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Cobra 2000 No Tx.

Discussion in 'General CB Services Discussion' started by Brian G, Mar 16, 2021.

  1. Brian G

    Brian G Active Member

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    Ok so first update---- the final does appear to be bad after checking with the meter. After removing, the base foil pad appears to be ok. It is showing around 0.25 volts at the pad. I am going to check the bias diodes next. But so far, what does this suggest besides a possible bad solder joint or bridge? Assuming I can find another choke to replace the one that burned up, I am worried I will just smoke that one.

     
    #16 Brian G, Mar 25, 2021
    Last edited: Mar 25, 2021

  2. Brian G

    Brian G Active Member

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    I unsoldered the wires that go in near the bias VRs (not the ground) . On the diode setting, the readings are around “612” on the driver and about 1200 On the final. I assume that 612 and 1200 are in millivolts. If that’s the case then both diodes are good. So then based on that and that the foil pad voltage at the final base is ok, then what is causing this issue? I also replaced the final variable resistor from another radio. I still also need to find a new Rf choke but I’m afraid I’m just going to burn it and the new final up if I don’t fix what was causing this. Is there a substitute for that rf choke I can get more easily?
     
    #17 Brian G, Mar 25, 2021
    Last edited: Mar 26, 2021
  3. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    Check voltage at the base leg of the final, in TX and RX, shouldn't be very much voltage, maybe a half a volt or so. If you get anything more than about .7V or so, then C152 has failed short, which is allowing the full voltage from the emitter of the driver transistor to hit the base of the final, which can smoke L39 in the process. Let us know how you get on.



    ~Cheers~
     
  4. Brian G

    Brian G Active Member

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    I pulled the final out because it was bad. The base pad is about 0.2-0.3 volts. I don't recall if it was ok prior to smoking everything. Perhaps I just change out C152 anyways in case. I think I do remember the voltage there being high.
     
  5. Brian G

    Brian G Active Member

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    Question: when that final and choke smoked, it seems to cause
    Question: when that final and choke smoked it seems to have caused power surge in the radio. I believe it smoked the voltage regulator. Could that be the case?
     
  6. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    It's doubtful, but possible. When you say it smoked the regulator, which regulator are you referring to? The regulator in the power supply circuit, the MB3756 8V regulator that controls things on the main PCB that's located near the audio IC, or the small 2SC496 located in between the driver and final transistor that controls the bias voltage/current for the final?


    ~Cheers~
     
  7. Brian G

    Brian G Active Member

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    the MB3756. The radio runs off a DC power supply, not the AC. I had replaced the one in the bias circuit previously (TR37 I think it is).
     
    #22 Brian G, Mar 26, 2021
    Last edited: Mar 26, 2021
  8. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    The failure of the final and L39 wasn't caused by the failure of the MB3756, but rather, C152 failing short. That capacitor passes RF through it, and I have seen the capacitor either fail short, or become leaky, allowing excess voltage to reach the base of the final transistor. The purpose of this capacitor is to couple RF from the driver to the final, so it passes RF, but blocks the DC voltage that is present on the collector of the driver. RF can do some weird things to disc capacitors, especially if they aren't rated for the job. As a measure to prevent it from happening again, I install a 470pf, 1kV disc capacitor in its place. Remove C152, and see if you still get high voltage at the base leg of the final, removing this capacitor will prevent any excess voltage from reaching the base leg of the final. Let us know how you get on.


    ~Cheers~
     
    tecnicoloco and NZ8N like this.
  9. Brian G

    Brian G Active Member

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    ok thanks will do I will let you know. Other problem is where the hell do I get another L39. They don’t make them anymore and I have no idea what an equivalent would be.
    Other question, can I check and adjust the final bias without the final actually being installed? I’d like to make sure that is normal before I put the new parts in.
     
  10. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    Yes, should be around .5-.6V or so at the solder trace where the base leg of the final solders to. You can usually get a suitable replacement for L39 out of a junk Cobra AM or SSB radio.


    ~Cheers~
     
  11. Brian G

    Brian G Active Member

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    I usually check the bias current putting an inline meter through the bias wire and those test point posts.
    Haven’t been able to find a choke and I don’t have an old radio. Damn, sucks
     

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