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Cobra 29 High Power mod with 2sc1969 transistor

Discussion in 'CB Radio Modifications' started by Moleculo, Dec 15, 2008.

  1. Moleculo Administrator Staff Member

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    I was asked by a few people how to do this mod, so I figured I'd just post it up.

    Please don't copy and paste this mod to other sites/forums, but feel free to link to it.



    Also applies to:
    Uniden 78
    Cobra 25 & Uniden 76 mod is similar, although you'll need to cross reference against the schematic.

    1: change the final to a 2sc1969
    2: change the driver to a 2sc2314.
    3: change the electrolytic capacitor by the audio IC (C42, 1000uf) to a
    3300uf, 16v.
    4: Take a turn off the output tuning coil (see tuning coil note below)
    5: Remove the AMC potentionmeter (VR4), and put a 1K resistor in it's place.
    (only two connections are used on this pot, use those two connections
    for the resistor)
    6: add a 1K in series with D11.
    7: Change R48 to a 33K ohm resistor.
    8: Change R55 to a 10K 1 watt resistor.
    9: change C37, and C68 to a .2 monolythic (this changes the A.F.
    response on TX)
    10: Now align the radio, start with the output coil, the the predriver
    stage for peak output.
    11: Tuning notes: Tune L17 for max with audio. Then, tune L23 for max output with audio, this will upset the radio's frequency. So, after L23 is set, adjust L24 to put the radio back on frequency.

    Tuning coil note: If the radio says "made in China" on the back, you must remove 1 turn from the output tuning coil with the slug in it. If it doesn't then you may or may not have to. Count the turns, if it has 4, leave it alone, if it has 5, it's too many and won't tune, remove a turn. You may have to add a 68pf capacitor from the left terminal of the coil to ground, with the solder side of the board up. This method produces better results than just removing the tuning slug.

    This mod will consistently produce output between 10-12 watts AVG and 45-50 watts peak. If I can find my video clips of Bird 43P power meter and spec analyzer I took after doing this mod, I'll follow up with those.
     
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  2. loosecannon break on through

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    moleculo, thanks for posting that!

    i am planning on purchasing a new PC78xl pretty soon, and im guessing it will be made in china.

    since im going to be running a DX400 after it, i dont want to replace the final with the 1969, but STEPS 3 and 9 should be very cool!

    if im not going to replace the final, should i still remove a turn of the output coil if there are 5?
    or is this change only done to work with the 1969?

    also, STEPS 5 and 6 are interesting.

    thanks for any info or advice,
    LC
     
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  3. ExitThirteen Grumpy, Cranky Repairman

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    Tag, I might find me a 29 to do this on... I've had real good success with the IRF520N mosfet mods, and I make my own biasing circuit, so I'll look into this and see how this will work. Cheers!

    -Exit
     
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  4. Moleculo Administrator Staff Member

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    Yes, you can/should still look at the tuning coil regardless. Doing it that way really lets you fine tune it. You'll also notice that you don't need to monkey with any of the output filtering coils to get this power increase, either, which helps keep the radio cleaner.

    ExitThirteen: If you wouldn't mind following up with posting how to do your bias circuit for the MosFET mod, that would be great!
     
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  5. ExitThirteen Grumpy, Cranky Repairman

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    Moleculo,

    What I do with the mosfet mods is basically the EKL way, but instead I use an IRF520N mosfet. It says to use an EN1230 for the bias, but I make my own. Take a look at the schematic and finished product below. All these parts can be found at any Radio Shack or electronic wholesaler, and it's very easy to do.

    Last radio I modded was a Uniden PC-68LTW, and when I was finished, it deadkeyed 4W with a 35W swing, and was clean on the scope with no flat-topping. Hope this helps!


    Sorry about the grainy pic, but it uses 1 - 470 ohm resistor, 1 - 3.3k ohm resistor, and 1 - 1N914/1N4148 switching diode.

    Also wanted to mention that I just used shrink tubing around the finished piece.


    -Exit

    [​IMG]
     
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  6. loosecannon break on through

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    thanks again moleculo!

    im not sure why anyone messes with L13 and L12 trying to get more power out.

    from what i understand, these coils, along with their associated shunt capacitors form a halfwave filter.

    its purpose is to filter out any harmonics, and is used instead of a pi filter because it has a 50 ohm input and a 50 ohm output. (or, put another way; it has the same impedance on the input and output)

    seems to me that if you spread or compress these coils, you mess up that 50ohm to 50ohm relationship and decrease the filters ability to block harmonics.

    sure, the wattmeter shows a bit more wattage, but that wattage is not at 27mhz.

    again, thats how i understand it, maybe i am mistaken, but i dont think so.
    LC
     
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  7. 1096 Member

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    Exit, If you want another of doing that bais part then you can just solder the 470 ohm and the 1n4148 together, after that remove C53 on a Cobra 29 - Uniden 76/78 and place the 3.3k there instead of soldering all three together.... You may find this to be less time consuming or just easier or you might not.... Just throwing out another way of doing it.... For a Cobra 25 - Uniden 66/68 you can remove C149 and put the 3.3k there...
     
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  8. DoctorMaster Member

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    Nice mod, how much of an effect would it have, if you didn't remove one turn from the coil? Is it possible to due without performing this step, or will the performance suffer too much? Is so, any idea how much?
     
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  9. Moleculo Administrator Staff Member

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    You won't be able to get close to the max power out of the rig. Most people just pull the slug out and throw it away, but that's not the best way to do it.
     
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  10. Robb Yup

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    What are steps 3, 7, 8, and 9 changes providing?

    I bought the resistors and capacitors; I just need to know if the changes will work with the IRF520N change asa well.

    Also - would it help to change out the driver to a 2SC2314 with the IRF520N?

    I'm a noob to modifying radios, and I'm about to give a 29LTD an IRF520N mod with the EN1230 home-made bias circuit - since I have those parts.

    And - do I remove a coil from the tuning coil from the top or bottom of that coil? Or does it make a difference?
     
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  11. Moleculo Administrator Staff Member

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    OK, I had to go over the schematic, and the ones on CBTricks are hard to read. I'm not that good at understanding what everything in a circuit does, but here's my take:

    Steps 7,9 go together. They change the circuit values around the Audio power amp and the AMC (modulation control) transistor. Basically it's widening up the audio response and allowing the radio to modulate a little more free.

    Step 8 : Not sure I understand it, but it looks like it's the first resistor on the collector of the final transistor just past the low pass filter. Adding more resistance to ground at this stage seems to me like it would allow more current to flow through the final to the load (antenna). This part of the mod is eliminated with the MOSFet mod.

    Step 3: Not sure...it's supplying more capacitance to the audio IC, but since I don't have a pinout handy of that IC, I can't be certain what part of the IC it's affecting. I'm guessing it's just providing a little more sustained drive to the driver and final.

    Maybe someone who understands the circuits better than me will come along and help me out.

    You can't change the driver to the mosfet if you're going to do this mod. You need to do one transistor design or the other but not combine them.
    However, it looks to me like you could do steps 3,5,7,9 with the mosfet mod. Regarding the output coil: It doesn't really matter which side you remove it from; you'll have plenty of adjustment range once you have the correct number of turns. You should be able to do this with the mosfet mod also and fine tune the output a bit better than just discarding the slug.

    If you do this, let us know how it turns out. I've been thinking about trying this out myself but just haven't gotten around to it yet.
     
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  12. Robb Yup

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    Although the mod sheet provided points out the use of the 1K ohm resistor with D11, it doesn't say if the resistor goes to the right of the diode banding. That is, looking from the front of the radio twards the back. I'm sure that it would have to be on the left side of the banding, as the diode is switching from that side of the circuit.

    If I'm wrong, please correct me before I start with that mod. My plan is to do one mod at a time. I'm going to do the MOSFET final mod first, then the TX/RX audio enhancement with the .2 monolythic/33K ohm, then the 3300mf cap/variable pot removal/1K ohm resistor replacement.
    But-
    Isn't it redundant to put a 1K ohm resistor in place of the variable pot AND the 1K resistor in series with D11 diode?
    I thought the variable pot WAS feeding D11 - or is that wrong?

    Q: Why wouldn't the addition of the 2SC2314 work? Doesn't the MOSFET final get fed from the stock bipolar driver as well?
    So, if the MOSFET gets a bigger boost from a better driver; wouldn't that potentially drive the MOSFET final harder?
    Or - would it be too much? Especially since I am using a IRF520N as Exit 13 suggests? But I can use either the ERF2030 or the IRF520N; as I have both. And either MOSFET can be driven with the same EN1230 - rrrright? My point in saying this is purely speculative and needs correction - that is why I saying it here to be examined and considered.
    What's right/wrong with these ideas?
    Please comment!

    BTW - Does anyone know when CB Tricks will be fully operational again? They still don't have everything available like they used to. That info provided from that site is indispensible and essential. It's too bad someone wrecked it for everybody else...
     
    Last edited: Mar 31, 2009
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  13. Moleculo Administrator Staff Member

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    Regarding the resistors around the VR and diode...I'll have to look at the schematic again tonight. I don't have it here at work with me and CBTricks is still down. Maybe someone else has it and can look at it. Remember, I didn't come up with this mod, I just documented it here.

    I think the main issue with changing the driver also (this is speculation on my part) is driving the FET too much. There was a thread about runnign these Mosfets too hot from too much current causing them to pop here:
    http://www.worldwidedx.com/tech-repair/26775-erf-2030-a.html

    CBTricks has a warning about running them with too much current or overdriving them here: CBTricks MOSFET Experimenter's Information Notes

    I'm not sure, but I would bet that changing that driver will push the FET too much. BTW, check out some more notes on increasing the carrier of the MOSFET mod here:

    http://www.worldwidedx.com/tech-repair/24863-mosfets.html
     
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  14. Robb Yup

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    I put in the MOSFET final kit. It keys at 1/2 watt and swings waaay over to 5 watts. Is it because the driver is working, and the final isn't?

    I checked my work before I turned it on; so I'm stuck now.
    I'm using the EN1230 bias; is it the right one?
    Maybe I should use the EF369, or change the driver.
    I can't use a diode checker on the MOSFET final, because they need at least 3v to open up - right?
    I gotta think about my next move first.
    Any suggestions?

    This 29 radio came with the 2SC2028/2SC2029 driver/final setup...
     
    Last edited: Apr 1, 2009
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  15. Moleculo Administrator Staff Member

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    Yeah, sounds like the FET isn't doing it's job. Use the link I gave you to that other thread and check everything out with a multimeter along the way.
     
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