New hobbyist needs mod help

[bsquinn]

Hello everyone. I am an eight year naval nuclear electrician (high voltage and big equipment) with a pretty solid electronics background and excellent soldering skills. Although I know what nearly every component (diode, cap., etc.) does individually, I'm a little rusty on how they function together as certain parts of an overall circuit. I have three Cobra 29's that I would like to experiment with by doing modifications using "Radio Shack" parts. The problem is I have seen so many different "expert" mods explained on the net that I don't know which ones are legit and which ones will do more harm than others. Can anyone out there tell me how or guide me to a site that explains how to do a high power mod on my 29's and explain what each step is actually doing to each part of say a block diagram of my circuitry. I have a factory schematic and service manual and from all the parts that I have been told to modify (i.e. D11, R53, C126, TR13, L14 etc.) I just can't seem to figure out what's actually being done to the radio. For example I've been told that removing the slug from L14 is a common part of one mod but that you will have no tank resonance or you can cut D11 but that's just eliminating the limiter. I need a refresher on what those terms mean. I don't need things explained to me in idiot's terms but I'm no genius either. Any explanations, links, pics etc... would be appreciated.

Thanks

 

[ExitThirteen]

Well, here's a little tidbit of info for you, that you may find useful.

Do NOT jump D8 on those Cobra 29's. This is an rf spike diode that sits in between the rf section and the audio transformer. Jump the diode and the voltage jumps up about .7 volts.... bad news. It can also cause distorted TX/RX audio as well. Stay away from this mod.

I've never been a fan of clipping out any limiters, would rather turn up the VR's to achieve higher modulation.... and in the event that you still don't achieve strong modulation in this manner, you can put a 1k-3.3k ohm resistor inline with the limiter diode, or lift the emitter leg on the limiting transistor and install a resistor inline there. Most radios will achieve great modulation without adding the resistor, so I'd just adjust the VR's first.


Here's a link on some mosfet upgrades that you may want to look thru.

http://www.cbtricks.com/miscellaneous/fet_papers/mosfet_experiment.htm

Hope this little bit of info helps!

-Exit

 

[speedracer6g]

The EKL mod is ok but it needs some help to get the full potential of the mosfet. With the cobra 29's I use the IRF520N mosfet instead of the ekl2030. They are Cheaper and work better as far as i am concerned. Since you are a tech guy a assume you have a ossillicsope. This comes in handy to check you output waveform to make sure you are not overmodulating or clipping you output of the radio.

I will list some step by step info I will get to the point since you know whats going on. FOLLOW THE STEPS FROM THE EKL MOD THIS IS FINE YOU JUST HAVE TO ADD A COUPLE OF STEPS.

- INSTEAD OF INSTALLING ERF2030 INSTALL IRF520N.

- ADDING THE 68PF CAP ON C59 MAY NOT BE NEEDED I USUALLY CHECK WITH AND WITHOUT TO SEE WICH IS BETTER.

-REMOVE C62 AND INSTALL A 390PF OR 480PF CAP I CANT REMEMBER WICH I WILL HAVE TO LOOK LATER OR JUST TRY AND SEE WICH WORKS BETTER FOR YOU. THIS JUST HELP INCREESE THE RF DRIVE TO INCREASE DEADKEY AND SWING.

-THE MOST IMPORTANT THING IS TO INSTALL A 10 OR 100K POT (WICHEVER YOU CAN GET IS FINE) BETWEEN THE GATE OF THE MOSFET AND THE POSITIVE SIDE OF C116 THIS WILL LET YOU SET YOUR BIAS VOLTAGE.

-SOLDER ONE WIRE TO THE CENTER PIN OF THE POT AND THE OTHER ON ONE OF THE LEFTOVER LEGS.

-TO SET YOU BIAS VOLTAGE MEASURE THE VOLTAGE FROM GROUND TO THE GATE OF THE MOSFET. TURN YOUR MIC GAIN ALL THE WAY DOWN. MAKE SURE YOU HAVE A LOAD OR ANTTENNA HOOKED UP TO THE RADIO. KEY THE RADIO AND SET THE POT TO SEE ABOUT 3 VOLTS ON THE METER. YOU CAN GO BETWEEN 2.7 OR 3.2 JUST RUNS HOTTER WITH HIGHER VOLTAGE.

-I ALSO REPLACE C71 WITH A 33UF 16VOLT CAP. THIS HELPS CLEAN UP THE AMC CIRCUIT. THIS ESPECAILLY HELPS WITH THE MOSFETS.

NOW THAT YOU DID ALL OF THAT YOU CAN PEAK THE RADIO I USE AN OSCILLOSCOPE AND A WATTMETER OR COMMUNICATIONS ANALYZER FROM HP OR MOTOROLLA. DEPENDS ON WHAT EQUIPMENT YOU MIGHT HAVE ACCESS TO.

-FIRST THING IS DONT MESS WITH THE OPEN COILS TO THE RIGHT OF THE MOSFET OVER BY THE ANTENNA PORT THEY ARE THE RF FILTER.

- KEY THE RADIO WITH THE WATTMETER HOOKED UP AND PEAK L17 YOU MIGHT NOT SEE ANY GAIN I USUALLY JUST CHECK IT TO MAKE SURE.

- THEN KEY THE RADIO AND WHISTLE INTO THE MIC TO GET YOUR MODULATION AND ADJUST L11 FOR THE HIGHEST PEAK READING.

-WITH ALL OF THAT DONE YOU SHOULD SEE ABOUT 40 WATTS PEP GIVE OR TAKE.

- THERE IS A DECENT RX MOD FOR THIS RADIO BUT YOU HAVE TO HAVE AN RF SIGNAL GENERATOR TO DO IT CORRECTLY. IF YOU WANT THAT INO JUST PM ME. OR DROP ME AN EMAIL. MIKE@MYNOVASECURITY.COM

LET ME KNOW HOW YOU MAKE OUT.

MIKE

 

[bsquinn]

Thanks

Thanks to both or the previous replies. They both helped a little bit. I would like a little more infor on D8. As far as I can tell by the schematic these radios have some AC (or pulsed DC I suppose) circuitry, correct? I guess that's what an oscillator's function is? If I'm right, is D8 part of this circuitry? I keep hearing that jumping it will only get you about 1 extra watt pa resistor in parallel with it so it conducts 100% on one half cycle and at a lesser value on the opposite half-cycle therefore not being as likely to do damage? Seems like kind of a pain for 1 extra watt but that watt could get you out an extra mile or two. What does everyone think? Am I way out in left field here? Also, is there a general rule of thumb as far as temperature or power levels that it is a must to add some sort of cooling system to these things?

Thanks

 

[speedracer6g]

As far as jumping D8 goes it is not worth messing with like you said. With it being jumped it can wipe out the audio section of you rig leaving you with bigger problems. You can get plenty of power without doing the jump. As far as heat goes with one single final there should be enough cooling to keep you happy. As long as you keep you bias voltager within reason you should be fine. You can go up to 3.6 volts bais voltage on these but they just run too hot for what heatsink is there. I personally stick to 3.0 volts on mine just too keepthings safe. have fun modding.

MIKE

 

[bsquinn]

Testing transistors

Another question I have- Is it possible to get a transistor (i.e. IRF-520 or 2SC1969 type) too hot with a 30 or 40 Watt soldering knife and burn it up during an install? Also, can a transistor be tested with a simple multi-meter or do I need a special tool?

 

[cbradiomagazine.com]

Yes it's possible to damage the transistor with overheating it with the soldering iron.

Your statement here should be reconsidered -

"Seems like kind of a pain for 1 extra watt but that watt could get you out an extra mile or two. "

The extra 1 watt or two you achieve will most likely not make any recognizable difference in the distance of your transmission.

People will spend infinite amounts of time and money adding different finals, cutting limiters and such to create a 40-50 watt cobra 29 thinking they will increase their transmit distance, problem is that they'll do all these mods and then use the radio in conjunction with a crappy 3 foot fiberglass antenna.

I can take a Cobra 29, adjust the modulation to max, add a nice power microphone and run it through some quality coax into a 102" whip doing 20 watts PEP and talk farther than the guy that has a Cobra 29 doing 50 watts PEP into a 3 foot fiberglass whip.

Cobra 29's can sound loud and clear without changing out any parts. There is no need "ever" to clip the limiter in a 29. With modulation adjusted to max and a good power mike the modulation is excellent.

You need to think carefully about what you are trying to achieve when doing modifications, otherwise you're just wasting money.

 

[Keystone's #1 Mudduck]

Modulation pot cranked wide open with a power mic huh...bet that sounds great.

 

[cbradiomagazine.com]

If he's looking for loud modulation and an little extra wattage it's a better solution than removing the limiter or trying to volt stuff and hacking up the radio.

Of course if he's looking for a clean signal he can run out and drop $500 on a ham rig, but you and I know that's not what he's looking for.

My goal is always to try to offer people positive solutions or options based on what they are trying to do rather than making snide remarks.

And yes, you can make a 29 with the pot cranked wide open and a power mic sound great as long as you don't run the power mic wide open.

 

[bushwacker]

Another question I have- Is it possible to get a transistor (i.e. IRF-520 or 2SC1969 type) too hot with a 30 or 40 Watt soldering knife and burn it up during an install? Also, can a transistor be tested with a simple multi-meter or do I need a special tool?

Test the transistors just like you test a diode... one way continuaty. The npn transistor is like 2 diodes.... one from base to emitter... the other base to collector. So you should get continuaty from base to emitter one way only.... continuaty from base to collector one way.. and no continuaty at all between collector and emitter.