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Cobra 29 NWST no transmit audio

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koolaid_kid
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Cobra 29 NWST no transmit audio

Post by koolaid_kid » Monday, 23 November 2009, 12:39 PM

Hi all. Just had one handed to me that has no transmit audio.
It'll key up, and receive, but no transmit audio at all. will not even show modulation on the service monitor.

The radio seems to be stock, ecxcept that the modulation limiter was cut.

Any suggestions?

Thanks


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koolaid_kid
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Re: Cobra 29 NWST no transmit audio

Post by koolaid_kid » Monday, 23 November 2009, 12:49 PM

koolaid_kid wrote:Hi all. Just had one handed to me that has no transmit audio.
It'll key up, and receive, but no transmit audio at all. will not even show modulation on the service monitor.

The radio seems to be stock, ecxcept that the modulation limiter was cut.

Any suggestions?

Thanks



I forgot to add that I was using a known working mic at the time, and still no transmit audio whatsoever.

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Re: Cobra 29 NWST no transmit audio

Post by TheCBDoctor » Monday, 23 November 2009, 12:59 PM

Hi koolaid_kid

Key up and take a wet finger and run it across the audio chip. If the meter shows an indication of movement, then it is possible that a 10 volt cap in the audio transmit circuit is shorted. This is a common problem with many radios with a 10 volt decoupling capacitor on the 8 volt DC supply.

The receive audio takes a different path at the audio chip input. You can measure the caps in the circuit. If you find one that is shorted replace it with 16 volt to prevent it from failing again. I don't have a schematic in front of me at this time. I have repaired a number of 29s with this problem. It happens most often when the limiter is disabled.

Good luck,
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Rick, The CB Doctor


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koolaid_kid
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Re: Cobra 29 NWST no transmit audio

Post by koolaid_kid » Monday, 23 November 2009, 14:01 PM

Ok, I will try that.

i have another one that is on the bench right now that was working fine. i installed an echo board in it, now the echo board works fine. Tehere is good strong modulation into the service monitor, but the output is only 0.01 watt, and the needle will not move at all. What the heck did I do? Note. I reversed the echo mod, and still no output. I haven't pulled the final yet, but this is my next step.


School me real quick on this Doc.

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koolaid_kid
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Re: Cobra 29 NWST no transmit audio

Post by koolaid_kid » Monday, 23 November 2009, 14:33 PM

koolaid_kid wrote:Ok, I will try that.

i have another one that is on the bench right now that was working fine. i installed an echo board in it, now the echo board works fine. Tehere is good strong modulation into the service monitor, but the output is only 0.01 watt, and the needle will not move at all. What the heck did I do? Note. I reversed the echo mod, and still no output. I haven't pulled the final yet, but this is my next step.


School me real quick on this Doc.


Nevermind Doc... Popped a final. How... I'll never know. motorola service monitor with 125w dummy load, good short cable (8 ft). Must have just been it's time.


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