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148 gtl issue with adjusting driver

Discussion in 'General CB Services Discussion' started by kaos513, Dec 2, 2018.

  1. kaos513

    kaos513 Well-Known Member

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    Hello guys i have a 1992 Philippine model cobra 142gtl issue i have is adjusting my driver with a bench top ma meter which works well on my other radios. any help would be great.


     

  2. kaos513

    kaos513 Well-Known Member

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    it's reading like 100 ma cannot adjust it up or down.
     
  3. loosecannon

    loosecannon Sr. Member

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    check the trimmer resistor itself.
    LC
     
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  4. kaos513

    kaos513 Well-Known Member

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    I replaced it as it was very loose it still will not adjust stays at 100ma.
     
  5. Low_Boy

    Low_Boy Well-Known Member

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    Easy mistake . Question may sound stupid but it has happened to myself and many others is the radio in sideband mode?
     
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  6. kaos513

    kaos513 Well-Known Member

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    yes , I did find something interesting, I saw resistors one for final and for driver they have bare leads on one end, I put the positive lead on that bare lead and I can adjust bias from there strange bit works .
     
  7. kaos513

    kaos513 Well-Known Member

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    Here is the picture how I managed to adjust the bias red lead went on to bare lead of resistor circled.
     

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  8. Handy Andy

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

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    Your Driver Bias DIODE ain't hooked up...

    Cobra148DriverBias.jpg

    Got one leg to Board (Foil) Ground?

    Cobra148BIASDriver.png
     
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  9. kaos513

    kaos513 Well-Known Member

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    Yeah I put it back, still have to measure at those points to get my readings.
     
    #9 kaos513, Dec 2, 2018
    Last edited: Dec 2, 2018
  10. kaos513

    kaos513 Well-Known Member

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    Tp 7 and tp 8 have 13 volts seem I would need 8 volts to adjust bias and driver and r180 and r182 carry 8volts.
     
  11. Handy Andy

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

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    Mostly I just adjust between 0.50~0.62 VDC at D50 - the mA reading requires a "break" on the line - easier to trim for voltage - should match reading.

    Measure FROM board to diode feed wire - by the Trimmer.

    Now, I also add a 180 ohm resistor ACROSS the D50 diode to "swamp it" - so the reading on that diode with this mod is no more than 0.33VDA which equates to about ~30mA. It is not necessary but I do this to preserve a driver If the operator uses AM mode a lot more - this reduces the bias and the heat AB class does - but I do it (or did it) to help people keep their radios working better longer...Less likely to fail when operator want to drive it hard.

    I do that mod to save the Driver (2166) - does not affect SSB drive a lot - keeps the Driver in AB and can survive a maul masher AM station operator if need to.

    If the Diode were ever to "open up" there is no divider in the Driver Bias line to keep 8 volts from "snapping" on the Driver all the time - damaging it. The 180 ohm resistor provides a cushion that other radios use in their lines to pare down the bias so it doesn't climb and damage the very part it's supposed to protect...

    Besides, if you like how other radios' do theirs' - it pays to take a look and see if their designs can help yours...
    Cobra138and146GTL180ohm.png
     
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  12. kaos513

    kaos513 Well-Known Member

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    so my question is that the right spot on picture to adjust the bias and final ?
     
  13. Handy Andy

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

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    mA no, Voltage no...

    For mA - requires BREAKING circuit at TP7 and TP 8

    BUT YOUR BOARD IS A DIFFERENT LAYOUT than that which I use for my own templates...I've attached my board so you can see how D50 and D49 are wired differently in my version - but both are essentially the same...

    Or IF you can - remove TP7 TX into Dummy load and ADJ mA reading on METER using Trimmer - in SSB MODE no audio - TP7 and TP 8 can be done this way...

    KAOSBias.jpg

    TP8 - WIRE TO DRIVER - NEGATIVE TO WIRE (Where you pulled wire) - POSITIVE TO BOARD
    TP7 - WIRE TO FINAL - NEGATIVE TO WIRE (Where you pulled wire) - POSITIVE TO BOARD

    Are you aware you may have a wiring problem...note your purple wire and where it goes to the board - that spot is GROUND for BIAS to D50 - there is a hole RIGHT NEXT TO L42 in photo - that this wire should go to - VERIFY FIRST - hate to see you lose a good radio from a mis-wired jumper...

    KAOSBiasD50.jpg

    For Voltage - try this...works and is far safer...

    Set METER POSITIVE to ferrite bead shown in photo... METER NEGATIVE to Foil ground - any convenient can

    KAOSBIASVoltageD50.jpg


    Hope this clears things up...

    On a separate note...
    Cobra148GTLD50locations.png

     

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    #13 Handy Andy, Dec 2, 2018
    Last edited: Dec 2, 2018
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  14. Low_Boy

    Low_Boy Well-Known Member

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    Good catch Andy.
     
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  15. kaos513

    kaos513 Well-Known Member

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    Adjusted! working great
     

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