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Cobra 148gtl, Unident Grant mods

Discussion in 'CB Radio Modifications' started by Moleculo, Oct 12, 2009.

  1. Moleculo

    Moleculo Administrator Staff Member

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    Here is a good set of power/audio mods for the Cobra 148GTL. I couldn't find these in the search, so I reposted it.

    Also applies to:
    Cobra 148GTLF
    Cobra 2000GTL
    Undien Grant XL
    Uniden Grant LT



    1. Carrier Adjustment: VR10. If lower carrier is desired, change R196 near VR-10. Try about 270 ohms.
    2. Change R114 to a 10K, and C46 to a .2 monolithic, this will widen up the audio on AM.
    3. The AMC is TR-26. You can remove it, and still have good results on AM, the ALC for SSB is not effected by this. If you don't want the AMC disabled, leave it. the other way to disable the AMC is cut R131. Check both to restore AMC from previous mods.

    4. If extra frequencies have been added, perform the following to widen up the band pass filters to recover lost power at the highest and lowest frequencies: C163 - 4pf, C41 - 3pf

    5. R180 - Jumper it. This brings up the gain of the TX mixer.

    6. IC TDA6130 - Pin 12 to ground through 100 ohm resistor. Upper left, pin 14 = ground.

    7. Remove the AM regulator, (usually a 2SC1419) and replace it with a 2N6487.

    8. C174 - remove, add 1000uf electrolytic. connect POS to base of yellow wire next to it. If the yellow wire is too far away, it might be easier to connect the POS leg to the leg of the AM regulator closes to the back of the chassis via the bottom of the radio.

    9. For 148F, change C18 to a 1000uf electrolytic, 16V
     
    #1
  2. loosecannon

    loosecannon break on through

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    im surprised that those mods couldnt be found around here through searching, but then again, i have had many troubles with the way the search feature works on this forum.

    i just have a couple of things i believe should be changed on the list.
    these are just my findings, and should not be taken as gospel.
    (unless you pray to the almighty loosecannon. then, go ahead LOL)


    i will address #4 last.


    5. the purpose in jumpering this resistor is to keep the bias from going negative.
    (these are justin depolis's words, not mine)

    6. this is the mod that increases the gain of the transmit mixer.
    also, you might want to include the older chip that is used here: the S042P.

    7. might want to mention the part number here. its TR41.


    8. probably some different wire colors used in different chassis. maybe add that the negative lead of the cap goes in the negative hole left open by the removal of C174, and that the positive lead of the cap goes to the base of TR41. (lead closest to the rear of the radio)

    9. dont know why this would only apply to the 148F.
    this cap is in the mic circuit. the stock cap wasnt rated for enough voltage and would fail. increasing the value of this cap will help keep the mic from squealing when audio gain in the radio is increased.


    now, for #4.

    C163 can be increased to 4 or 5 pF, but increasing the value of C41 actually makes the tuning of L6 too narrow.
    the effect will be that L6 will peak over a very narrow range causing the gain to fall off quicker towards the band edges than it did with the stock 2pF cap there.

    a better mod is to leave C41 as a 2pF cap and solder a 1pF cap from the emitter of TR14 to the lead of C41 that connects to L5.

    now, i must say here that these are my own findings, and i have never heard anyone else's opinion on this mod. i do know that this is what i observed personally.

    just trying to help clarify things,
    LC
     
    #2
  3. Klondike Mike

    Klondike Mike Active Member

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    LC, glad I came across your old post. I was going to follow along and change both c163 & c41 to 5pf. I'll do c163 for now and do the 1pf later when I get it. I'll let you know how it turns out.

    PS: the document I'm referencing mentions removing c161. Your thoughts?
     
    #3
    Last edited: Aug 13, 2010
  4. 74IN

    74IN Well-Known Member

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    The thief DePushdin really knew his stuff.

    His technical expertise was only second to his dishonesty, what a shame.

    Right click, save, lol.
     
    #4
  5. 74IN

    74IN Well-Known Member

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    I think it applies to the 148gtl, not the gtlf.

    I had a GTLF (frequency counter model), and it was not up to snuff compared to the GTL (my Grant XL).

    Sent it to DTB, got it back and was dissatisfied, until he explained that it was a completely different rig.
     
    #5
  6. loosecannon

    loosecannon break on through

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    CORRECTION!!!

    just wanted to correct some info i was just reminded of.

    in speaking about how to broadband L5, i stated to connect to the emitter of TR14.

    it should be the COLLECTOR (middle leg) of TR14.

    so the mod should read: connect a 1pF cap from the COLLECTOR of TR14 to the leg of C41 that connects to L5.

    sorry about the bad info.
    LC
     
    #6
  7. 15minigrass

    15minigrass Active Member

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    So this will lower my DK below 1.5 watts right?

    1. Carrier Adjustment: VR10. If lower carrier is desired, change R196 near VR-10. Try about 270 ohms.
     
    #7
  8. Needle Bender

    Needle Bender Well-Known Member

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    It should, I got mine down to 2/3w but it sounded like double sideband so I brought it up to 1.25w DK.
     
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