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Audio chip HOT No TX and very quiet RX Cobra29

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07dodgeram59
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Audio chip HOT No TX and very quiet RX Cobra29

Post by 07dodgeram59 » Sunday, 07 August 2016, 20:32 PM

I have a newer Cobra 29 WX NW ST the audio chip gets very hot with in a few seconds of power on (TDA2003) you mash your speaker up to your ear you can faintly hear WX and static I've replaced the chip with new one no cigar any help would be great I have tried to replace a few caps along the schematic trail but no luck

-- Monday, 08 August 2016, 19:45 PM --

Yes I have tried different mic and no change same thing out of pa very little sound volume turned all the way up should hurt your ear drum with ear phones in you can barely hear anything on WX


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443 Arizona
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Re: Audio chip HOT No TX and very quiet RX Cobra29

Post by 443 Arizona » Saturday, 24 September 2016, 18:49 PM

do you have an antenna hooked up when testing,(or at all times when on?) i ask this because on that particular radio(im going to assume is the same as the smallermodel18,18II and 19with Wx) the pll gets locked up and will exhibit those symptoms. i dont know but i suspect lack of internal sheilding, how are the external pots ?(another area of concern on these). also, it may take awhile but it may have to be re-aligned. their a very touchy unit and if anyone has screwdrivered it, that may be it.

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sonoma
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Re: Audio chip HOT No TX and very quiet RX Cobra29

Post by sonoma » Saturday, 24 September 2016, 21:43 PM

check for a bad cap at the audio IC. C48 can cause you a problem on the cobra 29 radios. I think that is the right one. doing this from memory. it is between the transformer and the audio IC.

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pabxone
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Re: Audio chip HOT No TX and very quiet RX Cobra29

Post by pabxone » Sunday, 25 September 2016, 4:02 AM

A diagram for Cobra 29 's with the TDA2003 audio amp is here :- https://downloads.cobra.com/CB/Models/2 ... ematic.pdf" onclick="window.open(this.href);return false;
Specs for the TDA2003 is here : https://downloads.cobra.com/CB/Models/2 ... 03data.pdf" onclick="window.open(this.href);return false;

Since the TDA2003 audio IC is getting hot one of two things is happening , either :
1/ The output of the IC is being shunted to ground (most likely)
2/ The input voltage to the IC is far too high (less likely)

First thing to check is that the TANG (Metal mounting tab) of the TDA2003 IC is not grounding to the chassis or PCB ground of the radio.
Is the insulating mica washer in place and intact without any damage?
Is the correct screw and insulating washer being used on the IC? - It will either be held on with a PLASTIC screw or a metal screw with a plastic washer to insulate the tang from the chassis.

The tang is internally connected to pin 3 of the TDA2003 - pin 3 is ground for the IC but all grounds are not the same in the radio and you do NOT want PCB ground and Chassis ground being connected together via the IC.

If that is OK , it could well be a shorted electrolytic capacitor - C44 is the most likely candidate for this problem as it goes straight from the IC's output to PCB ground.
If C44 is OK , check C48 , C42 , C73 , C35 , C39

Pin 5 on the TDA2003 is the supply voltage to the IC - looking at the diagram it is getting full 13.8 volt supply via the on/off switch so it's probably not worth checking assuming your power supply is only putting out around 13.8 volts ?


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