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Galaxy radio drift

Discussion in 'CB Radio Modifications' started by Larry A McClanahan, Aug 18, 2018.

  1. Larry A McClanahan

    Larry A McClanahan Active Member

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    Gentleman, I believe I have found the cause and the Cure for the Galaxy radio drift.
    I will use the clarifier on the ranger 2950 versus the Galaxy dx2547 for instruction purposes. The Galaxy's all use a zener diode in its clarifier section that is an SVC 251. It has a rollback voltage of minimum 12 volts. That means that at 12 volts or higher it becomes unstable. The ranger has a svc201 which does not become unstable until 16 volts or higher.


     
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  2. Larry A McClanahan

    Larry A McClanahan Active Member

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    This is major!
     
  3. nomadradio

    nomadradio Analog Retentive

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    Do you mean this SVC251?

    It's not a zener. It's a voltage-variable capacitance diode, or "varactor".

    Allows you to alter the capacitance in the crystal circuit with the DC voltage from your clarifier control.

    And the 2950 can't place more than 8 Volts onto that circuit.

    73
     

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  4. Larry A McClanahan

    Larry A McClanahan Active Member

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    Yep. It's the culprit. Replace with a svc201.
     
  5. Larry A McClanahan

    Larry A McClanahan Active Member

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    Agree, misspoke but going the try one. Will keep you posted. It's pretty much the only component in there that's different so to speak. Going to give it a shot on my 959 and see if it solves the problem.
     
  6. nomadradio

    nomadradio Analog Retentive

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    The SVC 201 has a higher rate of capacitance change per Volt of bias than the -251 part.

    Wouldn't that tend to increase temperature drift?

    It should increase the range of the clarifier.

    73
     

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  7. Larry A McClanahan

    Larry A McClanahan Active Member

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    Sounds plausible
     
  8. Larry A McClanahan

    Larry A McClanahan Active Member

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    Maybe the 201 is a more stable diode
     
  9. nomadradio

    nomadradio Analog Retentive

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    Each of the two data sheets has a graph showing the change in capacitance ratio with temperature at four different bias voltages.

    Both graphs look the same to me.

    73
     
  10. Larry A McClanahan

    Larry A McClanahan Active Member

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    What if it was never really plotted and some lazy ass just did a duplicate.

     
  11. Handy Andy

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

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    Didn't several older Galaxy radios use dual Varactor? One offset the other...

    Galaxy33HMLVCO.jpg

    Then there is also the Clarifier section too...
    SVC251.jpg
    At least you can see all the parts and every one of them has one type of impact or another in how well the circuit can stay on frequency.

    Didn't know if anyone caught this but the clarifier is buffered in the circuit by R150 and R157 - to isolate the interaction from the externals - will still have drift but clarifier "floats" after all the signal cleanup by C127, C132 and R153 ... weighted, smoothed and RF removed - sort of thing...

    :+> Andy <+:
     
  12. Larry A McClanahan

    Larry A McClanahan Active Member

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    I'm going to find out.
     
  13. nomadradio

    nomadradio Analog Retentive

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    The dual varactor is needed to get the incredibly wide coverage range you need for 8 bands. You won't see that in a 40-channel radio.

    I'll guess that two varactors in parallel was cheaper than a single part that has enough tuning range by itself.

    I used a single varactor in the 21 MHz PLL we used to sell for the Browning Mark 4 transmitter. The 16 MHz version we made for the Mark 3 transmitter used two of the same varactor in parallel. Needed the extra capacitance at the lower frequency. It was cheaper for me to use two of the same part than to buy two different types, one for each model.

    73
     
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  14. Larry A McClanahan

    Larry A McClanahan Active Member

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    Same circuit, different capacitor in the 2950 versus the dx959. And different diode. I have a junk 2950 that I will take the parts out of.
    What radio is that from Andy?
     
  15. Larry A McClanahan

    Larry A McClanahan Active Member

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    Interesting discovery. I pulled the two parts out of the parts radio, 2950 Ranger. They do not even use the SVC201 that the schematics indicate. There is a glass diode in it's place. I cannot read the numbers. I am going to take it by the jewelry shop tomorrow and have them use their magnifying abilities to tell me what the number is.
    I believe this is going to stabilize the frequency on the clarifier circuit. But there is another problem that also causes drift. Heat. Heatsink on the back of the radio that I'm using as a test bed solves that problem. I would like to install the diode right now but I need to take it to the jeweler to find out what it is.
     
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