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Cobra 148 GTL clarifier unlock.

Discussion in 'General CB Services Discussion' started by Low_Boy, Nov 29, 2018.

  1. Low_Boy

    Low_Boy Well-Known Member

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    I have a old beat 148 GTL that someone many moons ago unlocked the clarifier. I can align AM and LSB but USB bottoms out. Will changing the value of R236 8200 ohm give me more adjustment? Probably one of Andys little graphics would help
    From what I see this is the only resistor that goes to L59. Or can i add a adjustable cap. To the underside of L59?


     

  2. loosecannon

    loosecannon Sr. Member

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    messing with the value of R174 will shift the center of the clarifier range.

    also, trying different varactors in place of the stock one can move the freq up or down a bit.

    these are my standard techniques when one of the tuning cans won't line up.
    LC
     
  3. Low_Boy

    Low_Boy Well-Known Member

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    Thanks LC. Seems like usually the only one that does not line up is the USB.
     
  4. Dmans

    Dmans Well-Known Member

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    Low_Boy,
    My recent experience with a "Made in the Malaysia" 148 had the same issue though the clarifier was not unlocked and still "factory". I found a bad spot in the CT2 trimmer capacitor. Replacement allowed me to tune it in. FWIW

    73's
    David
     
  5. nomadradio

    nomadradio Analog Retentive

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    A tiny 5.6 uH RF choke that came with the old Expo 100 kits would get installed in series with the clarifier crystal's varactor diode. This would increase clarifier range, but it moved all the clarifier-center adjustments upwards. This trick would sometimes move the USB adjustment beyond its range of adjustment. USB would still be below the correct frequency with the adjustment set as high as it goes.

    Stretching the clarifier's range by changing resistors will also cause this. Try setting USB on frequency with the knob pointer at 3 o'clock instead of 12 o'clock. Good chance that all 3 modes can be set on frequency this way.

    Or make sure all those resistors that limit the clarifier's end-to-end range have been put back, and the short gets removed across R, umm.. um,

    Too lazy to look that one up. The "3 o'clock" solution is bound to be easier.

    73
     
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  6. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    I think R175 is the resistor Nomad is referring to... it's the resistor that ties one side of the clarifier pot to ground. Jumping R174 and/or R175 will shift the center slot. I usually "line up" my center slot at about 2 o'clock, then when I get all the modes lined up, I pull the knob off and "reset" the center at 12 o'clock.


    ~Cheers~
     
  7. Handy Andy

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

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    I won't bore the rest of the users with posted graphics, not sure if these have been posted here before - but here are some that may help you...
     

    Attached Files:

  8. 357magnum

    357magnum Well-Known Member

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    Will this work on a 140gtl ? I've had mine for years , I paid $50 from the termite guy spaying my first home , Has extra channels but I have no freq counter in the mobile and I think they put extra channels in and never un-locked it or should that be part of the process ? I'm no radio Tech just an Op .
     
  9. Handy Andy

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

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    Is your 140s' Voice Lock Unlocked now?

    Here are some cautions...

    Audio uses the 7.8MHz Xtal - and does not slide (not adjusted)

    Tripler for IF (and the UHIC007 VCO chip) uses 11.125MHz Xtal - and is the adjustable side - so there are limitations...

    The pot that adjusts Voice lock - uses 8 volts with no dropping resistors on it's LIVE FEED side. So this means it's very sensitive to audio mods that tend to pull more power and drop regulation when heavy modulated audio gets in. It also means that any typical mods to jumper resistors to obtain more slide is SUICIDE - as in DON'T DO IT!

    Since the adjustable side feeds a tripler, the effect of drift is far more pronounced and requires you to pay attention to the thermal and voltage aspects of things done to the radio. Read this as, the more stock the radio is, the more success you will have in making this work.

    Locate D35 and D36 - D36 should be by VR3 TX Adj. pot, D36 is removed and kept for later.

    Locate Voice Lock Pot, the wire that feeds 8 volts RX - would be the one that does not change (vary in voltage) at the control - that trace on the front panel board needs to be cut and a wire installed from + side of C66 By the PLL PIN 9 (Power feed for IC) to the pot.

    That unlocks the Voice lock, you don't have to adjust anything else - if it does change center slot, install the diode you took out of D36's position and install BANDED end towards Trace that feeds the pot - point it towards the Voice lock control. This would "drop" some voltage but also provides some thermal tracking. If that drops it too far, as in the Center Slot is too far off, then remove the diode and run power straight - the radios' Varactor it uses needs to maintain this voltage so any real adjustments to obtain larger levels of slide requires some resistors to be changed.

    Those are on the front panel...this can get tricky - so the less done to the radio - the better.

    I'll stop here to let you catch up...
     
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  10. 357magnum

    357magnum Well-Known Member

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    Thanks Andy ! Back to that saying .. " If it ain't broke don't fix it " ! Because right now , No problems , just wondering , someone like me ? I'll just leave it alone or have it done by a " Pro " .
     
  11. Low_Boy

    Low_Boy Well-Known Member

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    I have put more cobra 148 GTL's back to factory because of this problem
    I know USB does not get used on cb but i like things right. What about my idea of adding this to the capacitor side lf L59? Any thoughts why it would not work? That particular varicap is and I may have screwed up when I bought them 40pf.
     

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    #11 Low_Boy, Nov 30, 2018
    Last edited: Nov 30, 2018
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  12. Handy Andy

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

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    Actually you didn't. But you didn't, also, buy a ton of them either, which is smarter...

    Ok, You can use that cap as a means to tune - but you also should check the USB side of that 7.8MHz too...

    IF you find that you can not "center" the L59 coil properly - it means you have to tweak the other missing half that is forcing you to tune the coil in this fashion.

    So, instead of raising the bridge....
    ...Lower the water - ok, a Jerry Lewis film but hey- eh?

    I'm saying try removing CT 2 variap and replace it with this 40pF instead then retry to tune.

    Cobra148GTLCT2.png

    Don't focus so much on the L59 and thinking you have something wrong. You need to also look at the 7.8MHz and getting it right. These two work to offset and compliment each other - set up L59 first - set it's slug is close to mid to upper tuning range point (near the top of the slug but not all the way up) - then go tweak the 7.8 MHz side, via CT2 to move L59's audio curve to center slot...

    Once L59 is set to it's near peak - go back to CT2 (that varicap) using your new 40pF - and swing for the bleachers - a.k.a. - set it so you get L59 nearly right (without touching L59) - then go back and tweak the best of both to obtain USB center slot.

    See C125 - a 5pF? You can change this too. Works with CT 2 to offset it... so you don't have to even swap the CT2 if you wish.

    Ok, with everyone in bed, I have a moment to add more...

    Look at the snippet I am attaching to this post. There is a reason to work on the CT2 side inclusive. It's not all L59's fault. Well, maybe it is, but you'd have to locate the guy from Uniden that did this...

    Ok, L59 note it does not have or use a "DC Block" cap - like L23 or L22 - you can install a cap but I'm not sure you'd like the results - look at the circuit and all the parts involved.

    The 11.125 Xtal floats but L59 is used to provide ground for the other half of the Xtal - so in essence - it's the real tuning point and reference for the other two coils of L22 and L23. So before you go chasing down that guy or look elsewhere - look at the schematic and back to that radio - L58 and R146 are also a part of L59 and that tank circuit resonance. They provide the return path for the Xtal thru L59 if the other lines are active or not - they cannot provide a ground reference to the Xtal - L59 is the one that does that to stabilize the circuit.

    You can prove this to yourself by fiddling with L59 at the same time you fiddle with the L22 and L23.

    So, you can try that cap idea, but I'd look at CT 2 and fixing/replacing that part and retune. IT should help you locate the best balance for you to find center slot for all the modes.

    EDITS: L59 on the brain....
     
    #12 Handy Andy, Nov 30, 2018
    Last edited: Nov 30, 2018
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  13. Dmans

    Dmans Well-Known Member

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    It did work for me. FWIW

    73's
    David
     
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  14. Low_Boy

    Low_Boy Well-Known Member

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    Thanks Andy I will look at this again on my computer from home if I ever get there ha ha,
     
  15. Handy Andy

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

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    Bad weather?

    Drive Safe...
     

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