Hey,guys. my brother and law brought me his cobra 25 nw wx st to look at, well it had a what looks to be a original 1969 final so I thought I could do it a little good. it would key 3 watts swing to 25 spread some coils it would get worse put back to where it was 3w & 25 swing.worked with tuning slug forgot # but only get worse. any suggestions on mods that would work. made in china 2002.mod limiter already clipped took out tr13 put back in no difference.removed c6 no difference put back in. radio sounds fair on the air but not as good as my plain 29 with simple mods. I guess all radios are different. Thanks
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cobra 25 nw wx st power mods
- Thread starter daddyo
- Start date
Spreading coils may show a higher watt reading; but the truth is surprising. Many of those extra watts are not on the CB ch 1-40; they are at 54mhz. Surprise! Watt meters cannot tell the difference between 27mhz (CB) and 54mhz.
Cobra AM radios don't use the 1969 transistor; they used a 2sc2078 final and a 2sc1957 driver.
http://www.cbtricks.com/radios/cobra/25nw_st_25wx_nw_st/graphics/25wx_nw_st_main_sch.jpg
It may well have been modified.
The coils you spread need to be put back into their original shape/form. They are parts of the trap circuit for the 2nd harmonic of 27mhz, which is 54mhz (2 x 27 = 54). IOW, the radio is throwing off 'spurious emissions' and in violation of FCC regs if that filter is disabled w/spread coils.
Not to mention this mod seriously messes up your SWR readings, because SWR meters cannot tell the difference between 27mhz and 54mhz - either. The net effect is that the SWR meter is trying to read two different SWR's from two different signals coming out of the radio at the same time - and the antenna is only capable of being resonant on one of those frequencies (27mhz) and getting a bad reading from the 54mhz SWR. These mixed SWR signals - both good and bad - are being averaged and read at the same time in the meter . . .
Fix that problem?
Fix those spread coils . . .
Cobra AM radios don't use the 1969 transistor; they used a 2sc2078 final and a 2sc1957 driver.
http://www.cbtricks.com/radios/cobra/25nw_st_25wx_nw_st/graphics/25wx_nw_st_main_sch.jpg
It may well have been modified.
The coils you spread need to be put back into their original shape/form. They are parts of the trap circuit for the 2nd harmonic of 27mhz, which is 54mhz (2 x 27 = 54). IOW, the radio is throwing off 'spurious emissions' and in violation of FCC regs if that filter is disabled w/spread coils.
Not to mention this mod seriously messes up your SWR readings, because SWR meters cannot tell the difference between 27mhz and 54mhz - either. The net effect is that the SWR meter is trying to read two different SWR's from two different signals coming out of the radio at the same time - and the antenna is only capable of being resonant on one of those frequencies (27mhz) and getting a bad reading from the 54mhz SWR. These mixed SWR signals - both good and bad - are being averaged and read at the same time in the meter . . .
Fix that problem?
Fix those spread coils . . .
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Thanks, robb I put coils back where they supposed to be. final is a Mitsubishi 1969 after removing & scraping crud off to get a better look. limiter is cut for sure. I do not have any more to replace it . just used last one in another radio. after putting coils back DK1-2 swing 10-15.it does not splatter on the air or on other channels just not as loud as my 29's. driver is stock. will mods for plain cobra 25 classic's work on that radio.
1) 4x the carrier strength is 100% modulation. Anything more than that is wasted energy and will either cause you to splatter or will actually start to distort and hurt the quality of audio coming out of the radio.
2) Spreading coils might make a meter show more watts, but they're all over the place. You could hook your cell phone up to that meter and get a reading, it doesn't care "where" the power is at. Those coils are not power limiters put there by radio manufacturers, they are an important part of the radio that do any number of things like reduce the spurious emissions that land in ham radio bands and make that meter show you all kinds of big numbers. I got a radio one time with spread coils and I sat there and pushed them together, turned down the modulation to about 90%, (it was swinging like 10 times the carrier) and got audio reports from a friend on the other side of the county (about 10 miles away straight line). I ended up getting the watt meter to show HALF the output power compared to when the coils were spread and guess what it did on the receiving end? I didn't lose one number on his S meter and he said I actually sounded clearer.
Remember, cleaner is meaner. I have talked "through" people running several hundred watts on a radio with a clipped modulation limiter with my barefoot (4 watts swinging 13 or so) radio that has never been clipped, cut, screwdrivered, etc.. One of my friends said, and I quote, "Well, they're giving me a 30 pounder, and you're only hitting me at 20, but for some reason when you talk I can hear you over top of them, explain that."
Yes; I would re-install the Limiter and still get ~137% +/- modulation; which is still considerable.
And clean . . .
And clean . . .
I have never been able to get the sound tracker radios to put out as much as the straight LTD versions of the cobra radios. some one else may have found the trick to do this but as Robb has stated you will not know on the other end how many watts you are putting out. you have the WX ST version from what you stated in the 1st post which is the sound tracker. I know some guy's remove the sound tracker section on the cobra 29 radios to get it talking better. cobra had so many inquiries on this they put out a mod to remove it. have not seen the mod for a cobra 25 as of yet.
well I did a little research and found the 25 mod also for removing sound tracker. it was in the same bulletin as the cobra 29.
http://downloads.cobra.com/CB/TSB/TSB10012.pdf
well I did a little research and found the 25 mod also for removing sound tracker. it was in the same bulletin as the cobra 29.
http://downloads.cobra.com/CB/TSB/TSB10012.pdf
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woah daddyo!
you will never get those coils back to the value that they were before you spread them.
you may have gotten the power back up, but that half wave filter probably isn't doing it's job as well as it should.
NEVER EVER spread any coils in a CB radio. that is old school snake oil and only does harm.
I have explained why numerous times here on this forum, so do a search for "half wave filters" here if you want to do some learnin'.
there is a coil near the antenna jack that you should also never turn, and it looks exactly like the one near the final.
hopefully you didn't turn that coil too.
that radio was most likely already tuned and aligned for max output, because 2SC1969 transistors do not come stock with cobra 25's or 29's.
it is however a very popular mod to replace the 2078 final with the 1969.
this is what was probably done to your brothers radio.
25 watts PEP from a cobra 25 is good and does not need improving on.
remember, to increase your signal by 1 S-unit in someone's receiver, you have to quadruple your power output. this is true no matter how many watts you started with.
if you hit someone at S5 with your deadkey, and your deadkey is 5 watts, then you have to have a 20 watt deadkey to hit them with an S6.
you would have to have an 80 watt deadkey to hit them at S7.
you would have to have a 320 watt deadkey to hit them at S8.
see where im going with this?
while everyone wants their radio to do the max that it is capable of, there are limits to what you should and shouldn't try in your attempts at more power.
tweaking coils to try for an extra few watts will NEVER be noticed by a listener, but will be noticed by your TV and maybe even your toaster.
playing inside CB's is fun, and addictive, but there are a couple of books i recommend reading if you are going to start messing with peoples' radios.
The Screwdriver Experts Guide to Repairing CB radios-Lou Franklin
start with this one, as it will dispel a lot of myths you might hear on the air.
once you get to where you really want to know the whys as well as the hows, read
Understanding and Repairing CB Radios-Lou Franklin
check these out and start hitting the local garage sales for junkers to practice on.
LC
you will never get those coils back to the value that they were before you spread them.
you may have gotten the power back up, but that half wave filter probably isn't doing it's job as well as it should.
NEVER EVER spread any coils in a CB radio. that is old school snake oil and only does harm.
I have explained why numerous times here on this forum, so do a search for "half wave filters" here if you want to do some learnin'.
there is a coil near the antenna jack that you should also never turn, and it looks exactly like the one near the final.
hopefully you didn't turn that coil too.
that radio was most likely already tuned and aligned for max output, because 2SC1969 transistors do not come stock with cobra 25's or 29's.
it is however a very popular mod to replace the 2078 final with the 1969.
this is what was probably done to your brothers radio.
25 watts PEP from a cobra 25 is good and does not need improving on.
remember, to increase your signal by 1 S-unit in someone's receiver, you have to quadruple your power output. this is true no matter how many watts you started with.
if you hit someone at S5 with your deadkey, and your deadkey is 5 watts, then you have to have a 20 watt deadkey to hit them with an S6.
you would have to have an 80 watt deadkey to hit them at S7.
you would have to have a 320 watt deadkey to hit them at S8.
see where im going with this?
while everyone wants their radio to do the max that it is capable of, there are limits to what you should and shouldn't try in your attempts at more power.
tweaking coils to try for an extra few watts will NEVER be noticed by a listener, but will be noticed by your TV and maybe even your toaster.
playing inside CB's is fun, and addictive, but there are a couple of books i recommend reading if you are going to start messing with peoples' radios.
The Screwdriver Experts Guide to Repairing CB radios-Lou Franklin
start with this one, as it will dispel a lot of myths you might hear on the air.
once you get to where you really want to know the whys as well as the hows, read
Understanding and Repairing CB Radios-Lou Franklin
check these out and start hitting the local garage sales for junkers to practice on.
LC
I never had much problem getting the coils back into original shape and it worked out fine.
No problemmo . . .
What you talkin 'bout, Willis?
No problemmo . . .
What you talkin 'bout, Willis?
they may go back into the same shape, but the inductance value will not be the same.
maybe pretty close, but not the same.
that's why i said it's not doing its job as well instead of not doing its job.
of course i may be nitpicking, and the difference may not really affect much, but it will make some amount of difference.
like you might have to re-tune the TVI trap for minimum output at the 2nd harmonic, which can be tough to do without the expensive equipment.
thanks for the reply Robb, i suppose i did need to qualify my previous statement a bit.
LC
maybe pretty close, but not the same.
that's why i said it's not doing its job as well instead of not doing its job.
of course i may be nitpicking, and the difference may not really affect much, but it will make some amount of difference.
like you might have to re-tune the TVI trap for minimum output at the 2nd harmonic, which can be tough to do without the expensive equipment.
thanks for the reply Robb, i suppose i did need to qualify my previous statement a bit.
LC
Thanks guys big help. I did put radio back to stock concerning coils and limiter. Did not turn tvi trap. Did some experimenting of my own. when I put everything back the way it is supposed to be I drove up the road and talked to my daughter at the house two miles away and radio sounded the same with or with out any of those things done. you are right loosecannon I could not tell a difference. it is now 2wdk 10w swing on my radio shack meter. My bad on the final I ment it looked like a real 1969 not a fake. I watched the video of the guy from uk telling the difference between real ones and fake ones. Thanks Sonoma for the link I will check it out.That radio sounds different for some reason than my 29's. loosecannon I do have those books by lou franklin my wife ordered them for me and I received them today.
I sure like experimenting with them it is addictive.it is 12:30 am here and I am going to the building to play with a radio. I pave roads for a living so in the winter we get layed off till the end of march. Do you guys think the eleflow finals are as good as he says they are . would like to get some to try. thanks again guys you all are a big help.
I sure like experimenting with them it is addictive.it is 12:30 am here and I am going to the building to play with a radio. I pave roads for a living so in the winter we get layed off till the end of march. Do you guys think the eleflow finals are as good as he says they are . would like to get some to try. thanks again guys you all are a big help.
Do you guys think the eleflow finals are as good as he says they are . would like to get some to try. thanks again guys you all are a big help.
Good for a replacement yes. But to squeeze one more watt out of no.
As far as spread coils? I cannot tell you how many times I have set down to fix these issues. One has to remember these are tuned circuits. Getting them right is critical. If one really wants to squeeze out more watts look into coupling capacitors. Spectrum analyzer is your friend.
Thanks retro and radio tech. good advice glade to see there's people who want to help me. Not many around here. Just trying to get a little better at this to try and help some fellow coal truck drivers I used to drive with for about 4 years. What would be a little better meter than my radio shack meter. Money wise 100-200 dollar range max.
