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Cobra 2000 No TX power then whole radio died

Discussion in 'General CB Services Discussion' started by Brian G, Feb 27, 2021.

  1. Brian G

    Brian G Active Member

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    Makes sense. How would I mount the new version if needed?

     

  2. nomadradio

    nomadradio Analog Retentive

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    Ideally you'd like the body of the diode laying up against the plastic body of the transistor.

    Just one problem. The old part had really long leads. Extending them to reach the holes in the circuit board is a chore, but doable.

    The end of the MV1Y with the tiny color dot is the cathode, same as the end marked with a band on a glass diode.

    73
     
  3. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Got a TRC-485 anywhere?

    upload_2021-3-3_8-41-55.png

    They Glued their MV-1Y clones to the BODY of the part, then soldered the wires underneath to the Bias feeds.

    No, I said that right - they soldered the Diodes UNBANDED side - to the Bias supply voltage - thru a Resistor/Bead for the DRIVER on the Left, and fed the FINAL thru a Green bodied Choke, with a ferrite bead choke (2X the choke) on the FINAL - on the right.

    The BANDED side went to RF ground so they followed the Emitter leg down and soldered to it's pad on the main PCB.

    Each transistor had an individual lead to the Bias voltage - not thru the Base lead - just met there thru the jumpers and the Chokes, the Diodes never get close to the Base lead or it's input signal..

    So they buffered and choked off RF from returning back into the Bias diodes and altering the Class - but they Glued the Bias Diodes directly to the body of the Transistor.

    You were forced to install new diodes every time...

    In another thread, the above heat sink mounting hardware insulator - note the PVC and the "Curl" in one - this is a dangerous moment - for as these thing heat up, they expand and those insulators try to transfer the heat away but as that happens the expansion and contraction of all the components involved - it's gets toasted - Radio Shack - we got dinged for that every time one was returned.
     
  4. Brian G

    Brian G Active Member

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    What is the purpose of L44 in my radio. It’s a little black ring/circle with 3 leads tiny leads into the board but it looks like at least one lead is broke. Not sure how critical it is or how Ling it’s been like that. It’s so small don’t know if I can even do anything with it. Also do you happen to have a pic of the solder side of this board at L44? Thanks.
     
  5. nomadradio

    nomadradio Analog Retentive

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    L44 is a "transmission-line" transformer. The doughnut-shaped core is called a toroid, and it's made from a ferrite material. Serves to match impedance feeding out of the predriver transistor TR39 into the driver transistor TR38. If one leg breaks off, you should not get much power at all, if any.

    73
     
  6. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    Replace C152, and C157. These two are disc capacitors located in the driver/final section of the radio. C157 is .0047uF, and C152 is 470pf. Replace those, and let us know how you get on. I'm suspecting one of those capacitors have failed short, which is causing extra voltage to "leak" into the bases of the driver and final transistors.


    ~Cheers~
     
    Brian G likes this.
  7. Shockwave

    Shockwave Sr. Member

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    At least they were silkscreened onto the board, as a reminder to reinstall them after a new final or driver. Do you know if mounting the thermal tracking diode on the transistors case, made any difference or benefit in its ability to track faster? With all of that metal tab exposed out of the top of the TO-220 case, I'd have been tempted to find an "eye connector", that would fit snuggly around the insulator bushing and make direct contact to the tab. Physically attach the diode to that.
     
  8. Brian G

    Brian G Active Member

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  9. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    If you're referring to this...
    upload_2021-3-4_8-43-59.png

    Per @nomadradio - that is the part L44 is a Bifilar wound toroid. ​

    upload_2021-3-4_9-7-47.png

    Sorry this looks like hell but...it is from a Documentation project from the X

    upload_2021-3-4_9-31-36.png
    As a word of caution:

    Because this brought up quite a debate in a former site...

    The "TWIST" can be found in various orientations - some are tied at the foil pad that feeds the Collector of the Predriver, another may use the Power Feed side arriving from the 8V TX - in this instance the TWIST is on the output leg towards the cap that feeds the DRIVER - C157.

    In the above its from a Cobra 148.
     
    Shadetree Mechanic likes this.
  10. Brian G

    Brian G Active Member

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    Yeah I get all that about this part. The issue again is that it appears to be damaged. I need to know where to find another one. I don’t think this is a part I can buy like other components.
     
  11. Brian G

    Brian G Active Member

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    Yeah I get all that about this part. The issue again is that it appears to be damaged. I need to know where to find another one. I don’t think this is a part I can buy like other components
     
  12. Brian G

    Brian G Active Member

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    No I don’t have a TRC radio. Still unclear how to mount these. I’m hoping the ones there are not bad in the first place. I did switch them out from a Cobra 140 that I know was working prior. So I’m not going to touch those for now. What is the green and white dots on them? I assume that is differentiate them but is that a universal marking? Want to make sure I put the replacements in correctly.

    My bigger worry now is that L44.
     
  13. Brian G

    Brian G Active Member

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    Done.
     
  14. Shockwave

    Shockwave Sr. Member

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    The good news is that part was used in many different radios that had the MB8719 PLL chip. We have lots of people on this forum that have worked on that board and probably more than one with a parts donor. If you're stuck, I would just ask if anyone has an L44 that you can obtain. If the core in not cracked, the original part could be rewrapped with the same size magnet wire, as another alternative.
     

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