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Cobra 139xlr, I need help

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

  1. Sammitch

    Sammitch
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    Hello everybody,

    I got this off flea bay as parts only- non working and they were right.
    I recapped all electrolytics, sprayed all the switches with Deoxit and she roared to life in SSB mode receive, no transmit at all. Still had the stock driver and final, so I pulled them and checked with DMM and they show OK.
    Read all the tech bulletins, and my production number was high enough all but one should have been made at the factory. I replaced TR5, a C1730,D26 and D2 and all receive including AM came alive.
    Plus I changed out all C1419s for NTE152s. Checked my mic wiring since it is relay switched and all looked good. Plugged it in and as soon as I keyed, it all went dead, changed the fuse and just went ahead and swapped out for all new NTe152s again. Lights came back on and SSB receives but no AM, now when I key up the channel LEDs go out and meter lights go dim and no transmit. I have no idea what caused that to happen.

    I know I am fighting different problems, but if someone could tell me or point me in the right directions, it would be greatly appreciated. I am about to pull what little hair out I have left. I just received my Sams copy, but it was printed before all the tech bulletins.
    I have another NEC C1730 on its way, but at 7 bucks apiece, I would rather not pop another one.
    I am going to replace D26 and D2 again tonight, I have correct voltages on TR5 so hoping it will still be OK, but I have my doubts.

    Any help or suggestions will be greatly appreciated, it has never been modded and is bone stock.



    Thanks, Mike
     

  2. Xitami

    Xitami
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    Hmmmm, lights dimming smacks of a short to earth, my first angle would be gaze at the solderside for a solder short, some monkey may have had a go at it, but its a definite short in there somewhere.
     
  3. sonoma

    sonoma
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    change C179 a tantalum cap to a electrolytic of the same or higher voltage but the same UF rating and see if it comes back. that cap can be bad and cause the fuse to blow and dim as you are stating. it is back by the finals.
     
    #3 sonoma, Apr 5, 2017
    Last edited: Apr 6, 2017
  4. ExitThirteen

    ExitThirteen
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    Sonoma beat me to it. C179 is a known failure component that will cause the symptoms you're having. Do as he says and you should be able to get things going.


    ~Cheers~
     
  5. Robb

    Robb
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    That flat ribbon cable between the channel LED and the PLL have issues at the PLL end. If it gets dirty or has poor contact surfaces (common with these radios that I've worked on), then you could have problems with the LED reading incorrectly or the channel selector not working at all.

    You do not have to replace C179 with another tantalum, you can also use a regular electrolytic cap. But you should replace it - even if it isn't bad. If it isn't bad now, it will be. Common failure point as already posted by others above
     
    #5 Robb, Apr 6, 2017
    Last edited: Apr 6, 2017
  6. Robb

    Robb
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  7. TM86

    TM86
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    What these guys said, plus, check your NTE 152s. I had 2 out of 6 that were bad when I went through one of my 139 XLRs. Then again, I bought the NTE 152s at Fry's Electronics.
     
  8. Sammitch

    Sammitch
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    I did change c179 first with a new tantalum, then to an electrolytic and no change.
    I am wondering if one of the new caps went bad, [I have always heard, if electronics go bad it will happen within a few hours and if they last that long they will last a long time]
    I don't own a capacitor checker, so don't know which one is likely to cause such a problem.
    I am afraid to leave it keyed to long for fear of damaging the PLL or more components because, like you all say, It is something shorting to ground. I have been over the solder side of the board with a magnifying glass looking for a solder screw up with no luck. I even went back and checked to make sure none of the electrolytics I changed were in backwards.
    I know I have no voltage going to the finals or driver, could not even check the bias, no voltage to the test pins.
    I think I might be in over my head on this one, and it sure is a nice cosmetic looking one, and like I said, it was never modded or messed with that I can tell. Sure would like to sort out the problems. This one is a keeper if I can.

    Thanks, Mike
     
  9. Sammitch

    Sammitch
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    Is there any wiring inside the mike that needs to be changed around? I read somewhere on the internet that on a relay controlled radio if the wires are not right, when you key the mic, you are making 2 connections to ground and it would be lights out. I have just a standard Cobra coffin mike and also a D 104 with TUG-8 base, it has a switch in the bottom for electronic or relay, which position should it be in?

    I changed D26 and D2 again with no luck, now I have SSB receive and extremly weak receive on AM, no transmit on either.
    The LED channel light does not go out in AM, but does in SSB when the mic is keyed. My new C1730, { TR5 } should be here by Monday, if that does like the last one, receive will all be fixed, but as soon as you key the mic, it will short out again.

    Does anyone have anymore suggestions?

    Thanks, Mike
     
  10. Robb

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    #10 Robb, Apr 8, 2017
    Last edited: Apr 9, 2017
  11. sonoma

    sonoma
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    does the relay click when you key the radio? is it stuck in some mode between RX and TX? if the relay is not switching when you key the mic or is it a constant clicking. there is a diode right by the relay that can go bad and cause some problems also. forget just what number it is. but it sets right beside the relay. also check the am,fm,switch and see if you have voltage from it in each location as you turn the control.you may have a dirty switch not putting power to the main board for each mode. as Robb stated you need to start checking voltages to see what has power and what does not.
    it may be time to take it to some one that knows what to look for. these old radios are prone to have the old VR's go bad could be why you do not have voltage to the finals and driver.
     
  12. Sammitch

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    I have installed a brand new relay and it does click when you key the mic, no chattering, just a good solid click. Yes, I am going to have to check each and every component in the transmit section for voltage as when it is keyed is when it destroys TR5, the little RF Amp transistor, in receive everything is fine. Looks like it is not going to be a fast and easy fix. Everyone around me that did radio repairs has passed on, so it is either box it up and wind up putting in probably somewhere between 1 and 2 hundred dollars including shipping or spending countless hours of my free time poking and prodding inside.

    Robb, I have the Sam's copy of the 139xlr and it has the voltages in it. I ordered it from Sam;s and was extremely disappointed in it. I thought it would be just like the old Sam's books, but they are not, just a pile of copied and poorly printed pages for an ungodly price for what you get.

    Thanks, Mike
     
  13. Robb

    Robb
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  14. Sammitch

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    I used a 1N4148 in D2, and D26 a 3.7 volt Zener as one of the tech bulletins recommended reducing D26 down from 5 volts to the 3.7 volt.

    For that Tech bulletin, I used 2 1N4148's for the mod on L3, and I have been wondering if that is not causing my problem, just have not had the time to check this weekend to see if they shorted to ground, as it looks like L3 feeds voltage to TR5 on the schematic, or at least to me, I am not real great at following the way things go on one.

    If I did wrong, let me know.

    Thanks, Mike
     
  15. Sammitch

    Sammitch
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    I have completely run myself ragged on this radio.
    Let me refresh everyone on it. When I bought it, it had no transmit or receive. I used Deoxit on all the switches and that brought SSB receive back to life loud and clear,
    I replaced TR5 and that brought AM receive back loud and clear, receive worked great.

    So I started working on the transmit problem using a stock cobra handmic, then I thought these were wired for electronic switching and the radio is relayed and that may be the problem.
    So I dug out my D104 with the TUG 8 base and flipped the switch from electronic to relay, keyed the mic and the lights went out, blowed the fuse, when I replaced the fuse, all the lights came back on and had receive on SSB but not on AM. Replaced TR5 again and got back AM receive. Put the hand mic back on, still no transmit and blew TR5 instantly, also now the channel selector light goes out and meter lights dim when the mic is keyed in SSB, but stays on in AM.
    I have tried locating the connection for TR5 in AM and SSB, somewhere in there is a blown component when in transmit. I am having no luck.

    Please, someone should have run into this before, I would appreciate any and all suggestions or info. I have had to just lay the radio aside and walk away till I can get some help, I am almost positive it is nothing to difficult, but either I am overlooking something or It is more serious than I am thinking.
    If you have any ideas?

    Thanks, Mike
     

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