1. You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.

Cobra 29 No TX Output

Discussion in 'General CB Services Discussion' started by Derrick Walls, Sep 13, 2017.

  1. Derrick Walls

    Derrick Walls
    Expand Collapse
    Member

    Joined:
    Jul 19, 2016
    Messages:
    29
    Likes Received:
    4
    I have a Cobra 29 on the bench I'm having a little trouble nailing down the problem on. I'm getting a good signal to the base of the driver but not to the final. The signal on the collector of the driver less than half of the signal on the base. For instance on 19 when keyed the signal on the base reads 27.185 which is what I would expect. RX works as it should. I believe the problem is in the PA section of the radio but at this point I'm stumped.


     

  2. loosecannon

    loosecannon
    Expand Collapse
    Sr. Member

    Joined:
    Mar 9, 2006
    Messages:
    2,609
    Likes Received:
    849
    sounds like the driver isn't amplifying.

    check to see that you have 13.8 volts going to the collector, and after verifying that is there, unsolder the leads of the driver transistor from the board and test it using your multimeter.

    here is a link to an article on how to do it if you need it: http://www.learnabout-electronics.org/transistor_faults_02.php

    if you don't find the 13.8 volts on the collector of the driver, unsolder the collectors of the final and driver one at a time and check the PCB trace for that voltage.
    If it is there when the transistors are out of circuit, but is not there when they are in circuit, then one of them likely has a short.

    if that voltage is not there even when the collectors are lifted, then trace back to D8 which may be blown, or the modulation transformer itself.
    LC
     
    LeapFrog likes this.
  3. Derrick Walls

    Derrick Walls
    Expand Collapse
    Member

    Joined:
    Jul 19, 2016
    Messages:
    29
    Likes Received:
    4
    Ok I'll give it a try and let you know how we turn out.
     
  4. kaos513

    kaos513
    Expand Collapse
    Active Member

    Joined:
    Apr 10, 2014
    Messages:
    764
    Likes Received:
    92
    I would start by checking bad solder joints some cobra 29s are know for that.
     
    loosecannon likes this.
  5. Derrick Walls

    Derrick Walls
    Expand Collapse
    Member

    Joined:
    Jul 19, 2016
    Messages:
    29
    Likes Received:
    4
    d8 diode checks good also getting 13.8 voltage to the collector of the final and driver. I put two new transistors in. still no output. Note: I have a oscope, multimeter and a bench freq counter. Not as familiar with the Cobra radios as i should be because I rarely work on them
     
  6. Derrick Walls

    Derrick Walls
    Expand Collapse
    Member

    Joined:
    Jul 19, 2016
    Messages:
    29
    Likes Received:
    4
    ok found part of the problem. had a bad driver. just low output now. Radio has a mosfet installed on the final. getting little less than half watt deadkey with a swing up to 2 watts.
     
  7. sonoma

    sonoma
    Expand Collapse
    Well-Known Member

    Joined:
    Sep 14, 2005
    Messages:
    1,462
    Likes Received:
    495
    you said you put 2 new transisters in and then you turn around and say it has a mosfet in for the final. if it had a bipolar in the final spot and you just put a mosfet in place you will have bad output until you set up the radio for a mosfet. it is not a drop in part.
     
    Robb likes this.
  8. Derrick Walls

    Derrick Walls
    Expand Collapse
    Member

    Joined:
    Jul 19, 2016
    Messages:
    29
    Likes Received:
    4
    Sorry I was a bit tired last night. It does have a mosfet final but has all the parts install and removed for it. Reason I put a mosfet final in originally was because the customer wanted a rfx85 amp added to the radio and was using it to drive the amp. Radio is the 29LX been working great for him up until a couple of weeks ago. I'm beginning to think something is shorted between the final and the antenna connector. I know in most cases the final or driver would be bad. I ruled out the rfx85 amp as I disconnected it and hooked the antenna connector back up to just the radio. I'll find it eventually.
     
  9. Derrick Walls

    Derrick Walls
    Expand Collapse
    Member

    Joined:
    Jul 19, 2016
    Messages:
    29
    Likes Received:
    4
    Correct me if I'm wrong shouldn't the final have some gate voltage to turn on the mosfet so it would amplify the signal. At this point it has none when keyed
     
  10. sonoma

    sonoma
    Expand Collapse
    Well-Known Member

    Joined:
    Sep 14, 2005
    Messages:
    1,462
    Likes Received:
    495
    it should have around 3.5 volts at the gate. do not go much higher or it will pop. the part is rated to go to 4.0 volts but they pop at that voltage. some put them 3.50 to 3.75 so they do not blow the final.
     
    Robb likes this.
  11. Derrick Walls

    Derrick Walls
    Expand Collapse
    Member

    Joined:
    Jul 19, 2016
    Messages:
    29
    Likes Received:
    4
    Ok my problem is there I have no voltage on the gate of the final.
     
  12. sonoma

    sonoma
    Expand Collapse
    Well-Known Member

    Joined:
    Sep 14, 2005
    Messages:
    1,462
    Likes Received:
    495
    I would lift the leg 1st and check if you have voltage with the leg raised. some times you can have a shorted part and it will draw the voltage down to zero.
     
  13. Derrick Walls

    Derrick Walls
    Expand Collapse
    Member

    Joined:
    Jul 19, 2016
    Messages:
    29
    Likes Received:
    4
    yep that was it. the bias component was shorted to ground. got it fixed. The rfx85hd is shot. the transistor is gone in it. radio works fine without it. Is there a feasible replacement for a on board amp?
     
  14. LeapFrog

    LeapFrog
    Expand Collapse
    Vocoder Frog

    Joined:
    Feb 15, 2016
    Messages:
    1,255
    Likes Received:
    378
    AD-203 stinger by RM, never tried one though... (heard they tend to self-oscillate)
     
  15. loosecannon

    loosecannon
    Expand Collapse
    Sr. Member

    Joined:
    Mar 9, 2006
    Messages:
    2,609
    Likes Received:
    849
    This statement is a bit confusing, as one end of your "bias component" goes to PC board ground.
    Unless you just put a piece of wire in place of R56, i can't see how this could happen.

    As for the RFX85, there are two amplifying transistors in it, and both are available so it shouldn't be too hard to repair the RFX85 unit.
    LC
     
    LeapFrog likes this.

Share This Page