Cobra 29LTD Classic (Stock) Help

While the military experience should be great background for working on HF AM rigs (also exotic modes at UHF, etc.) having an understanding of what the components do in a given circuit is still not likely enough to know how much to tweak a given component (or multiples) without an o-scope and (ideally) a spectrum analyzer on the bench. CBs are made to a price point, and the best CB around won't be anything like the quality of a Flitefone or Narco or King TSO'd Com radio out of an A-10 or similar. It's *really, really* easy to tweak an inexpensive rig from an "OK" performer into a wideband noise generator.

BTW, did your section leader encourage just taking the hood off a piece of avionics gear without reading through the theory of operation and tweak anything substantial without the manual open? I grew up in amateur radio (starting at age 9) and commercial land mobile (starting at age 20), and I'd have been killed by homebrew transmitters as a kid, or fired from my Motorola tech job, if I'd just stuck a diddle stick into some new piece of gear without having current docs open in front of me -- even if those "docs" were nothing better than the 1963 ARRL Handbook from which I was homebrewing stuff with 500VDC supplies......

And speaking of "swing" (I have my own term for it, and it's unprintable), it's really an inertial artifact of analog meters that develop momentum during a sudden burst of very high level audio. Average audio doesn't move a power needle much. Using a proper tone generator (NOT shrill, fast whistles!) adjust the sine wave so that smooth peak of the composite signal at the tops (and bottoms) of the now-modulated carrier, and you'll see (but you know this) that when they null out, there is no span along the zero line -- no place where the sine waves at their minimum do not smoothly begin their next excursion.

SSB, of course, is way different: no output without audio, and just a fraction of the plate current running as bias. There, the entire output envelope IS swing, but still it gets adjusted with a two-tone generator running into the mic input stage.

Counting on some bored guy who's got nothing better to do but drink a bottle of hot YooHoo and spank his monkey during his break give you an objective signal report is worth, well, rather what it costs, I believe, and he may indeed just be getting you to adjust your rig to sound like ca-ca (technical term there) for giggles and grins.

Please let's not keep falling into the trap of "swing' to judge our transmitters' performance. If the carrier isn't up there with the audio, then only the very loudest peaks are going to be heard at distance.

A lot of my ham gear here at the shack is boatanchor AM stuff, running around 240 watts output, and when it's adjusted to spec, the average power meter hardly moves. That particular rig is an EF Johnson Viking I, using a pair of 807s to drive a 4D32 final, and it sounds like broadcast audio. I've also got ARC/5 pairs for AM & CW on 40m and 80m at greatly reduced power.

The CB rig in my service van is a Cobra 29, and the Little Woman has a Cobra 25 -- all stock, but tuned on my bench to factory spec. Well, the modulation is set to 94% instead of the paltry 80% in the manual.....

Hope this helps, and that I didn't step on any toes.

73

 

Find a shop that knows how to do a proper alignment procedure. Dont let anyone cut,snip,spread,add,delete or modify in anyway to make a happy meter look impressive, been down that road and end result is not good. There are some posts on this forum as to recommended cb shops and those to avoid. Personally i would of left vr4 adjusted up slightly, all that did was increase modulation and with stock microphone it probably helped.

 

Handlebar your knowledge is very impressive and i learn something everytime you submit a post. My question is why then is everyone so preoccupied and obssesed with pep swing numbers if there importance is really minimal in the overall performance of a cb radio. So if i have a cobra 29 that has a carrier of 5 watts and swings to 20 watts, and a general lee that has a carrier of 5 watts and swings to 40 watts there shouldnt be much difference in the range, correct.