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2000 GTL Frequency Synth Error

Discussion in 'General CB Services Discussion' started by LeapFrog, Mar 15, 2020.

  1. LeapFrog

    LeapFrog Wielding Hanlon's Razor

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    Radio had been through the ringer, changed electrolytic and tantalum capacitors, rebuilt PSU and PA.


    Everything seems to work ok, though it's one channel low & occasionally won't transmit. I can get to 27.285 by going to Ch. 29 and adjusting clarifier.

    Now and again the frequency jumps, I might be clarified to 27.285 and all of a sudden I'm almost a channel off. Sometimes the VCO goes out of Lock and I don't have any transmit or receive.

    Slight trouble adjusting L19 to see 3.0 Volts on 40 and 2.0 Volts on Ch. 1, though managed to get approx 2 Volts on Ch 1 w/ 3 Volts on Ch 40.

    AM Mode Ch. 1, Freq. counter to TP1, was only able to obtain 34.550 MHz, attempted adjusting L23 for correct freq. (34.650), core bottomed too soon. (was screwdrivered/chipped years ago)

    The traces around the MB8719 have been repaired, initially I thought issue could be related to programming input pins but they look normal. (Haven't verified the N code)


    Unmodified the radio, reverted it to stock and had it nearly aligned after making some changes, fine clarifier was out 1/4 of a turn.

    VCO block could have unseen corrosion, a few pins on the original PLL eroded away (it's been swapped now)

    It's possible that I changed crystal (X3) out when I undid the mods.

    My memory is not the best and honestly this point I really wish my friend did not ask me to unlock the clarifier after I had reverted the radio back to normal, but It is what it is and need to get it solved..

    Pulled R44, D52.
    Removed brown wire from pot.
    Took red to constant 8V.
    I wasn't looking for a bunch of slide so I did not short out R174, nor did I take the yellow wire directly to ground.

    I don't have the frequency synthesis theory memorized... I can tell you at one point the counter showed 34 megahertz!?



    I believe all of this is due to the fact that several of the ferrite cores have been chipped, I'm having an issue with this alignment.
     
    #1 LeapFrog, Mar 15, 2020
    Last edited: Mar 15, 2020

  2. Handy Andy

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

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    Make sure they didn't make any changes to the IF section. For if by your symptoms - it sounds like they left out a part.

    Here's what I got...
    Cobra148GTLAMUSB.jpg

    Note Clarifier below Red line...

    Clarifier Ground lead needs to have 3.3K on it, if it went straight to ground you can find yourself unable to tune straight up...it may be why you're off.

    Same goes for R146 - over by the VARACTOR in the above graphic, if it's been taken out - it can put you into nickles and 2.5MHz off setup - and things won't line up very well that way either.

    Only other option was to check to see if 8V TX side is "leaking in" forcing a jump in your Clarifier voltage.
     
    LeapFrog likes this.
  3. LeapFrog

    LeapFrog Wielding Hanlon's Razor

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    Thank you.
    I did not short R174, and left the yellow wire alone (believe a resistor to gnd is down circuit on pcb)..

    I will inspect to make sure the resistor is actually in circuit and not bypassed, thank you Andy!
     
  4. LeapFrog

    LeapFrog Wielding Hanlon's Razor

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    Checking Pin 8 of IC3 for 10.2400
    My friends vintage counter read 10.2394XX instead of 10.24000.
    I believe the counter was loading the oscillato
    r, causing an inaccurate measurement;
    Those Chinese test leads offering a simple direct current path to the BNC input.


    After reading some posts regarding clarifier problems I noticed I should be
    adjusting the VCO for 3.2 Volts on Ch 40 not 3.0 Volts as I read in my pdf.
    (after adjustment I am reading 2.22 Volts on Ch 1)

    With fine clarifier positioned center and the course positioned at "3:00"
    (I read that in a post) re-adjusted L23 to obtain channel frequency;
    the ferrite core was about 1/2 way down the L23 can, before when
    I was centered
    I would have to bury the core in L23 (all the way down) to get close to being on freq.

    After these alignment steps the PLL remained locked,
    clarifier has functionality and the correct channel positio
    n/N-code.
    (Fine adjust control doesn't do much, but it works)

    I Compared the internal freq. counter reading
    to the external counter (checked against SDR)
    the internal reading was a little high of the actual transmit frequency,
    about 18Hz high but within +/-20Hz tolerance.


    The driver and final have been properly biased.
    Thought it might be ready for delivery....
    *Then.. Suddenly...*
    Like the Cheetah pouncing towards a Gazelle.
    Murphy's Law strikes again!

    The radio jumps down about a whole channel,

    Closer to 27.2750 than 27.2850, all while Ch 28 is displayed by the channel indicator.
    At that moment the Delta Tune no longer responded, the internal frequency
    counter remaining still; I was left feeling indignant.

    CFobra2Kpll.png
    Photo sourced from Google web search.

    I decided the clarifier mod must be the cause for unlocking, but reassured myself of
    the fact that people have been running poorly performed mods like
    that for years and the radio function ok...


    Initially the clarifier seemed okay, and TX frequency of carrier looked stable.
    27.2850 on Ch 28 ( and verified externally) I keyed down for a lil bit
    (a minute or two) & noticed the TX freq is actually moving.

    Without touching or turning the clarifier (zero modulation)
    it was moving/drifting (slightly) up and down.
    CH 28 TX mode
    27.28510, 27.28512, 27.28518,
    27.2822, 27.28524, 27.28526
    then back down
    27.2824 27.28522, 27.28520 27.28515, 27.28510

    I believe the VCO card is stock and may be having an issue, due to the glue, or solder joint
    I'll have to locate my copy of Sam's photofact so I can read over the expected voltages/signals.
    CFobra2Kpll.png
    I decided to tap, rap, & slap this area, & the radio jumped back to 27.285 MHz with the clarifier functioning again, I reflowed the joints, will have to double check all the caps involved right there.

    With the modified clarifier circuit, in order to align the knobs to center during alignment I'll need to work on the component values used in varactor "clarifier" circuit.
    With the radio covers on and radio warmed up the internal freq.
    counter was subtly bouncing between 27.2850 and 27.2851 in RX mode.
    We decided to let it sit running for a while to see if it goes out of lock.

    If the voltage regulator I.C.(s) are good & produce solid rails
    will the radio's last digit of the counter still drift a little no matter what?

    Occasionally I see 34.XXXMHz (I think 34) flash up on the internal counter,
    since my last attempt at going through the alignment steps I haven't seen it pop up again.

    The broken ferrites, corrosive glue damage are not helping this restoration.
    it's possible that I've overlooked an intermittent failure somewhere, perhaps a modified component value (flipped a broken slug) loose wire or failing IC.
     

    Attached Files:

    #4 LeapFrog, Mar 16, 2020
    Last edited: Mar 16, 2020
  5. LeapFrog

    LeapFrog Wielding Hanlon's Razor

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    At first (because of loss of TX/Lock issue) I suspected the clarifier mod was causing VCO to quiet oscillating.

    I will have to go through entire alignment procedure and see if the VCO Card is causing an intermittent failure (if the radio "fails" and acts strange again).

    It could be that there is nothing wrong with the radio, and it maybe just needs a final alignment; seeing 3X.xxx MHz flash up on the counter caused me to raise an eyebrow but I haven't seen that happen again after a little alignment.
     
  6. LeapFrog

    LeapFrog Wielding Hanlon's Razor

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    Update:

    After a complete re-alignment the radio is on channel and stable.

    Thank you WWDX memebers who helped me get here!!
    20200214_144637.jpg
     
    tecnicoloco and TM86 like this.
  7. Robb

    Robb Yup

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    LF:
    Do you use the Kenwood software with your TS-2000?

    Has the encoder been tested and eliminated.

    Just curious questions . . .
     
  8. LeapFrog

    LeapFrog Wielding Hanlon's Razor

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    Robb, I've been a bit scrambled lately.

    The clarifier was off-center after it was all said and done, values needed to be changed in the circuit. If you tuned the clarifier far-left it would pull too hard on the vco causing the pll unlock!

    Sorry, I don't recall the Kenwood.
     
    #8 LeapFrog, Apr 14, 2020
    Last edited: Apr 14, 2020

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