cobra 29LX no mic Audio

[dss56]

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

29lx audio prob_resized.jpg

[nosaj]

How about trying the simple stuff first? Like another mic or trying your mic on another cb?
Mic's in general take the most abuse as far as the cb goes, get thrown around dropped, etc etc.

nosaj
N1EBC

[dss56]

Thanks but the mic is in working condition tried on another radio and I tried another mic on this radio and same results i have to scream AHH AHH into mic to get the needle move just a bit on an external test meter. Now I have replaced c70 c67 c71 c72 tr118 tr117 check r122 so running out of parts to check

[nosaj]

Symtom make me think an amplifier transistor of some sort in the mic chain.
What do people on the other end hear if you talk normally?
nosaj
N1EBC

[dss56]

nothing no audio just a little buzz noise when mic is keyed monitored by a radio on the same bench good power out but not audio.
I replaced tr 17 and 18 same problem

[nosaj]

only thing I can suggest at this point is to inject audio at the mic connection and signal trace it and see where it stops.

nosaj
N1EBC

[nk14zp]

Dyna mike turned up? Dyna mic pot good?

[dss56]

Thats my next move. I have a cobra 29ltd similar chassis and was comparing and tracing some audio but will have to inject like you mentioned and see if I can find the problem.

When I key mic and monitor on another radio I get a hum and buzzing slightly. If I yell into mic ahh ahh i can hear on the other radio behind the hum and buzz. Well replaced and checked parts so tomorrow i will Inject.

[dss56]

Dyna mic is up all the ways up I will jump that out tomorrow also

[MDYoungblood]

Did you try a external speaker plugged into the PA jack to see if the audio regulator was okay before changing transistors?

3's

Greg

[dss56]

Hi Greg.

There is good receive audio good carrier 4 watts just very low mic audio out.
I was just in my shop and replaced tr 17 and 18 and did some signal injection and the audio chain seems to be low getting to the audio chip ic 4 TDA2003. Compared to a cobra 29LTD.

[dss56]

Checked the dynamic pot and its good. So I have checked almost every part in the audio chain and just cant seem to find the problem. I have been checking it against a good working 29 and big fat O. If I yell real loud into mic the test mod meter will go up about 1 /4 but you have to really yell ahh ahh ahh.

thanks so far for some suggestions

[Bobcat]

I didn't see an answer to this and it is a pretty important question in cases like this...

Have you put a speaker in the PA jack and tried the PA?

[Bobcat]

Have you got a scope or a signal tracer?

I'm taking a bit of a stab here.... looking at the schematic....
TR17 is the mic amp. TR18 appears to be the modulation control...

The Same audio chip serves the PA and the RCV audio so we have a good sign it is working.

What I am wondering is if something in the modulation control is turning TR18 on as it's collector will pull down the mic audio. I have not found it yet..... but the Mod Control may be disabled in PA .... so it would be interesting to know if it does work on PA.

Either way, you are going to need to do some probing in that area (TR18, TR17) because you have already changed them... and that wasn't it....
It would be interesting if that little C68 (.01uF) was shorted because that would take your mic audio down badly. It would be interesting to pull TR18 and C68... and see if audio gets in then. If so, either something is telling TR18 to kill the audio or that cap is shorted and killing it for you.

[dss56]

I didn't see an answer to this and it is a pretty important question in cases like this...

Have you put a speaker in the PA jack and tried the PA?

There is no pa port in this radio cobra 29LX BT

[MDYoungblood]

I didn't see an answer to this and it is a pretty important question in cases like this...

Have you put a speaker in the PA jack and tried the PA?

There is no pa port in this radio cobra 29LX BT

Sorry, I missed it is a bluetooth model, that in itself creates a new set of problems. Using the menu make sure all of the bluetooth functions are turned off, it is possible the radios is stuck on "Hold", (cellphone mode). After resetting everything back to default check to see if there any audio out of the "TalkBack"?

3's

Greg

[Bobcat]

Thats odd.....because the drawing up there..... has a CB/PA switch in it.... close to the jack.

[dss56]

Sorry I could not find the schematic for the 29LX BT.

The 29 chassis are about the same in the audio section.

The 29lx is i guess too new for a schematic to be out.

So Im using the next closest schematic, sorry for the confusion

[Bobcat]

Not at all! I can understand that, I just hope that the Bluetooth functionality DOESN'T cut in to this area... because we wouldn't know it if it did!!!

You seem to be finding the components on your radio.... so hopefully it is close.....

[Bobcat]

Looks like the other forum is down. How are things going on the 29LX ??????

Bob

Sorry I could not find the schematic for the 29LX BT.

The 29 chassis are about the same in the audio section.

The 29lx is i guess too new for a schematic to be out.

So Im using the next closest schematic, sorry for the confusion

[nosaj]

Maybe it is similar heres schematics for 29LTD BT. Tie in points should be similar.
[Please login or register to view this link]
nosaj
N1EBC

[Bobcat]

Thank you SIR!!!! I totally missed seeing that one and it's readability is MUCH better than what I had!!!

Thanks a bunch!!!!
Bob

Maybe it is similar heres schematics for 29LTD BT. Tie in points should be similar.
[Please login or register to view this link]
nosaj
N1EBC

[dss56]

Thanks guys I have been sick the past 3 days temp and bone and have no desire to work on it I hope maybe Sunday night might be able to get back into it.

thanks again for all the help so far

[dss56]

29lx bt.jpg

Here is a picture of the radio in question

[Bobcat]

I'll be waiting. Take your time and get well!