Cobra 29 NW LTD Classic Mods? (New guy)

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tdkask
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Cobra 29 NW LTD Classic Mods? (New guy)

Post by tdkask » Monday 23rd Jan 2012, 19:57

I'm relatively new to CBs and though I know electronics from years as an avionics tech in the Navy, I don't know jack about this circuit and am not trying to reinvent what's already been done. Digging through here and other sites I've seen plenty or mods for modulation, final amp, RX pre-amp, etc... not including things like echo and roger beep, etc.

I'm the kind of guy who can't leave anything alone... I have to tinker and modify and improve... so want to dive into this radio myself instead of taking it somewhere. If there was a more local shop I'd go to him for the parts and advice and do it myself with his suggestions, but where I live is a ways away (Norfolk, VA), so I find myself here on the web.

Basically, Id like the input and advice as to what are the most effective mods for the time/money?

Is a RFX 75 really as good as advertised, or are the other Mosfet mods going to be plenty for the price? Is the 520 or the 2030 going to do better? Or should I stick with the 1969, even? Is the 2sc2312 same/better?

Does the 2SC2999 RX upgrade (less noise?) work on a 29? Is it worth the time/trouble?

Also, for the modulation mod, I saw mention of one with a NTE331 or similar with variable pot, diodes, etc... is this worth it, or would I be as good with simply the normal cap/resistor? For variable, what should be the max/min levels? Can this cause problems after a final amp mod? What should I avoid?

What about the RX boost of PLL (switched, with variable resistance of 100K-500K between pin PLL P14 and R11?)


I see a lot of disagreement about clipping limiting diode D-11... Where would his help/hurt?

Additionally, this is in an '80 Jeep with very noisy power when the fan/wipers/heater blower is on... considering direct power from battery (or relay switched by ignition, but from battery) to clean it up some. What else component setup would be a good filter?
I will be adding simple variable talkback (50-150 resistor)

Will I be okay with a Radio Shack SWR/Power meter for these mods? Please realize I'm not opening up a shop here, just my own single radio. Also, I do have an o'scope, freq counter, external power supply, and soldering iron, but NO watt meter or RF or AF signal generator.

Thanks for any help. I'm sure there will be lots of good knowledge to share. However, please don't tell me to go get a better radio that doesn't require mods. For a couple reasons, this is the right route for me. 1-I already have this one. 2-I like the tinkering aspect of making this one better.

lbcomms
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Re: Cobra 29 NW LTD Classic Mods? (New guy)

Post by lbcomms » Tuesday 24th Jan 2012, 0:06

what are the most effective mods for the time/money?
With that radio, not too many. 29's are made to look pretty and be a convenient size for modern SUVs, but as a radio they are fairly mediocre.
Is the 520 or the 2030 going to do better? Or should I stick with the 1969, even? Is the 2sc2312 same/better?
By that, I'm assuming you want to increase the transmit power. Don't bother.
To double the range of a radio, you need TEN times the transmitter power. To quadruple the range, you are going to need a HUNDRED times the power. A 2SC1969 / 2312 / 2030 will boost your output from 4 watts to 15 or so.
The result? Only your power meter can tell, other users on the channel won't be able to hear ANY difference.
Does the 2SC2999 RX upgrade (less noise?) work on a 29? Is it worth the time/trouble?
In a base station at a quiet location, maybe. In a vehicle, it will just make the noise and static louder.
What about the RX boost of PLL (switched, with variable resistance of 100K-500K between pin PLL P14 and R11?)
Playing with your local oscillator injection levels will just make the RX performance worse with simple transistor mixers like those in the '29.
I see a lot of disagreement about clipping limiting diode D-11... Where would his help/hurt?
Your call. The "swing kit noobs" will always say YES, as your distorted voise will sound "louder" (because overdriven, distorted audio is always "louder")
Experienced users like clear modulation, so they will buy a decent radio or add a linear amp (or both!), but will never clip a limiter. It's there for a reason! If "clipping the diode" really made your radio work better, the makers wouldn't have put it there in the first place.
very noisy power when the fan/wipers/heater blower is on What else component setup would be a good filter?
The noise is being generated at the motor, and traveling through the air to the antenna and then into the radio.
Filters in the radio power line / powering the radio from a different source will NOT stop the noise, you have to add a suppressor at the motor itself. Nothing else will work.
However, please don't tell me to go get a better radio
It depends what you want to do. If you are chatting in a convoy situation, the '29 is fine. Just don't expect high performance from a $59 "radio".

You haven't mentioned your antenna system. This can have a huge impact on range.
Hams have known this principle for years - $10 spent improving an antenna will give better results than spending $100 to "improve" a radio.

I won't repeat earlier answers here, read a couple of my old posts if you want to know more about power and modulation. Links:

http://www.radiomods.co.nz/forum/viewto ... f=2&t=2684

http://www.radiomods.co.nz/forum/viewto ... f=1&t=2914

You'll have to copy and paste these into your browser, as linking no longer seem to be working on this forum.

Regards,
Sue

tdkask
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Re: Cobra 29 NW LTD Classic Mods? (New guy)

Post by tdkask » Tuesday 24th Jan 2012, 6:47

will take it from there, and tweak for most reasonable signal level with best envelope. Thanks. All other findings welcome too.

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