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Cobra 139 XLR resistor between B&E

Discussion in 'General CB Services Discussion' started by Low_Boy, Jul 21, 2021 at 9:18 PM.

  1. Low_Boy

    Low_Boy Sr. Member

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    Why in the world would there ever be a resistor between the base and emitter on a 1307 final on a cobra 139 XLR?


     

  2. nomadradio

    nomadradio Analog Retentive

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    To stabilize the bias circuit, pretty sure. Cobra published an update. Said to add that resistor and some other stuff to any radio getting a blown final replaced.

    73
     
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  3. Dmans

    Dmans Sr. Member

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    These 2 Service Bulletins.

    73
    David
     

    Attached Files:

    tecnicoloco and NZ8N like this.
  4. Low_Boy

    Low_Boy Sr. Member

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    Thank You.
     
  5. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    upload_2021-7-22_8-13-17.png

    I do 180 ohm but - eh?
     
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  6. Low_Boy

    Low_Boy Sr. Member

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    Andy, possibly you did this one. I think that is what was in here. I will have to take another look.
     
  7. nomadradio

    nomadradio Analog Retentive

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    The other thing it does is to make the setting of the bias trimpots less "touchy" to small movements. Typically the correct trimpot setting before the update is nearly all the way to one end. This puts all the stress onto a tiny portion of the trimpot's resistance element. Tends to wear out the trimmer pot. Any time this setting seems way too sensitive, it could be time for a new trimpot. We see a lot of them that need to be replaced.

    Some things just don't last more than 40 years.

    73
     
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  8. Low_Boy

    Low_Boy Sr. Member

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    Thanks Nomad. Yes with it in the bias set nicely. I noticed another problem one of the voltage regulators must be moved to the back of the radio. TR36 because it really heats up the resistor behind it.
     
  9. Dmans

    Dmans Sr. Member

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    This service bulletin for that problem.

    73
    David
     

    Attached Files:

  10. Low_Boy

    Low_Boy Sr. Member

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    I bet you have worked on one or two of those before. First time I was ever in one and it reminded me of the washington.
     

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