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Cobra 142 over volted on DC power and smoked :(

Discussion in 'General CB Services Discussion' started by WINKDOGG, Nov 15, 2019.

  1. WINKDOGG

    WINKDOGG President CFO & CPFO WorldwideDX Radio

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    I have a pretty sentimental 142 gtl that I pulled out to fire up . I had no empty AC plugs in reach at the bench so I hooked it to my 50A power supply and when adjusting the voltage (I run 14.5 normally on exports ) the pot was glitchy and I spiked it with probably 18-20 volts . Now I have no power up on AC and if I try DC the two large coil wound resistors connected to the DC input jack start smoking .The AC power supply is still producing clean 120v power and 13.8 DC on output side . I see no burnt parts or traces so looking for help saving this radio as it was modded and left to me by my pop !



    Thanks and 73's
    Four-Two-Zero In the Garden State in DX land
     

  2. kopcicle

    kopcicle Sr. Member

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    D407 is shorted , then ...?
     
    WINKDOGG, 2NC995 and midnight special like this.
  3. WINKDOGG

    WINKDOGG President CFO & CPFO WorldwideDX Radio

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    I’m hopping that’s just a diode. So just remove and replace thank you so much I’ll give it a try !!

    Brian
     
  4. guitar_199

    guitar_199 Well-Known Member

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    Good luck. Keep us in the loop....
     
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  5. WINKDOGG

    WINKDOGG President CFO & CPFO WorldwideDX Radio

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    Have not had the time to track down the diode suggested will get to it in the morning but need to sleep ! Thanks so much guys and 73's

    Brian
     
  6. WINKDOGG

    WINKDOGG President CFO & CPFO WorldwideDX Radio

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    Hello DX land I am having a hard time finding D407 inside this 142gtl ? I had the flashlight and the glasses on and everything but no luck yet ;( copcicle thanks for reaching out ! I have found a few schematics but they are so blurry I can see the board better ! If I'm being an idiot and I'm not supposed to be looking for a diode please just let me know but ill be "reading the mail" until I find it !! Thanks and 73's Brian
     
  7. 2NC995

    2NC995 DAN

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    I am not personally familiar with the inside of that radio, but it should be at/near the DC input. Maybe even right across the pins.

    Edit:

    Looking back at your original post - check across the smoking coils - that’s exactly where it should be.
     
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  8. 2NC995

    2NC995 DAN

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    Oh, and it might not look much like a diode if it is cooked.
     
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  9. WINKDOGG

    WINKDOGG President CFO & CPFO WorldwideDX Radio

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    10-4 10-4 loud n clear will give her a look and post back asap ! Much thanks and 73's the skip is Rollin tonight and the connex 3300hp is talkin all over the country !!

    4-2-0 NJ told ya that !!
     
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  10. nomadradio

    nomadradio Analog Retentive

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    D60 shows in the schematic as a protection diode.

    This tech note shows that they changed it to a 18-Volt or 16-Volt zener diode after the first few production runs.

    http://www.cbtricks.com/radios/cobra/142gtl/graphics/cobra_142gtl_tech_note_1226.pdf

    That part will almost surely fail as a dead short from excess supply voltage.

    Good chance this is your culprit.

    73
     
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  11. kopcicle

    kopcicle Sr. Member

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    ?? was I looking at the wrong schema?
     
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  12. 2NC995

    2NC995 DAN

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    No, a different protection diode.
     
  13. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

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    Winkdogg I wish you the best of luck on your radio. Over voltage is akin to a lightning strike in damage.
     
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  14. kopcicle

    kopcicle Sr. Member

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    So, different schools separately?
     
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  15. 2NC995

    2NC995 DAN

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    I’m seeing the same as you...I believe what Nomad references is a separate over voltage protection diode.

    Or I just don’t have a clue...which is always a distinct possibility!
     
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