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Cobra 139xlr, I need help

Discussion in 'General CB Services Discussion' started by Sammitch, Apr 5, 2017.

  1. ExitThirteen

    ExitThirteen
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    Grumpy and Cranky

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    Ok, try this, desolder the base and collector legs of the driver and final and disconnect them off the main PCB, then key the radio again in SSB, see if the channel lights dim. Let me know how you get on.



    ~Cheers~
     

  2. sonoma

    sonoma
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    C179 is a tantulum cap back by the finals that will do just what you are having. put an electrolytic in its place of the same UF and voltage. this will work just fine.
     
  3. ExitThirteen

    ExitThirteen
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    Grumpy and Cranky

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    Sonoma,

    In post #8 he did say that he replaced C179 with a tantalum, then an electrolytic, but no change. Didn't know if you missed it or not. :)

    ~Cheers~
     
  4. sonoma

    sonoma
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    exit 13 I missed that. thanks.
     
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  5. Sammitch

    Sammitch
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    I unhooked the base and collector legs and all lights stay on in SSB and AM, no dimming at all. Seems I have a shorted driver and final, but I would have sworn they checked OK when I pulled them from the radio earlier.
    I still have good receive on SSB, but very weak on AM because I am down to my last transistor for TR5, so I left the bad one in for this test.
    I can order new NEC 1306 and 1307 from RF Parts, little on the expensive side, but I don't know of any replacements.

    There is a resistor on the final that measures 47.7ohms,it goes from the collector to the base, I cannot find it in the Sam's schematic at all, seems to be factory as it has a plastic sleeve over it. But I don't know.
    Also there is a 100nf ceramic disc cap going from the emitter leg of the driver to chassis ground, but on the final the same size cap is going from the collector leg to ground, I cannot find these on the schematic either.
    Could these be in the wrong place causing the final and driver to cause all these problems?

    Thanks ExitThirteen, this is what I found when disconnecting the legs of the driver and final. Please let me know if it is the final and driver causing all my problems, and if the caps and resistor are in the right location for them. If I need to look further let me know.

    At least I feel back on the right track.

    Thank you very much, Mike
     
  6. unit_399

    unit_399
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    EL CAPO

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    The 47 ohm resistor is part of a factory mod. The "technical bulletins" for the factory mods are shown here.

    http://www.cbtricks.com/radios/cobra/139xlr/index.htm

    The 47 ohm resistor and the caps are not shown in sams #127 because that volume was published before the service bulletins were issued. The bulletins call for the final bias to set at 15 ma rather that the 70 ma called for in the sams #127 alignment. For more info on this, check out this site.

    http://unit399.wixsite.com/858ssb.

    Good luck. 73s.

    - 399
     
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  7. Sammitch

    Sammitch
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    Thank you 399, I have all the tech bulletins from cbtricks printed out. But cannot find any reference for a cap to go from the collector on the final to chassis ground, or for one to go from the emitter to ground on the driver, although that one can do no harm as the emitter on both the driver and the final are on the PCB ground. But why one from the collector to ground? Right now, I have looked and probably overlooking this, can you reference the number of the bulletin? I found bulletin 1208 referring to the 47 ohm resistor it says from base to ground, mine had been soldered in from base to collector, I think somebody has screwed this all up. My production number is high enough that the only tech bulletins that should have been done after the build is the one referencing the 2 diodes on L3, and that had not been done.

    Yes I know about keeping the bias to 15 ma, down from 70 ma. I am going to have to replace the driver and final before I can set any bias though. I am wondering if this is the only problem though.

    Thanks, Mike
     
    #22 Sammitch, Apr 19, 2017
    Last edited: Apr 19, 2017
  8. ExitThirteen

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    Grumpy and Cranky

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    Well, it may not be the only problem, but this fixes a major problem you were having. You can install a C2166/C1969 driver/final pair and those will work in place of the C1306/C1307, since genuine C1306/C1307's are tough to find. Get the driver/final replaced and key it up again and see if the lights dim again, and let us know how you get on.

    ~Cheers~
     
  9. Sammitch

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    Will do. I want to thank everyone for all the info and help. I have tracked down new NEC 1306 and 1307 they say were manufactured in the late 70's, I don't know if I should believe it or not though. Kinda pricey but don't have much choice I guess.
    Thanks everyone, will post after I get new parts and let everyone know the outcome.
    Mike
     
  10. ExitThirteen

    ExitThirteen
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    Grumpy and Cranky

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    Also, the 47 ohm resistor you mention is supposed to be soldered from base to ground, so if yours is soldered from base to collector, there's where the problem lies. With that resistor in its current position, it's feeding the base of the transistor with close to 13V.... and that's really bad. That will destroy the transistor. Before you replace the final, make sure that 47 ohm resistor is soldered from base to ground. :)


    ~Cheers~
     
  11. Sammitch

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    I have the new final and driver on the way. Just to clarify, the 47 ohm resistor will go from base to emitter, which the schematic shows as ground, or should it go to chassis ground?
    One more question. Are 1n4148 diodes OK to use for the Tech Bulletin #1210?
    I assume, that resistor being in the wrong place is also the cause of TR5 shorting as soon as you key the mic, correct?
    I thought this was a virgin radio, but I am going to have to really give it a good look over, some monkey has been in it for the resistor to be in the wrong place.

    Thanks, Mike
     
    #26 Sammitch, Apr 21, 2017
    Last edited: Apr 21, 2017
  12. ExitThirteen

    ExitThirteen
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    Grumpy and Cranky

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    The 47 ohm resistor should go from base to PCB ground on the final. The capacitor you mention that goes from collector to ground, should go from PCB ground to chassis ground. 1N4148 diodes are fine to use for the bulletin #1210, just make sure to observe polarity per the bulletin and you're good.


    Let us know how you get on. :)


    ~Cheers~
     
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