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Author Topic: 11 meter question - sorry guys  (Read 1571 times)
KD0CQG
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« on: November 13, 2012, 11:34:12 AM »

Hey guys. Back in the old days when I was a kid, my dad bought me an SBE Trinidad III. I enjoyed it alot and yes, I was a punk on it. But dont worry, I have matured into a peaceful HAM.

But I still get nostalgic from time to time. So I got on EBAY once looking for another Trinidad III. Well, I found one for a good price and bought it. When I got it and plugged it in and keyed up it has a horrible squeal when I keyed up. My elmer and I were checking it our and found that in the audio circuit around the automatic modulation control/mic amp circuit there were three things going on. There was a short around the circuit, and the circuit was effectively "snipped" out of the unit. There was a variable trimming pot broken out and a diode snipped. There was also a little wire under the main board that provide a run around that part of the circuit.

Well, after a buncha measurements we just figured that someone decide to bypass this control portion of the circuit. With the restoration of that circuit, it works fine and dandy.

What is puzzling me is "why". What is the mentality of bypassing this little control circuit thereby making this thing useless? Was it to make them "Loud and Proud"? or do you think there was a problem and they thought they could bypass it and still keep operating?


Anyway, it is back to stock with power and freq settings and this little control circuit and all is well. But I just cant help but wonder......Why? What were they trying to achieve?
« Last Edit: November 13, 2012, 11:38:33 AM by KD0CQG » Logged
W8JX
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« Reply #1 on: November 13, 2012, 12:02:27 PM »

What were they trying to achieve?

The mentality was the louder the audio the better you were heard, like shouting to be heard in a crowd but that rarely works too.
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KE3WD
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« Reply #2 on: November 13, 2012, 12:15:38 PM »

Defeating the modulation limiter circuit is/was a common modification done in order to drive those illegal linear amplifiers. 

Typically most would just clip the diode limiter, but you may find all sorts of circuit bypass, circuit defeat and even attempts at modulation enhancement from time to time. 

Sounds horrific without an amp, but if driving a few hundred watt linear or more, actually brings the low level AM modulation up enough to "swing" the amp output like they want. 

Before everybody piles on, NO -- it is not good practice to do it this way, of course. 

Just telling you WHY it was done.  A very popular "mod" among the illegal CB "LINYAR" crowd...


73
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KA4POL
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« Reply #3 on: November 13, 2012, 12:17:01 PM »

CB logic: The louder the audio the higher the output power in AM...
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AC5UP
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« Reply #4 on: November 13, 2012, 12:17:41 PM »

Whatcha' got there is a Charlie Brown radio with the BIG MODULATION enhancement. Gives you more swing on the meter and (unfortunately) is standard for radios that have been tweaked & peaked down t'the truckstop...

Without the ALC the radio will over modulate which makes it a splatterbox. But it will sound loud. When it doesn't squeal. If you need a schematic you should be able to find one here that's very similar.

The quick & dirty setting for an ALC circuit is with a 'scope. On a dummy load, hang the test probe near enough to the SO-239 to pick up enough signal for 3-4 units on the screen. Then whistle it up to full modulation. Adjust the ALC trimpot until the RF envelope just barely starts to flat top (clip). That isn't textbook, but it's better than what you started with...  Wink
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KD8GEH
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« Reply #5 on: November 13, 2012, 01:35:56 PM »

Oh the memories! 

That radio was "Strapped and Snipped". The old SBEs were pretty cool old CB's. Good job restroring it back to factory. I have a few ri9gs restored packed away for the same reason. Lafayettes, Uticas, and even a old SBE Brute (SBE-34).

Those old screwdriver jockeys did some intereting things. I even remember a mod called a "christmas tree" they did to white face Johnson tube rigs that bumped up the power Smiley

Anyhow, thanks for the trip down memory lane.

73, Dave
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KD0CQG
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« Reply #6 on: November 14, 2012, 12:30:15 PM »

Hey Guys,
Thank you for your very informative feedback regarding this subject. You answered my question exactly as I wished. Today I purchased From "Fistel's a couple of trimming pots to replace the one that is gone. I got two incase one was outa spec or something. So we should have it running back to normal in a couple of days as time permits the soldering iron to heat up.

you know? over here in Denver we have one of those old electronics supply shops that has "EVERYTHING". It is like an electronics hoarders dream. There is barely enough room to walk down the isles. I hope I am never there if it catches on fire, cause I wont be able to get out. It was established in 1934 or there abouts and probably has some parts from then. Each time I go in there they have the building up for sale and say that someone back east is checking them out and that they will not be around after the end of the year. That has been going on for a few years now. But one day it will be true. I will hate to see an old shop like this go away. I love the smell and the surly counter folks at these kinds of places. It is too hard to make a living off hobbiests with the internet.

Well guys, Again thank you for the education. I hope all your keys are productive.
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