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139 XLR Help Please

Discussion in 'General CB Services Discussion' started by packrat, Apr 21, 2010.

  1. packrat

    packrat Active Member

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    I recently picked up a Cobra 139XLR in decent condition. It received and transmitted both am and ssb, but output and modulation were both low. I got on CBtricks to get the adj. points for the radio, and when I started to adj., I started with VR8, which said it was AM power. I started the adjustment into a dummy load, and the power started to come up to around 5 watts. So I said, Cool. But when I keyed up next time to recheck it, I had no power or modulation. Relay kicks, and I did clean the contacts. Receive is fine. Changing the adjustment makes no difference. That was the only adjustment I made. When I key up on ssb, the radio clicks and the lights dim, kinda like a short?

    Can anyone out there clue me in to what I did? Seems everything I touch lately dies. Im afraid to take a leak!!


     
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    Last edited: Apr 21, 2010
  2. Robb

    Robb Yup

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    Just some thoughts...
    Turn the AM power VR back down to where you found it and re-check. You might have even snapped the final. Hook up an external power supply; maybe the stock one is ready to die...
     
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  3. packrat

    packrat Active Member

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    Robb,

    Doesnt matter where I turn the VR, it still has the same problem. When I key up in am, it trips the relay, but has no audio or power out, no dimming or other signs. In sideband, it seems like it is clicking the relay on and off, like the radio turning on and off repeatedly, and dims the lights.
     
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  4. tecnicoloco

    tecnicoloco Active Member

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    Check the AM Regulator C1419 and Capacitor C-179 2.2microfarads tantalum cap,you can replace the cap with a regular capacitor,watch the cap polarity.

    Cap is located behind the relay and the regulator is bolted on the side.


    The first thing to do with this board is look for the most common failure. It is C179. That is a small blue tantalum cap near the final. Right in front of the final and close to the White wires feeding the Final and Driver, you can see it next to a somewhat large RF choke. Here are the symptoms:

    On AM the lights dim with no output.
    On SSB the radio cuts out completely then the relay starts to chatter.

    Removing C179 will solve this problem. You can simply remove it if you wish. Modulation will increase somewhat so you may want to just leave it out of there.

    Credit to N9EFJ.

    PHP:
    http://trc458.n9efj.net/
    Check the above page for schematics and more info for your radio.(y)


    Tecnicoloco
    73
     
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  5. packrat

    packrat Active Member

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    That was it. Thank you for the info .

    When I was in there, I noticed that both CI 54 and CI 55 were cut. Any ideas why?

    Thanks again for all the help.

    PR
     
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  6. packrat

    packrat Active Member

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    I also see that ssb power is pretty low. Any tune up points I should try for this?

    PR
     
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  7. Robb

    Robb Yup

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    Cobra 139XLR
    Couldn't help but notice all of the service bulletins on this radio. They even had correction bulletins for the service bulletins - yikes. Either it had a lot of problems, or there were so many sold that a large database had been created. Probably both. Never owned one of these myself.
     
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  8. packrat

    packrat Active Member

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    Thanks for the link Robb. I think they sold quite a few of these radios. They are one of my favorite radios.
    I will look into the link.
    PR
     
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  9. loosecannon

    loosecannon break on through

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    for SSB power, adjust CT7 while speaking into the mic.
    it is near the relay.

    here's a really cool easy trick to do on this chassis.
    check R43, its near VR5.

    if its a 1.5 Meg ohm resistor, great, the upgrade has already been done.
    if its a 680K or something around that value, replace it with a 1 or 1.5 Meg ohm resistor.

    this will increase the ANL effectiveness and makes the receive sound better when the ANL is on.
    try it, you'll like it.
    LC
     
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  10. packrat

    packrat Active Member

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    Another question: Power is not balanced between 1 and 40, so how do I get it centered? Can you point me to CR6? I see everything but CR6 on the board, so it must be hiding somewhere???

    PR
     
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    Last edited: Apr 23, 2010
  11. packrat

    packrat Active Member

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    I am getting 4 watts on my peak reading watt meter on sideband, and CT-7 does next to nothing to adjust the power. I have right at 100% modulation with a power mic, and about 70 % with non amp mic. Any ideas on getting more from it? It should do 25 watts on SSB easily.
     
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    Last edited: Apr 23, 2010

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