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Cobra 29 LTD Classic. low volume.

Discussion in 'General CB Services Discussion' started by Low_Boy, Apr 22, 2018.

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  1. Low_Boy

    Low_Boy Sr. Member

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    This started out as a great receive but no sound. Audio amp good, but bad C123 near T2. Now the volume is barley heard. PA Full blast works great (even before the no sound). If I turn up the signal generator I seem to loose volume. The squelch seems to work in one spot only but I can not jump it to test it.

    So I have checked many parts all check good. How do I check T2 transforme? I know C123 ties directly to it so possibly the shorted Cap. has something to do with the two parts.

    Shouldn't I be able to jump the squelch or am I doing something wrong?
    Also rf gain has nothing to do with this problem. It will make the receive signal on the meter go up and down.
    Thanks,


     

  2. kaos513

    kaos513 Sr. Member

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    does your radios light dim when you key up?
     
  3. Low_Boy

    Low_Boy Sr. Member

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    no. dimming
     
  4. kaos513

    kaos513 Sr. Member

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    k so no power issues means a good sign, might want to check recieve diodes
     
  5. Robb

    Robb Yup

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    If the PA is loud but the receive volume is weak; then you know it is a problem with the receiver and not the audio amp. Receive might be out of alignment; or the AGC circuit may have a problem. Just first blush guesses.

    You might try the static method of aligning the receiver and see if receive comes back up - for starters. If it is an older radio and you already found one bad cap; then there well may be more bad ones.
     
  6. nomadradio

    nomadradio Analog Retentive

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    I'm trying to get a handle on the sequence of events here.

    There was no audio, but that's fixed now.

    Before the auido got fixed the S-meter would show strong receive signals with no audio.

    But now that the audio is fixed, the S-meter won't show anything.

    And that's what I'm wondering. Did the S-meter's readings fall off when the new problem started?

    73
     
  7. Low_Boy

    Low_Boy Sr. Member

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    The S meter is still working and never stopped. there is super low receive sound. good meter reading. I changed the volume/squelch switch. same problem. The sound does not get louder when the volume is turned all the way up. I only hear sound when the squelch is half about way. If i turn it to the right or left I get no sound. It did the same thing before I changed the potentiometer. I tried adjusting the receive no difference. I put everything back to where I started.
     
  8. nomadradio

    nomadradio Analog Retentive

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    That's odd.

    Does it have full transmit audio?

    73
     
  9. Low_Boy

    Low_Boy Sr. Member

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    Full transmit audio. Full PA. audio. I have to turn the mic. gain down or I get a squeal when testing PA. Same thing with extension speaker or internal speaker. All caps around audio amp have been changed. All diodes and transistors that I thought would cause this problem also checked. That is why I am wondering how a bad T2 transformer would act? The one cap. that failed and caused the no receive audio was tied directly to T2. Usually a problem like this is a bad solder joint but everything seems to check good.
     
  10. sonoma

    sonoma Sr. Member

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    have you changed C48? I had one a few years ago that had low audio and C48 was going bad.
     
  11. OldTech03

    OldTech03 Sr. Member

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    Seem to be chasing your tail? Listen you need to first find out if there is enough audio signal being fed to the volume control from the detector to make the speaker sound off. inject a signal from your audio gen at the wiper of the volume control be sure the volume is turned up so it doesn't ground the gen output also you only need a 1/10th of a volt from the gen to blast it into next year so be sure the audio gen is turned down. If you are not sure what you are feeding in terms of audio inject at R240 then you can use the volume control to control it. This should tell you if the path to the audio chip is unobstructed if you get a good response here then you need to troubleshoot from the volume control back to the detector stage a sope or good ole audio tracer will help do that.
     
  12. Low_Boy

    Low_Boy Sr. Member

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    I will try that right now. I've been poking around here with a signal Tracer and haven't had too much luck finding anything that's not making contact thanks.
     
  13. Low_Boy

    Low_Boy Sr. Member

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    okay injecting a signal at the wiper of the volume control does not make the output any louder. Now I'm a bit confused that means it's the signal going into the volume control that is not strong enough?
     
  14. Low_Boy

    Low_Boy Sr. Member

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    First ones I changed
     
  15. OldTech03

    OldTech03 Sr. Member

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    I assume you injected an audio signal at least at an amplitude that would be sufficient to create sound if you did then you should have heard that 1000hz tone "or whatever frequency the audio generator is set to" pretty loud coming out of the speaker if there are no issues in the path from the volume control to PIN 4 of the input of the audio chip. So if you are not getting a solid output from the speaker doing this then the problem would be between the volume control and pin 4. There are numerous things in that path that could obstruct the signal. You need to just look at it like a roadmap. Anything in that path that is shorted/Open etc could be the issue.

    If all the things you say in previous postings is true then the problem is pretty much confined to the path from the detector stage to pin 4?
     
    #15 OldTech03, Apr 23, 2018
    Last edited: Apr 23, 2018
    Shadetree Mechanic likes this.

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