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Cobra 29 High Power mod with 2sc1969 transistor

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  #1  
Old 12-15-2008, 03:33 PM
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Default Cobra 29 High Power mod with 2sc1969 transistor


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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Old 12-15-2008, 09:38 PM
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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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Old 12-15-2008, 09:40 PM
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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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Old 12-16-2008, 09:37 AM
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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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Old 12-16-2008, 10:58 AM
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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

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Old 12-16-2008, 06:52 PM
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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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Old 12-24-2008, 07:00 AM
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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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Old 03-15-2009, 09:29 AM
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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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