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142 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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    Cobra 142 GTL failure to TX / RX

    Loop Osc counter @ TP1
    USB reads 34.75726
    LSB reads 34.75724
    AM. reads 34.75725

    Counter @ TP3:
    USB reads 7.80248 MHz
    LSB reads null
    AM reads null

    VCO voltage measured @ TP9
    1.7 VDC, adjustment of L13 changes nothing.

    Any insight is greatly appreciated
    73


     

  2. LeapFrog

    LeapFrog Wielding Hanlon's Razor

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    Update/Correction


    TP1 Tripler

    USB: 32.61215
    AM: 32.61235
    LSB: 32.61225

    TP9 VCO Voltage
    1.7 Volts (doesn't change w/channel)

    TP3

    USB: 7.80250
    AM: 0
    LSB: 0

    Clarifier Pot:
    1 leg 8.3 Volts constant
    Wiper to VCO
    Last leg directly to GND!!
    (Here I placed 8K Ohm)

    @ TP1 max rf with L14 slug almost all the way flush with the top of the can.

    Time for me to look at the schematic, critical thinking, perhaps a little simple math!
    20200315_184520_Film3.jpg
     
  3. kaos513

    kaos513 Sr. Member

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    I would recap that section 1st then check it again
     
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  4. kaos513

    kaos513 Sr. Member

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    Also look for any bad solder joints as well in that section
     
  5. Handy Andy

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

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    On top of that, you have several other issues with Age I think you'll need to be addressing...

    The UHIC chip - note the discoloration at the base leads going into the board - looks like something is going on reactively to the leads. See Pink Arrow.

    You're getting lower MHz readings - you supposed to get a free running 35MHz + so you 3MHz low usually this means other problems too...

    Note the Orange arrow pointing the your L19 - may mean an internal cap failed - so you know. Nothing serious but for best "Hints from HELLEN" - turn the slug in two turns and try a retune around it. IF the Frequency won't raise up or even get close to 35MHz free running (you may or may not get lock on this) - if you don't get anywhere - remove the coil and check the slug problem - you may have one in that one too.

    Yellow Circle and Arrow, you have the "glue" that others have discussed as possibly reactive - which if you try to unsolder this - be gentle - it's not welded on there - those components are soldered to a simple board internally.

    CobraRadioLEAPFROG.jpg
     
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  6. nomadradio

    nomadradio Analog Retentive

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    Did you check the 10.24 MHz crystal? If it shuts down, so does the MB8719 PLL chip.

    73
     
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  7. kaos513

    kaos513 Sr. Member

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    Agree with Andy on that glue it’s notorious for eating up leads
     
  8. nomadradio

    nomadradio Analog Retentive

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    1977 date code on the MB8719 PLL chip.

    Might be more than just one issue in a 1978 or 1979 radio.

    Never learned to troubleshoot a PLL without a 'scope.

    Seeing 10.24 MHz on pins 7 and 8 is where I would start, followed by pin 17.

    Those are the two input signals the chip depends upon.

    73
     
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  9. kopcicle

    kopcicle Sr. Member

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    teaberry_stalker_ix_xv_sm_sch_pg33.jpg
     

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