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Cobra 25 LTD mods?

Robb

Honorary Member Silent Key
Dec 18, 2008
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Silicon Valley CA, Storm Lake IA
A local guy is asking me to give this radio an alignment. So I will do that. But while I have it open, I was wondering if anyone knew some trick mods for it.

Like which diodes/locations can be replaced with Shottky diodes in the receive and the NB circuit?

Which transistor # for the first IF stage replacement with a 2n2999e?

What is the NPC mod for it?

Schematic page?

Any other kewl mods/info?
 
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25 ltd mods

http://www.angelfire.com/fl2/cbradiorepair/cobramods.htm

Cobra 29 LTD super modulation modification

http://www.714xpress.net/25ltd_sch.html

Cobra Radios Super Swing Mods...

COBRA 25 LTD AND GTL, UNIDEN PC66: REMOVE AND REPLACE TR1 w/2sc2999 D4 AND D6 w/schotty

ON THE COBRA 25 AND UNIDEN 66, TR1 CAN BE FOUND NEAR THE REAR OF THE RADIO IN FRONT OF THE HEATSINK TO THE RIGHT OF THE FINAL AND LEFT OF THE ANTENNA JACK.

DIODES CAN BE FOUND IN THE CENTER OF THE RADIO 2.5 INCHES BACK FROM THE FRONT PANEL. YOU WILL SEE A TRIMMER CAN AND A SMALL BOARD STANDING ON EDGE D4 AND D6 ARE IN FRONT OF THESE PARTS.
 
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http://www.angelfire.com/fl2/cbradiorepair/cobramods.htm

Cobra 29 LTD super modulation modification

http://www.714xpress.net/25ltd_sch.html

Cobra Radios Super Swing Mods...

COBRA 25 LTD AND GTL, UNIDEN PC66: REMOVE AND REPLACE TR1 w/2sc2999 D4 AND D6 w/schotty

ON THE COBRA 25 AND UNIDEN 66, TR1 CAN BE FOUND NEAR THE REAR OF THE RADIO IN FRONT OF THE HEATSINK TO THE RIGHT OF THE FINAL AND LEFT OF THE ANTENNA JACK.

DIODES CAN BE FOUND IN THE CENTER OF THE RADIO 2.5 INCHES BACK FROM THE FRONT PANEL. YOU WILL SEE A TRIMMER CAN AND A SMALL BOARD STANDING ON EDGE D4 AND D6 ARE IN FRONT OF THESE PARTS.


For the sake of the other users on 11 meters, do NOT do a "Swing Mod" or any of the "Super Modulation" junk...

Here is why:
 

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I think I remember reading a thread somewhere on the forum that suggested to put a diode inline in series with a like diode already on the board. It is supposed to be the diode that is found after the audio transformer in the schematic. I think Freecell or loosecannon said that you can run two or three diodes in series to lower the dead key even farther. In addition, it would allow full swing when modulated w/o having to re-adjust the modulation pot.

Know what I am talking about here?
 
But if you lower the deadkey and let the audio pass at normal levels, it will overmodulate.

Even with the AMC functional...
I don't doubt you for a second, Bow.

What I am looking for is the real NPC/RC mod for it too.

Already did the alignment on this 25 LTD and gave it back to the neighbor. It had a bad protection diode (D17), receive need some peaking, meter adjustment on RX, turned up the mod a tad, and it was off freq a little bit. Right as rain now. Nice little AM radio with no frills.

But I may get some more of these coming in. Just was hoping someone knows how to do the NPC mod to it. On this radio in particular, I'm betting that it would do wonders.

Didn't mash down the 54mHz filter coils in it. It transmits clean. It came in keying 4 watts swinging to 8 watts (wow!) and after I turned up the modulation I got it swinging to 10 watts. Not even a 4:1 ratio; so giving it the NPC mod and tweaking the modulation might be the way to go.

Thoughts/info?

Here is the article I was talking about - BTW:
http://www.worldwidedx.com/cb-band-cb-radios/88445-help-need-lower-deadkey-2-watts.html
 
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From what I understand about the NPC/RC, it can not be done on a circuit like the Cobra 25/29, due to the way the voltage is regulated through the Modulation Transformer.

The radio that normal get the NPC/RC are circuit such as the 148, Galaxy, Ranger, etc that use AM Voltage regulators and Darling transistors for modulation.

BTW Robb, I sent you a PM...
 
I ran across this first from a dead site & saved it until today as i got both a cobra 25 & a pc-66 in box yard sale leftovers both work & i was going turn 1 down for a little mosquito ! if i just add the cap & resister to lower the wattage will it still work as I know how to up the audio ?
 
Welcome! The cap and resistor mod is usually used to lower the dead key for driving an amp that is not linear. Without an amp, it will give crunchy audio as already stated with the pinched carrier. Hook up an amp and adjust the resistor to give you a dead key out of the amp of 1/4 of your peak wattage and it should clean up nicely. Now that I see Bow's information, I am going to go back and put my little amp with my Cobra 29 back on the scope and play around with it some more.
 
Yeh I was writing big post & got call and lost it but I got one those rm Italy kl35 I'm going put on one ether the 25 or pc-66 the boards are the same from the looks but I have not had ether opened to make sure ! Along with the box I also got swing & mod kits the guy didn't know anything about it as it was left by his dad some years back 2 kits got larger finals with caps resistors ect I'm no tech just wanted try doing some re-educating myself some it won't be any loss if I screw them up and I got a great tutor that guides me along !
 
The KL35 is a complete waste of time. With a nice peak and tune you should e able to see 25 ish watts from either radio. Spending all that time de-tuning them to run with that dinky amp to only gain 10 watts is not worth the time. You must double your watts for anyone to even notice.

If you really want to do a mod on one of those radio's search here for the 50 watt Cobra 29 mod. The same should apply to your radio's for the most part.
 

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