet the receiver like the 29 ltd I have sitting next to it

Signal meter works ok with the same s units on the 29ltd next to it

 

[nomadradio]

Sure does make it sound like a radio that got slammed by a strong transmit signal nearby.

Diodes D1, D2 and D3 can be damaged by a nearby radio with a lot of power. A driver who pulls up at the truck stop alongside a truck who keys up his BandSmasher 5000 can suffer damage to the receiver front end.

TR6 doesn't go bad very often, but it can do this as well.

And there are two electrolytic caps in this circuit, C11 and C12. Again, not a common failure, but on the list of suspects just the same.

73

 

[dss56]

The customer is a truck driver and He bought the radio brand new and then this happened. I will check those parts later tonight.

thanks Bob

 

[dss56]

Nomadradio

I replaced the 3 diodes and the 2 caps also tr 6 and still the same problem.
I checked the pa good clean audio
incoming audio has some static with it

I have a mike plug with a toggle on it to switch 4 pin mic radios to transmit or receive.
So I have the chrome 29 and a regular 29 on my other test bench about 8 feet away.
On the regular 29 I have the toggle mic plug and I key up the radio with a dummy load and the chrome on same channel 1. The audio should be total silent but the same static sound is there like the receive with no close keyed up radio.
There is something bad in this radio and I have been at it about 3 hours replacing the parts Nomadradio recommended. I put both radios side by side almost the same and did some voltage checks. Can't seem to come up with the problem of this static on the receive. Its not the audio switch as I put it in pa mode wit an external speaker and is good. Also radio has talk back and works good clean audio. Something is not filtering out the static noise. I'm going to try and upload 2 files if I can on here

I guess I cant upload a mp4 audio file here

anymore suggestions

 

[Handy Andy]

I'll take a stab at this...

The RF Gain control - does it control the level of static or just stays at the same volume?

You said "little bit" so there is some effect but this may be related to an overload condition or a poor tuning job...

Because...

• Scratchy audio but able to control the signal strength, HOWEVER not it's noise - indicates an IF or further down stream from the RF Amp. Means RF Gain works, just not able to remove noise.
• Scratchy audio and not able to control signal strength means the feedback loop from the rear of the detector - is not looping back towards the RF amp - and you have no control over any signal or noise.
• One good thing is PA and TX sounds good and not scratchy or noisy indicates the problem is isolated to just the RX strip.
• Every little bit of evidence you can give us in this trail of bread crumbs helps us to help you...

The 29LTD Chrome is not that much different than a regular 29 LTD, but there is a Board number silkscreened to the PCB board on the component side - in the little "depression" area where the speakers' cone resides when it's all put together. It usually KEPC (KOREAN ELECTRONICS ) with a hyphen along with a 3 or 4 digit number...



We can help you more if we know that number, for there are several "variants" of the LTD floating around and the values used in critical areas are different than others.

Now, why this comes up is due to some caps that are critical to the RF gain function and uses power from the incoming signal to flow across them and the output is a type of voltage the radio can use to control the level of amplification the radio uses. This is a feedback loop process, AGC is what I'm getting at.

 

[dss56]

Hi Andy here are some pics of the pc board 29LTD chrome someone wrote black marker over marking but you can read KEPC-1106-L2 10 APR. 2015.

I was just gonna post this and saw your reply.

Anyone out there have any suggestions where to look next as I have replaced d 1- 2 -3
tr6 and C 11 and C 12. As nomadradio suggested

I was going to change out TR 1-2-3-4-5 next Noise Blanker stage and see if that helps.

Again the rf gain does not seem to work it just lowers the signal a little does not work like a regular 29LTD.

Any more suggestions welcomed.

 

[Handy Andy]

I did notice your VR1 - there is a "gain adjust" for the IF - that VR1 controls IF gain.

Have trimmed the Receive "quality" using that control yet?

For the rest of us ...

The KEPC 1106 - L(evel) 2 board is not the original Chrome one - this one supercedes that one and uses the 24-Pin that replaced the original PLL 22 pin 2816.and note the "29LNW" Nightwatch boards "compatible" if that makes any difference.

This board is the revised NightWatch boards - they made some changes to the PLL and Channel selector.

Note, they also don't have a Rotary Channel Selector - it's now an encoder...

The KEPC board, if you can locate schematics, uses a newer board #1140 is about as close as I can get...

Do you have the YOUDA Audio Chip in there? Else it's the TDA2003 - so by symptoms and since clarified, your RF gain works, but static and lack of volume control for it- refers to something further downstream or towards the Audio Chip - can be one of those "Green" mylar Chiclet caps - heat the solder up on the foil side of these kinds of caps - if the noise goes away and returns after cooling - then the cap needs replacement- it's gonna be a noisy fix, but I can pretty much presume that one of the many green caps in the audio line, is what is making your noise - for it's is past the Volume control and if Squelch can't tame it - it localizes it further to TR13 and then onto the Audio Amp. C37 (Yes it's the Mic amp but the line ties into the Audio feed-line as well as C41 and C38 and C39.
and its' past TR13 and into Audio Amp chips' main audio line).

Each and nearly every board since the KEPC-060 - uses the same schematic, but only extras are for the communication display items - ANT Warn ST light - stuff like that - you don't have TR25 in place but that's for another model of this which is the BT model - uses TR25. They started revising the Audio Amp section when the Chrome came out with it's Youda chip. They now use the TDA2003 chip.

Not that there is a big difference but this board is using the newer PLL- so you may be on your own - so look for soldering joint problems below on the foil side and it can be something as simple as a cold soldering joint on the RX side. And those Green Chiclets....

Gosh Darn It...

 

[sonoma]

down on the bottom of the list here is some info for 2 of the newer radios Andy. see if one of them match up with his radio.

http://downloads.cobra.com/CB/Models/29-SERIES/

 

[dss56]

Andy back up from shop and I replaced c37-c38 c39 and tr13 still the static problem.
You mentioned c41 and I could not find it maybe you meant c40. So the static sounds like frying bacon crackling noise.

You know when some one throws a dead carrier with no modulation dead quiet.
This static comes in with the carrier and is all crackling.

so replaced D 1 2 3- c38 c37 c39 Tr13.

Also did the heat on all the green mylar caps in the audio area and did not change the static.

Appreciate your time with this problem.

And this does have the TDA2003 audio chip.

Im about to give up as these type problems are a pain to track down

 

[Handy Andy]

Im about to give up as these type problems are a pain to track down

I'm not, and I get paid when I don't - because if I gave up - I'm not getting paid for jobs like this.

The scratch can be a simple reflow of solder onto a tightening of a screw.

The "Crackle" can be many things and thanks for checking what I normally would just remove and replace as part of the job.

So since the crackle occurs - like you'd hear in a dead carrier - how is the mike socket and mic wires?

Again thinking an open ground (mini-antenna), poor soldering or even broken wire, we know it's in the audio chain - for if it wasn't - it'd make the RF meter swing like crazy - you'd see it. Even the sockets for the EXT and PA speaker jacks can cause similar problems.

 

[dss56]

Andy here a youtube post of the sound If you see the meter its up from the dead key from another radio with no modulation just a dead carrier.

Here is the second with just receive if you see the meter jump with the static on the meter

thanks Bob