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Tried to add Talkback to Uniden68 and...

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CBAR815
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Tried to add Talkback to Uniden68 and...

Post by CBAR815 » Saturday, 11 July 2009, 21:54 PM

ended up with very little to no audio output from the internal speaker after adding a 100 ohm resistor across the pa and ext. speaker solder terminals. Could barely hear anything at all on max volume after this mistake. The radio is a Uniden PC68 Elite. I ended up fixing the problem but I'm still a little confused. I have come to the conclusion that I added too much solder and it went through to the other side of the board and shorted out the external speaker jack which also shorted the internal speaker. I feel stupid for making this mistake. After this happened I measured across the solder terminals of the 2 jacks with my ohmmeter and it was a direct short. I removed the excess solder after some serious cursing and tried the radio again and it worked like new. But when I measure across the 2 solder terminals I now measure 20 ohms and the radio works fine. I was under the impression that this measurement should be open. My Cobra 19 Ultra III is open, and from what I can see from a Cobra 25 LTD schematic, this measurement should be open also. Is this schematic close to the same as the 68 Elite? I am looking for some clarification on the external speaker and PA output jack circuits and trying to figure out what exactly happened and what I should do different next time. I know this is a simple circuit, but I have been very confused while trying to troubleshoot this problem even though it works fine now I still wonder what exactly happened and why. I ended up putting the radio back in the vehicle and being happy that it still works.


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Re: Tried to add Talkback to Uniden68 and...

Post by TheCBDoctor » Sunday, 12 July 2009, 6:50 AM

Hi CBAR815,

If it is working fine now I would not worry about it. You should be careful with solder bridges, you could have damaged the audio chip if the radio was something other than a Cobra 68. Idealistically the resistance measured across a speaker should read Zero. A speaker is a purely reactive component. There will always be some resistance across a speaker. The less expensive the speaker the less critical the resistance. Especially; in a speaker that is used strictly for audio.

Most speakers of that type measure around 4 to 16 ohms depending on the speaker. Reactance can never be measured directly. That is why speakers are measured by Impedance. That is the vectorial sum of the reactance and the resistance. Again, this can not be measured directly. If the radio audio sounds good on Transmit and Receive, then the radio is fine.

Respectfully,
Best Regards,

Rick, The CB Doctor


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CBAR815
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Re: Tried to add Talkback to Uniden68 and...

Post by CBAR815 » Sunday, 12 July 2009, 8:00 AM

Yes I have decided to leave it alone since it works fine now. Just one of those things that was aggravating and couldn't figure out exactly what happened. I still cant figure out why the resistance between the 2 solder points is 20 ohms and not open like the schematic shows. Thanks for the info!

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CBAR815
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Re: Tried to add Talkback to Uniden68 and...

Post by CBAR815 » Sunday, 12 July 2009, 8:21 AM

I just dont see in the schematic what is connecting the 2 solder joints where I am measuring 20 ohms between.

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Re: Tried to add Talkback to Uniden68 and...

Post by Texas Mobil 219 » Sunday, 12 July 2009, 8:27 AM

Have you tried an exturnal speaker yet? My 78 ltw sounded like that to when i had the TB installed. I just put an exturnal speaker in and it got loader and could hear better. Recieve and TB. Also. I had to turn my Mic gain down alittle to take out the feed back.
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CBAR815
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Re: Tried to add Talkback to Uniden68 and...

Post by CBAR815 » Sunday, 12 July 2009, 9:55 AM

No I havent tried the external speaker. I took the talkback out because of the problem. The problem was still there when I removed the resistor. I had to remove some excess solder to get it to work. It seems to me that there is still a problem in the circuit with the 20 ohm measurement. If you add a 100 ohm resistor in parallel with this, the resistance is still going to be less than 20 ohms, so I'm having a hard time understanding why it still works. Maybe I am thinking wrong.

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Re: Tried to add Talkback to Uniden68 and...

Post by rayray1963 » Sunday, 12 July 2009, 12:25 PM

I see you used a 100 ohm resistor but what was the wattage of the resistor? Should be 1 watt. Just a thought.

Ray
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CBAR815
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Re: Tried to add Talkback to Uniden68 and...

Post by CBAR815 » Sunday, 12 July 2009, 14:03 PM

Used 1/2 Watt.


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