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Cobra 2000 GTL with audio issues

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Husker
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Cobra 2000 GTL with audio issues

Post by Husker » Monday, 15 January 2018, 11:13 AM

Good day!
I am looking for help with a very strange Cobra audio issue. I have a 2000, it has just been recapped and was running very well. Now for some reason the audio for no specific reason will mute and not come back, or come back for no reason.
Below is what I have done so far to attempt to troubleshoot his crazy issue.
1. The radio has all new electrolyte caps in it.
2. When the radio goes into it's flaky mode I have checked IC4 (voltage regulator) and all voltages are perfect. IC6 (audio chip) and all voltages are correct (I even verified I could use the PA and I could). PLL (pin 6) checked out correct as well.The issue sounds like when the radio goes muted as you turn the channel selector. When it is flaking out, if i turn the channel selector there is no mute or change in audio.
While the audio is muted i still can transmit fully and the S-Meters move fine.


So the issue looks like the muted audio when you change channels.
Before I take this very clean radio out back and beat it to death (just kidding) I am looking for any and all ideas please!!!

Thanks




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MDYoungblood
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Re: Cobra 2000 GTL with audio issues

Post by MDYoungblood » Monday, 15 January 2018, 14:32 PM

It could be possible the speaker switch is bad or dirty contacts causing this, just a hunch.

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Re: Cobra 2000 GTL with audio issues

Post by Husker » Monday, 15 January 2018, 18:47 PM

MDYoungblood wrote:
Monday, 15 January 2018, 14:32 PM
It could be possible the speaker switch is bad or dirty contacts causing this, just a hunch.

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Greg
Thanks Greg, but no. I even tried using headphones to bypass the speaker jacks. I tried a different mike, still the same issue. Pretty soon this awesome shape 2000GTL will be in the spare parts pile



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Re: Cobra 2000 GTL with audio issues

Post by MDYoungblood » Tuesday, 16 January 2018, 6:40 AM

Was it doing this before the cap change, possible bad (new) cap showing up. Did you change out the tantalum caps in the receive section as well, they can be replaced with electrolytic of the same value. Last check the wiring, make sure all the connectors are tight.

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Re: Cobra 2000 GTL with audio issues

Post by Bobcat » Friday, 19 January 2018, 9:06 AM

I don't know if you did the cap change or had it done.... but I'll say this... I hope that nothing upstream of the audio chip (RF strip, demod) has one of the new electrolytics in backwards. Sometimes, if the voltage is not too extreme, I think they will behave for a while... before they give up the ghost.

Let me ask this, when it goes away ... does it ever come back?? If so.... on it's own? Or do you have to PowerOff/PowerOn to recover it?




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Re: Cobra 2000 GTL with audio issues

Post by Bobcat » Friday, 19 January 2018, 9:11 AM

One more thing....

Do you have a scope available?
If not, perhaps you could find plans for an inexpensive little RF/demod signal tracing probe...

What I am thinking of is the ability to backtrace from the audio chip to the demod and on through the receive strip to isolate where you are losing the audio.


OHHH... and just for clarification... you say the S meter moves fine. Are you saying that, when it mutes... you can see the S meter wiggling with the incoming signal/audio??? Just double-checking.... if so.... the audio is getting IN to the radio.... we just have to figure out where it is going....




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