Cobra/Uniden 29LX BT no mic audio

[dss56]

Sorry - correct I did the red arrows the other day and since then I did pull tr18 and c 69

Look on lower right of schematic if I put my audio tracer on C74 the non grnd side on the GOOD radio great audio and I would say its amplified audio.

On the bad 29 onc74 non ground side the audio is low and a bit muffled. I would say like you said something is sucking the audio down. So all the red arrows including c69 have been either unsoldered from the board or replaced.

I have never come across this problem before and of course these are the worst type.

 

[guitar_199]

Let me think on this and look at the schematic a bit when I get home from dinner!

 

[dss56]

Thanks so much for the help and suggestions

 

[Unit 75]

Don't know if that arrow is pointed at C67 or not. If it is shorted, it will cause your problem.

 

[dss56]

I did check cap 67 also. I think im going to check all the audio parts in the audio chain and try 1 more time.

c74-R122-C68-C69-C125-C70-C71-D11-D8-TR17-TR18 GOING TO TRY THESE PARTS AGAIN

 

[guitar_199]

I have a couple of marked up schematic sections but I can't get them to scan in right here. I can scan and send them in the morning.

What I will say after looking at this......
Just above and to the right of TR18 is a resistor....R122 it is a 2.2K.
You should see audio there. Let me say it like this...
If you put the mic on the GOOD 29 and check on the RIGHT HAND SIDE of that resistor....the side TOWARDS the pot....and YOU SEE AUDIO....
then you should see it at the same place on the SUSPECT 29 too.
That point is way before the modulation control. Just be sure that you are on the right side of the resistor.....as it goes back to the Dynamic pot... NOT the side that goes to TR18. If you SEE audio THERE on the GOOD radio.....and DON'T see it on the bad one..... your problem is before this point.

Look back around the mic jack.....there are two caps.... C78 a 0.01 uF cap and a little lower down.... C74 looks like a 470 pF cap.....if either one of those is shorted or shorting...... they will kill your mic audio too.....before it ever gets up into the area around TR18. But after the audio leaves C74 the next stop is that resistor....R122... looks like a 2.2K.....make sure the audio is getting there.

I'll look at this some more later.

 

[dss56]

I have not tried the C78 will try in the morning, C74 i checked and seems ok not shorted to ground. C74 on bad 29 has low audio at this cap when I checked with audio tracer. On the good radio all the audio you want real loud on audio tracer. So maybe c78 could be the villian. I will check

 

[guitar_199]

Again, double checking my understanding....

I believe that you are saying that, on the GOOD unit... when you signal trace at C74 you are getting PLENTY OF audio.....

BUT

On the bad one... signal tracing the same C74... the audio is very low.

C74 is coming right off of the Dynamike pot .. top end (they feed the mic input in to the wiper!). If audio is low there.....but IT does not appear to be shorted.... that doesn't leave much.....

The Dynamike pot,
C78 shorted,
Connector is hosed up somehow..... OR
where the connector solders on to the board has shady solder joints!

Yeah, I would signal trace at C78 and if it is low there.....pull it out. If the audio comes up....THAT was the problem.

If it doesn't....back up to where the connector solders to the board and give a good visual inspection. Might be a bad solder joint there, or a bad wire connection going into the pin on the connector itself....

But this appears to be WAY back there......towards the mic connector!!

 

[guitar_199]

From the other forum....nosaJ saved me!!!!
He found one that is really close....
Cobra 29LTD BT
Got the owner's manual, bluetooth operation sheet,radio schematic and board layout, bluetooth board info....

and in nice shape.

 

[dss56]

hi all been sick for the past 8 days bad flu.
I dug more into the audio problem and still no good news. I found out that pin 2 on mic chassis mount does not go directly to the main pc board pin 2 yellow goes to a board that is connected to the front panel board.
I have no schematic for this board. I can't find the correct 1. I have been using the Cobra 29 bluetooth the closest to the LX. The cobra 29 main boards are about 99% the same.
I have replaced almost every part in the audio chain and still no results.
The board that has the yellow wire says MIC on the pc board so the mic audio travels through this board first. Also has 2 relays on the board which i hear no clicking at all when radio is keyed.

So I have decided to put it all back together and send back.
I have never had this happen before with a radio that has no audio and cant get the correct schematic.

Guess you cant fix them all

I want to thank you all for trying to help/much appreciated

Bob here in Boston Ma.

 

[guitar_199]

I'm sorry it comes down to that..... but without some info on that board.... you are in the dark. It probably is something to do with "audio intercept" to feed it to the Bluetooth logic and it is either bypassed (in CB mode) or NOT bypassed (Bluetooth connection paired or active!).

It would have been interesting if you could inject a tone where the wire leaves that little board and attaches to the main board... and see if things on the CB side start working. At least you'd know for sure it was that board.

Good work though......
Bob (in Houston!)

 

[dss56]

Hi Bob

Well I have not put the covers back on yet. I will do that tonight and inject a tone into that wire. That board I posted is flat connector to the face plate logic Board so there are 2 boards.

Yes without the schematic its not good. On a regular 29ltd pin 2 goes directly to the main board near the mic chassis mount.

And also I went through the menu and there is only an on/off for the Blue tooth. And its set to off.

So this is the latest cobra 29 model that is out there. And no schematic for it.
Interesting radio as the main board is all regular parts and the flat front board and the face plate board are all surface mount parts.

thanks again.

I have a sister in law who has a big farm in Houston and 1 near Dallas
Hope you were not near that tornado beginning of the week.

Ill try 1 more time tonight.

 

[guitar_199]

No sir. The worst I heard of was up near Dallas. I've seen pictures of a Home Depot with the ceiling caved in.

At least they should have everything they need to fix it!!!!!

Hi Bob

I have a sister in law who has a big farm in Houston and 1 near Dallas
Hope you were not near that tornado beginning of the week.

Ill try 1 more time tonight.