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Yet another cobra/irf520 question

Discussion in 'CB Radio Modifications' started by Timekiller, Feb 7, 2012.

  1. Timekiller

    Timekiller
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    Ok, so I did the IRF520 conversion (with companion parts) and the results are poor. Im getting 1.5w DK ~4.5W peak swing (pep) I tried diagnosing the problem, using another 520, and checking values at various points.



    I did find that the gate voltage is sitting at .2 volts, with modulation (whistle) it hits .33 volts. From what I understand this should be around 3.5volts correct? All pots are tuned for max output (L14,L17,L18) I also switched the driver to a 2314 and output decreased.

    So, can/where can I grab a TX only voltage to feed the gate with proper voltage?

    Any help would be greatly appreciated... I know this thing should be outputting more.

    I started this project to learn the workings of RF amps, and have learned a lot, but Im at a halt.
     

  2. Timekiller

    Timekiller
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    Ok, so I did find this thread, good to know someone else out there had the same exact problem as I.

    Cobra 25 ERF2030 upgrade

    I guess Ill just have to hunt down a TX only 8 volt supply, I'd rather do that than run a constant off of a 12 volt supply.

    Anyone know where I can grab said 8V?
     
  3. Timekiller

    Timekiller
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    I found a TX only 8v... If I put 3.5v at the gate of the IRF520 the wattage goes down?!? I used a pot (0-20K) and If I dial it in for max performance, Its at around 1.25v at the gate, and it seems the only thing the extra voltage is affecting is the DK (goes from 1-5w swing, to 4-5w swing with the added voltage)

    Now Im really lost.
     
  4. Timekiller

    Timekiller
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    Just to add to the confusion, I removed the bias pot I added to the gate. Hooked up the radio to a running vehicle and Im getting 4.5W DK and a swing to just above 10W. The bench ps was a 12V deep cycle battery that all previous testing was done on. Im assuming the difference was just in the 14.2V as opposed to the 12.3V from the deep cycle battery.
     
  5. mr_fx

    mr_fx
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    what parts did you use? what instructions did you use?
     
  6. Timekiller

    Timekiller
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    I used the instructions on CB Tricks, along with this info. (replace R38 w/ 10K, C46 to 390pF, & wire from C114 (+) with a 470ohm.)

    I first used a IRF520 with the pre made EN1230, and I also made a companion part using a diode and 3.3K/470 resistor combo. All polarity has been observed. I have also tested with another IRF520, and an IRF510 just to see if the FET was blown.

    Like I said, the best I was able to get was with a TX only 8v running to the gate (left leg if looking at the face) This was the wire that was at one point run to the + of C114, as stated above.

    Another thing is, with the stock 2075 final, I was only able to achieve ~6-7 watts max with proper tuning.
     

: bias, cobra, irf520, mosfet

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