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Cobra 2000 GTL help with SSB lower ouput

Discussion in 'General CB Services Discussion' started by Adamf, Jan 20, 2016.

  1. Adamf

    Adamf
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    Hi all, I am new to the forum, could use some help. I just acquired a Cobra 2000GTL in nice shape. Has extra channels, good receive, AM output is 4 watts and swings to about 10 watts but SSB only PEAKS to about 10-11 watts output. I looked over the radio and see that TR24 has been removed so it should have much more output. I did a quick alignment but it did not help too much. A couple of questions, in the Cobra 2000 GTL service manual it say the driver bias should be 30ma and the final bias should be 60ma but I have seen post showing different values. To properly align the radio does any one know what values I should really use? Also, any other suggestions for getting more SSB output? I am using a good meter reading PEAK output, in to a dummy load and I have tried a couple of different mics. Any help appreciated. Thanks, Adam


     
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  2. Robb

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    Your output looks normal for a standard 2000.

    The bias current figures are correct for that radio.
    Don't turn them up or they will fail with a costly part replacement.

    To peak a radio correctly, you also need - at least - an oscilloscope to do it right.
    Not to mention a bit of experience too, Otherwise, you may well get too much distortion out of the radio . . .
     
  3. Adamf

    Adamf
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    Thanks, I appreciate the input. Adam
     
  4. Robb

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    BTW - I'd put tr24 back in the radio and readjust the AM modulation. It may sound a little louder with it out; but most of that is distortion. If you had a scope you could see that. Additionally, it really screws up the SSB audio something terrible. I always put it back in if/when I find another radio being sold locally and go through the radio.

    ANY tech worth his weight does the same thing for precisely the same reasons. It is better to have the radio peaked with a scope; rather than hacking them like that. You will find out . . .
     
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  5. ExitThirteen

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    It's common on the 2000's to have low SSB output, most radios after a good alignment will only produce about 16W or so PEP on SSB.

    Do what Robb suggested, and re-install TR24. Replace with a 2SC945 (labeled C945 on the transistor itself) and restore proper AMC/ALC function. To verify that the limiter circuit is doing its job, you should be able to adjust VR11, which is your ALC (SSB output). If that adjusts, you're good.

    L20, L45, L46, L47, and L48 are your TX adjustments, with L45 and L46 affecting TX output the most. You'll want to adjust all of these, along with L38, for maximum SSB output.

    There's also a TX mixer stage mod I have used for years that works very well for bumping up output in the TX (both AM and SSB) without sacrificing audio clarity.

    If you would like this mod, let me know and I will post the info.

    For now, get TR24 replaced in the radio, and do a TX "chain" alignment on the "L's" listed above. Use a ceramic or plastic screwdriver, do NOT use metal. You'll also need a device that can produce at least a 1khz tone so you can properly tune. Let us know how you get on.

    ~Cheers~
     
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  6. w9cll

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    Power can always be increased with an amp and or a good antenna system. Crappy audio in will always crappy out.
     
  7. Robb

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    Yes please.
     
  8. ExitThirteen

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    TX Mixer Amplifier stage mod:

    Locate IC5, which is labeled S042P. This is the TX mixer amplifier IC.
    Change R190 from 100 ohm to a 47 ohm. R190 is located next to IC5 on the right.
    Flip the radio over, locate IC5 again.
    Solder a 330 ohm resistor from pin 5 to PCB ground as shown below.

    [​IMG]

    Also, check the value of R183, it is located just to the right of TR39, which is the TX pre-driver transistor (labeled C1973). If R183 is 12 ohms, change it to 10 ohms. The older 148's and 2000's had a 12 ohm resistor at R183, the newer radios had a 10 ohm resistor.

    This should bump up SSB output 4-7W, depending on the radio. You may see about a 1/2W or so deadkey change on AM, but your modulated output will be higher.


    ~Cheers~
     
    #8 ExitThirteen, Jan 21, 2016
    Last edited: Jan 21, 2016
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  9. M0GVZ

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    Which is quite frankly pointless and I doubt the other end of the conversation would notice but your finals definitely would as they slowly cook themselves to death.

    I don't know what this obsession with trying to get an extra few watts out of a radio is. The minimum power output increase even remotely worth a damn is quadruple stock because that gets you an extra S point and you're not going to do that just by tweaking the finals in a radio.

    Instead of upping the power, lower it and use the radio to drive an amp with a meaningful level of power such as 50-100W.
     
  10. ExitThirteen

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    I am very well aware of how much output is needed to to gain an "S" unit on RX and such, as I used to be an extra class ham years ago.

    The OP wanted information on how he could increase power on his 2000, and I have provided this information.


    ~Cheers~
     
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  11. Robb

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    BTW; is this mod workable on the new version of the 148GTL?
    Is this the area and cause of their weak signal on SSB?
    I can tell they choked it down somewhere; but was never able to track it down.
     
  12. ExitThirteen

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    Grumpy and Cranky

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    Not workable on the new 148's; they redesigned the TX mixer amplifier stage and they use a different IC now. Can't be modded. The "choked down" you are referring to lies within the redesigned +8V regulator circuit, since the MB3756 voltage regulator IC is made of Unobtanium. When Cobra redesigned the +8V regulator circuit they made a mess of it and it's simply too unstable, hence all the gargling/warbling on SSB when you try to turn up the ALC.

    Sad but true. :(

    ~Cheers~
     
  13. Adamf

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    Wow, thank you all for your posts! I will install a new TR24 when I get it and go from there. Thanks again! Adam
     
  14. Adamf

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    Quick question, I am going to adjust in SSB mode, correct? I guess that is why I need to produce the tone, yes? Thanks
     
  15. Robb

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    Yes.
     

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