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Cobra/Uniden 29LX BT no mic audio

dss56

Active Member
Sep 27, 2010
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I have here a cobra 29 LX BT radio that has no mic audio good receive audio.

I have to scream into mike to make the needle move up a bit on my test meter.

So far I replaced tr 17 and tr 18 the same
replaced c71 and c66 the same.

here is schematic section of the audio

Any suggestion appreciated
29lx audio prob.jpg
 

Ahhhhh.... so you have not looked at C69 yet? (I think I said C68 on the other forum...but I could see it better here!)

My bet... or my path......remove TR18 and C69 both....turn it on and see if you have modulation then..... youve changed the transistor so that wasn't it....

If the audio on the collector of TR18 is getting killed there....either something is telling TR18 to squash it.... or....C69 may be shorting/shorted sinking it to ground.

If removing those two doesn't bring it back.... I know I would want a scope or signal tracer.... because the audio just isn't getting in from the mic to the audio chip which we know works.... as you have receive audio..... that doesn't leave much.....but it may be tough without a way to test components on the path....
 
Good. I don't have it all down pat just yet (and I am at work right now so can't dig in deep!) but..... the way that TR18 circuit works...... basically.....

If you look to the right of TR18 that is all the bias circuit for it. It should be set up to bias it to where TR18 is off.... or only just BARELY on...

(by on, I mean the top lead of TR18 ... the collector... is internally dragged closer to the emitter....which is ground. So the way TR18 works is... when it is OFF, the collector is essentially floating... so the audio on that mic line above it is left unmolested... and allowed to pass through to TR17, the mic amp!)

Back to TR18... if you look just a little more to the right... you will see a diode (D11) and a cap (C72). Those actually form a little demodulator that takes in a sample of incoming RF and converts it to a low pass AUDIO representation. The BIGGER that signal is... the more it turns TR18 ON..... pulling the collector DOWN towards ground.... to knock the modulating audio DOWN. Think of it as an "audio regulator". When the demod audio gets bigger, TR18 comes MORE ON and tries to pull it down. When it gets littler....TR18 turns MORE OFF.... trying to let the mic audio rise up. TR18 is not biased to act like a switch... (meaning it is either ON or OFF) .... rather it is like a variable control....(meaning it turns ON little by little and turns OFF little by little). All of that bias goop to the right of TR18 should be working to make TR18 change to hold the mic audio down....thus preventing overmodulation. Right above it....VR4 (500 ohm linear pot) should be the modulation control setting.

If anything in that chain is wrong....it may be causing TR18 to turn on and stay on... all of the time..... which would keep your audio down..... ALL OF THE TIME!

As mentioned that cap C69 that ties on to the collector....could also be shorted. That would have the same effect...because it's other side is ground. So...if it is shorted.... it will pull that mic audio down too.

Good luck.... see what you can find out!
Bob
 
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By the way..... THAT is why I suggest pulling out TR18 AND C69. If you pull them out..... ALL of the REGULATION is "disconnected". If they aren't there...then they can't POSSIBLY be pulling the signal down.

When you power it up..... you are operating without any modulation control..... BUT..

.if it WORKS... you know the problem is one of those two things....

either C69 shorted.....
or something in the mod control detection is TELLING TR18 to limit you.

otherwise, if it DOESN'T work..... something is wrong in the chain from the mic audio pin, to the Dynamike pot... up to TR17 that is not allowing/amplifying the mic audio that it feeds up to IC-4 pin 2. And you KNOW that works (the audio IC) because you have "receive audio"!

"Divide .... and conquer!!!"

Good luck.

I will check that in the morning. tr18 and c69

thanks will update then
 
guitar-199

I did remove the tr 18 and c69 and still low mic audio. I have been comparing with another 29ltd.

I put my scope on middle of working 29ltd and on transistor 17 there is low meaning no amplified audio on the middle leg of tr 17 on the 29LX there is no audio at all.

Im going to make a youtube video and post here later today.

This radio has no PA it has blue tooth and talk back.

the talk back of course is also low as its not picking up any audio with mic audio being low.

thanks for Helping
 
I'll watch for that...
I think I am missing something.....

Did you say that on the WORKING one....

"I put my scope on middle of working 29ltd and on transistor 17 there is low meaning no amplified audio on the middle leg of tr 17"

this sounds like you are saying that there is no audio on the base of TR17 on the WORKING radio? Is that correct?
 
The working one has audio at tr 17
I'll watch for that...
I think I am missing something.....

Did you say that on the WORKING one....

"I put my scope on middle of working 29ltd and on transistor 17 there is low meaning no amplified audio on the middle leg of tr 17"

this sounds like you are saying that there is no audio on the base of TR17 on the WORKING radio? Is that correct?

the non working has nothing and I checked surrounding parts
 
Just adding a little knowledge to my side here......

So, would you say that, with TR18 and C69 removed......
You still see no audio at the base of TR17??? If so, the problem is before that..... like.. back down towards the Dynamic pot. Perhaps PC board foil, or a corrosion issue,
bad solder on the pot.....

Again, I remind you... I can't see what you are seeing.... so I am kind of "guessing out loud".

If I bother you.... let me know.... I'll shut up!!!! :)
 
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Guitar-199

I know you cant see whats going on im going to try to do some utube videos later and post tonight
 

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