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MOSFET conversions for upD858 and MB8719 boards

Discussion in 'General CB Services Discussion' started by ExitThirteen, Mar 10, 2011.

  1. Robb

    Robb Yup

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    Using a zener - like Galaxy radios use?
    Sounds like a plan.



    I run the DX-959 @4v; gets great reports for clarity and crispness w/heatsink = 24w+.
     

  2. Robb

    Robb Yup

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    This MOSFET replacement you posted has proved to be an inexpensive replacement solution compared to the CB-20 by Eleflow ($20!). Cost to replace all parts was less than $3 for me - if you make the biasing device yourself. I made one but didn't use it - BTW.

    [​IMG]

    Removing/replacing a couple of parts wasn't that hard to do. The old glue/shmegma on the board made it hard to find L40 tho; so I traced it out using a schematic from CB Tricks.
     
    #77 Robb, May 23, 2014
    Last edited: May 23, 2014
  3. Robb

    Robb Yup

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    Whoa, hold on a tick....

    Just as well that I didn't use the biasing device I made, read this:

    http://www.worldwidedx.com/general-...anion-cbtricks-errors-drawing-correction.html

    [​IMG]

    Since PsiDOC did the math, I used his formula of .000185 / (desired bias voltage) = ~47k ohm resistor for the value of R2 in the EN369FN biasing device for a 4v gate voltage. 20k ohms will give the more desirable 3.75v gate voltage, as shown in the above graphic.

    Soooooo........Using a 100k ohm VR @ R2 in parallel with a 20k ohm fixed resistor will make the gate voltage variable for the EN369FN?
    That's a guess BTW; but it seems logical tho . . . OR adding a 100k ohm VR between the gate and source pins using the EKL EN369FN device . . .
     
    #78 Robb, May 23, 2014
    Last edited: May 23, 2014
  4. Robb

    Robb Yup

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    Sorry PsiDoc; you cannot be correct.

    I experimented by trying a 100k ohm VR across the EN369FN device. It cannot be 20k ohms for R2. The VR did work; but as high as the gate voltage would go was 3.5v with max resistance from the VR. So R2 would have to be greater than 100k ohms . . .
     
    #79 Robb, May 23, 2014
    Last edited: May 24, 2014
  5. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    100k for R2 would be a sufficient value. That coupled with the 100K variable will give good results. Let us know how you get on. :)



    ~Cheers~
     
  6. Robb

    Robb Yup

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    Took out the EKL part and measured the resistance putting the neg lead on the positive pole to read just the resistance of R2. It was 19.5k ohms just like PsiDoc said; so he was right! Then I put the pos lead on the pos end of the device and read 21.5k ohms.

    Now the fwd resistance thru a 1n4148 diode is ~180k ohms when tested. So in parallel with a 20k resistor, it should read 100k ohms. What is it that I don't understand? Doesn't make sense; what did I miss? What about the diode resistance; is that where I got the wrong picture?

    :confused:
    :unsure:
    :oops:

    Don't have any 100k resistors ATM . . .
     
    #81 Robb, May 24, 2014
    Last edited: May 24, 2014
  7. Robb

    Robb Yup

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    Made another homebrew EN369FN using a 22k resistor for R2.
    After testing it, it has the same/exact values forward/reverse as the OEM EKL part.
    PsiDoc was right on the money . . .
     
  8. Robb

    Robb Yup

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    Tested the MOSFET/Cobra 148GTL with the Autek WM-1 watt meter and 50 ohm dummy load:

    AM: 5w dead key swinging to 20w+.
    SSB: 24w

    The MOSFET barely gets warm with short keying. If you key up for 1 min, it does get a bit warm. That is with the bias at the stock EKL device/3.85v setting. Didn't get any 100k resistors yet; prolly tomorrow. Looks fine on the scope - too . . .
     
    #83 Robb, May 26, 2014
    Last edited: May 27, 2014
  9. Heatwave1972

    Heatwave1972 Member

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    exit thirteen where does the 56k go excactly?
     
  10. Heatwave1972

    Heatwave1972 Member

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    from the far side of L35
     
  11. Birddog2009

    Birddog2009 Member

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    Exit thirteen why does the mod sound trashy in my 858 SSB rigs? PM me or post on here or text me and tell me what i am doing wrong.
     
  12. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    What's your voltage at the gate of the mosfet? Should be around 3.75V or so.

    Sounds like a low voltage condition. Is the bad audio in all modes, or just one mode only?



    ~Cheers~
     
  13. M0GVZ

    M0GVZ Sr. Member

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    So you're doing all of this to get an extra 6W or so? What is the point? The receiving station can't tell the difference. Even if you double the power its only half a S point increase. You'd have to increase the power fourfold to be remotely worthwhile.
     
  14. Robb

    Robb Yup

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    As it was stated earlier, the reason is not so much the small gain in power; but the availability, time, and cost factor. I can tell you firsthand, that using a commonly found IRF-520 and a few paltry parts and taking very little time, one can make use of that substitution for less than $3 as opposed to using an Eleflow for $12 plus shipping and the loss of time waiting for it to arrive in the post. Not to mention it works like a treat, and the power gain is just so much more icing on the cake.
     
  15. GnG8d

    GnG8d Well-Known Member

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    That's about what your Moxon does over a dipole, and the mosfet conversion takes considerably less time.

    But like Robb said, it's mostly a parts availability issue in trying to keep some of these radios going. I know some will remove a perfectly good 2sc1969 and replace it with an irf520 looking for more power, but most realize how insignificant that power gain really is.
     

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