COBRA 29 with IRF510/ IRF520 / ERF2030 Question

[RFEverywhere]

Hi guys you like I have a cobra 29 that I am playing around with, and doing different modifications on, hoping to learn a little bit more about them.

I have so far burnt quite a few parts anf have been able to repair it over and over.

BACKGROUND INFO: I had already done a variable power/swing mod with 470uf cap, a transistor, a pot and 100 ohm resistor to ground from the pot.

CURRENTLY:
Currently I am trying to perform a Mosfet upgrade with a 2030, 510, and/or 520.

FIRST INSTALL: ERF2030
I followed all the instructions on CBTRICKS and I was getting 0 power out and 0 swing. At this point I removed the variable power/swing mod & went back to the jumper only. I started to play around with the supposed 68pf capacitor. When I started to change the value of the 68pf capacitor I realized that I was starting to see some power. I ended up getting the highest possible DK, along with the best swing when I ADDED a 220pf cap instead of the 68pf. This would give me a close to 1 watt deadkey and 6W to 7W PEP using a Palomar 500 meter and 60W dummyload.

Switched out the 2030 with a 520 and increased deadkey to ~5W and ~9.5W swing PEP. Same result with the 510.

At this point I keep the 520 on there. Wanting to get a higher deadkey, I ran a 4K7 resistor from the positive of a cap which the number I can not recall right now. This worked well and increased my deadkey to about 4.5W or 5W on the same dummyload / meter combo BUT still swinging approximately 9.5W PEP. Then I started to play with the resistor right before the base of the driver. There was a 10 ohm resistor there so i added another 10 ohm to the solder side to decrease the resistance down to 5 ohms...... That did not change anything. Somwhere along here I started to see smoke coming from the driver and sure enough the driver got fried. I replaced it and also removed the extra 10 ohm resistor and I was back at 4.5W DK and 9.5W swing. By the way, I did replace the last resistor right before the PL239 with a 33K resistor and noticed that regardless of just taking that resistor completely out or keeping it at 33K my results would not change, so I left it out. Also, noticed that I got the best results when I leave the L14 inductor slug in but unscrew it all the way to the end where it's almost all the way out.

This is where I had to stop since I would have missed my flight if I continued.

Here are a couple of questions:

1. Does changing the resistor value or the value of any other component on the base side pin of either the driver or final change the output power or are only the components on the collector side of a transistor responsible to change or modify the Po (powet output) of a transistor?

2. I read all the time people swinging 40W or 50W. What seems to be my problem? I just realized that after getting my 4.5W DK and 9.5W swing, I did not redo the cap/resistor change at the jumper. Do you think this may be the reason why I was not able to swing it to approximately 40W or even 30W for that matter?

3. I would much rather have the highest possible deadkey with no swing than have mediocre DK and super high swing. What is the highest possible DK I can expect with a 520?

4. I have not done a full tune up based on Cobras service manual yet. I do have the oscilloscope, frequency counter, and other various tools and equipment to be able to follow the step-by-step instructions on the service manual and properly align everything. The only thing I am lacking is the proper and professional RF generator to be able to adjust modulation on. I am hoping to get one. Any recommendations for a RF generator for this type of work? Will I see a good increase after doing a transmit realignment based on Cobras Service Manual?

At this point, I must really really thank you very much for reading through this extremely long post, and thank you in advance for making any helpful suggestions.

ALL HELP, CRITICISMS, COMMENTS, SUGGESTIONS, RECOMMENDATIONS, ETC.... WELCOME.

 

[Robb]

http://www.worldwidedx.com/cb-band-cb-radios/73873-radio-shack-irf-510-cobra-29-impressions.html

Have absolutely no idea why you want a high dead key. What about the 4:1 modulated watts to DK watts ratio? Otherwise, the final will just fail faster with max DK. Not to mention it will sound like rubbish.

Don't need to mess with the driver because it already has enough drive for the final. However, you might use up to 470pf total capacitance to couple the driver to the final.

What kind of wattmeter are you gauging your results with? Using a quality meter with a true peak reading circuit is the best/only way to gauge your results (not a Dosy). Decent quality 100w/50 ohm dummy load is a must - too.

I've experimented with using a 1000uf/25v cap with a 160 ohm(?)/1 watt/5% carbon resistor for the swing mod and saw ~150% modulation on a Oscope into a dummy load. So long as the mic circuit wasn't being over driven, it worked very well. Of course the modulation/DK ratio was closer to 6:1 this way IIRC. Left D11 intact.
http://cbradiomagazine.com/March 2007/Cobra 29 LTD Super Modulation Modification.htm

 

[RFEverywhere]

I'm not talking about the max to push the final super hard. What I meant I guess is the max it will handle or be able to put out without any obvious stress being placed on it or pushing it extremely hard and without any sort of audio distortion. Basically with it sounding good and not being pushed to the max.

I am using a few different power meters. I have a Radio Shack digital power/swr meter, a Comet 2300, old Micranta 3 way tester and several others but the readings I mentioned are from my Palomar 500 meter which has an AVG and PEP switch even though it is not marked on the unit. Do you know if in fact the old Palomar 500 meters are true PEP meters? I had read that they were but am unsure.....RE: Dosy - Quite frankly, I have never used a Dosy meter but from what I have read about them, why would anyone want to use it unless they like having false information. If you want false readings, then just make it up as you go and don't spend any money on buying a meter that does not give you correct information. When I decided to buy the Comet 2300, I looked at the Dosy very closely but based on what I read, I did not buy it. As mentioned before the dummy load is a 60W one (from OPEK) and seems to work welk so far. I tested it on my MFJ259B and gave me a strait 1.0:1 to 1.2:1 across all bands from 1.8MHz to 175MHz.

Thanks for the tip on the couplung capacitor. I will have to play with that one when I get back in town. I have never messed around with that particular cap. I will also test the 1000uF for the swing. I've used between 100uF to 470uF and have not noticed any difference unless I was not looking properly. Am I suppose to see a difference when using different values for that capacitor?


TGIF

 

[Robb]

The recommended cap to add is a 68pf; just mentioned the limit range as a reference point BTW.

The 1000uf cap for the swing mod is just for fair measure for extra reserve. You could put a 2200uf in their too. Got best results from finished work with a 1000uf. The resistor is best replaced with a metal film resistor; can't remember if it was a 120 or 160 ohm that brought the DK to 1w ATM.

Your Comet should be used, as it is probably the most accurate of the bunch. FWIW, a true peak reading meter will need to have supply voltage to power the peak reading circuit. Of course, this does not mean that meter lights won't have a battery/AC supply. The pdf/manual points this out. But it is probably still the most accurate of the bunch for working with radios - IMO.
http://www.dxstore.com/download/CMX-2300T_Ver2.pdf

Don't work on many AM/only radios; just every once in a while I get asked to go through one (mostly work on SSB/Export radios).

Once again, this link is still the best reference for the 29/MOSFET mod, so I would go over it all carefully:
http://www.worldwidedx.com/cb-band-cb-radios/73873-radio-shack-irf-510-cobra-29-impressions.html

 

[RFEverywhere]

Thank you again for the info. Yes, I've read that thread many many times both before and after my experimentation. It is very helpful.

Well.... I can't currently use the 2300 meter because it is packaged up to be sent to Comet for repair. Since I got it from day one, regardless of how much power I put through the meter, it always shows me 2.5W. Kind of weird. Since I had bought it brand new, I figured I should not open it up. Normally however, I would have opened it and tried to figure out what the problem was. Maybe the germanium diode or something.... but I'll let them figure it out and calibrate it properly.

One thing I don't like about using just a resistor for the swing instead of a pot, - which you or someone else may be able to chime in and comment on - is that I've noticed when I am in the car, I'm going down the interstate, my dead key seems to drop lower when I press on the gas pedal, and when I take my leg off of the gas pedal, the deadkey goes a little bit higher. Depending on the radio, I've had up to 1.5 watts fluctuation.

Now the opposite of that would make sense but this does not. I can see it going up when I press the gas pedal but can't see why it would go up when I take my leg off of the gas pedal. For example, I put my deadkey on 0.75W the dead key goes up to 2W or sometimes maybe 1.5W when I stop pressing the gas pedal (all with no modulation obviously). However if I lower my pot resistance (~4W-5W DK) the fluctuation does not happen

I am wondering if anyone else has that experience or if there is a simple explanation for it. Maybe I am just a little bit dumber than I thought and am either overlooking something or not doing something right but I figured I'd mention it since we are on that subject.

 

[RFEverywhere]

Just A Friendly Bump

I've noticed when I am in the car, I'm going down the interstate, my dead key seems to drop lower when I press on the gas pedal, and when I take my leg off of the gas pedal, the deadkey goes a little bit higher. Depending on the radio, I've had up to 1.5 watts fluctuation.

Now the opposite of that would make sense but this does not. I can see it going up when I press the gas pedal but can't see why it would go up when I take my leg off of the gas pedal. For example, I put my deadkey on 0.75W the dead key goes up to 2W or sometimes maybe 1.5W when I stop pressing the gas pedal (all with no modulation obviously). However if I lower my pot resistance (~4W-5W DK) the fluctuation does not happen

I am wondering if anyone else has that experience or if there is a simple explanation for it. Maybe I am just a little bit dumber than I thought and am either overlooking something or not doing something right but I figured I'd mention it since we are on that subject.[/QUOTE]

Just a friendly bump....

 

[CBSlicer]

Sounds like an SWR fluctuation and / or grounding issue to me. just a thought.. probably not anything to worry about.

 

[2FB327]

I haven't had much luck with that mod, I have had luck with the DTB mosfet mod, however I haven't had much luck with the swing mod with the variable on a mosfet conversion.
What I get is that it works on the bench but in a vehicle it is hard to keep the dead key set. You'll set the variable to say two watt, use the radio a couple of times and look at the s meter and the needle is in a different place, wide open or zero. When I put it back on the bench it works fine.
So maybe it's just the meter being fooled I don't know, but it caused me to rip out my few remaining hairs.
Never heard of the accelerator issue, it's either like stated a swr issue,( loosing ground) or perhaps loss of voltage.
You could hook up your DVM to the power cord and work the accelerator and check voltage.
Good luck.

 

[RFEverywhere]

You could hook up your DVM to the power cord and work the accelerator and check voltage.
Good luck.

Yeah. Not a bad idea. I need to try that while going about 60 to 70 miles per hour down the highway. LOL. But I would be almost positive to say that when I let go of the accelerator, the voltage should drop from lets say 14.xxV to 12.xxV or 13.xxV because the alternator is not turning as fast as it would with higher RPM.

Sounds like an SWR fluctuation and / or grounding issue to me. just a thought.. probably not anything to worry about.

Yeah. I know. not that worried about it. But, I am Mr "Must Know or Must Figure Out why". So I will eventually get to the bottom of it.

It's a sickness really. It's some type of disease. I'm sure of it. It has to be. It's not normal. <More audio>:headbang:glare::bored:

 

[psycho]

Did you post on this forum as "DualAntennas" " in the last year? Your MO sounds awfully familiar.

 

[2FB327]

I was just trying to help you. I said work the throttle, not drive the car.
With the car in park and brakes on, the point is that the supply voltage be constant, just a shot in the dark.

 

[222DBFL]

Where is antenna mounted and how? What type coax is in use? What gauge power wire? How is radio grounded? How well is antenna grounded? How well is the cab grounded to the rest of the vehicle? Where exactly was the reading taken? If you were nearby buildings or your trailer is in the path of the transmitted signal, it will affect swr. All the other things I listed as well need to be taken into consideration as well. Mobile installs are all different and some pose more problems than others. There are just too many variables. Also is the radio tuned and aligned properly? With the proper test gear? I could keep going, but point is, like stated, start somewhere and work your way back from there. Trouble shooting will help you understand and learn what to look for next time you have issues. JMO. Have a great day.

 

[RFEverywhere]

Did you post on this forum as "DualAntennas" " in the last year? Your MO sounds awfully familiar.

Please excuse my stupidity, but what is MO?

And to answer your question: No. I just started to post on forums about a month or so ago.

Who is this guy DualAntennas? Must be one heck of a smart guy. ;-)

I was just trying to help you. I said work the throttle, not drive the car.
With the car in park and brakes on, the point is that the supply voltage be constant, just a shot in the dark.

No, I appreciate the help and I hope you did not take any offense to what I said. I was just trying to be funny. But I guess it didn't work out too well.

I have already decided to go ahead and install a small LED voltmeter (the cheap ones from China on ebay) on my power splitter. See below for my setup. I would like to add an ammeter also but those only goes up to 10 amps I believe and I am drawing more than that at times that I have my 2m/70cm radio and amplifier turned on too I think.

Anyway, yes, I have tried testing it at idle. But it gave me the opposite results (the way it should be.) . Basically, pushing the petal increases power ( both voltage to radio and output power from radio to antenna) and letting go slowly decreases it. This issue that I described only happens when I am accelerating on the road.

In regards to the antenna, I have had it mounted everywhere. First I had a fiberglass whip mounted on the very top of the roof, in the rear on the driver side. However, it did not give me the best results (But the best out of any other location on the car).

Then I purchase a Trucker 2000, and realized that it works best when I put it in the center of the trunk. Both my trunk and the hood of my car are fiberglass, with the exception for the roof of the car. Also none of the fiberglass whips work anywhere on my car, the only antenna that has worked well is the Wilson 2000. I had even tried to put one of the fiberglass whips at the same location as my current antenna which is the center of the trunk with a trunk mount, and terrible terrible results. Very high SWR, and I have tried six different kinds/brands/size of fiberglass whips. Even a no ground plane system. Anyway, that issue is now solved as I have low SWR on all low channels and a high channels, and I have tested it with an antenna analyzer MFJ259B as well as with 3 different SWR meters. The cable is what came with my trunk lip mount. It is good old RG58. I believe about 12 feet. This is the mount:
http://i.ebayimg.com/00/s/NTk0WDExMDA=/z/XTEAAOxyNmZTkEr6/$_3.JPG

I will try to answer all of your questions. Basically my setup is as follows:

12 gauge wire coming from battery on positive, and negative from under the hood from the body of the car where another connection is from the manufacture. That runs into a little 10 way home made splitter made from binding posts / banana plugs and 12 gauge wire inside a project box. Need the splitter because I also have a Baofeng UHF VHF radio with an amplifier that needs to be plugged in as well. Anyway, this is what the CB radio side of things looks like: CB Radio 》 10W PEP POWER/SWR/MOD Meter 》Galaxy 225 amplifier 》SWR/ANTENNA TUNER/POWER meter 》 Antenna. All using 12 inch to 18 inch RG58 coaxial cable.

The antenna is mounted to the center rear of the fiberglass trunk but to the metal frame of the trunk by the rear glass and I have tested it for continuity and is making a good connection with a 1.5 to 2 ohm resistance which is caused by the long test leads on my multimeter. I have also disconnected my negative power line from the power splitter and the radio and amp all come on just fine so I did not bother to ground the radio or the amp or the antenna any further than they already were both through the antenna connections and through the negative power line.

I added variable power to the radio (29LX) and did a TX and RX alignment using my scope, frequency counter, AF/RF generator.

I think I have the installation almost near perfect. And I don't think that this issue is that big of a deal, but I was just wondering if anybody else had experienced the same thing just because there is no good explanation for it, and what it is doing does not make sense. The opposite however, would make sense.

 

[M0GVZ]

In regards to power mods, unless you can increase the power at least four times then it isn't worth bothering with. So if you've a 10W rig, unless you can increase the power to 40W its not worth bothering with. It'll get you a S point on the receiving end.

For most people operating mobile, redoing their installation, using a decent antenna and installing it so it works properly and efficiently would see an increase more than upping their power fourfold. Not only that, they'll benefit on receive as well through stronger received signals with less interference.

Please go to www.k0bg.com and read the entire website or at least the bits on antenna efficiency, antenna mounting, bonding and wiring & grounding. Everything you need to know to solve all your problems is on that website. You're doing absolutely everything wrong, even running the negative wire directly from the battery which is an absolute no-no yet most people do it.

Low SWR does not equal efficiency. Don't be fooled by thinking a low SWR means its working properly. You use a MFJ antenna analyser. What does tell you how efficiently it is working is the value of R when X=0, the resonant point of the antenna. Should be 37 Ohms for a perfect installation. Anything above or below that shows you've got losses. If you're getting near 50 Ohms where X=0, you've got a whole lot of losses.

No idea why you need an amp on the Baofeng. 10W on my installation in my car gets me 60 miles easily.

 

[Captain Kilowatt]

Please excuse my stupidity, but what is MO?

Never watched any cop shows hey? MO= modus operandi which means the way in which someone characteristically does something.

And to answer your question: No. I just started to post on forums about a month or so ago.

Who is this guy DualAntennas? Must be one heck of a smart guy. ;-)

No.....not really. In fact he was somewhat of a real dickhead troll actually.