The resistor placed in line with the driver stage's collector supply dates back to 23-channel radios that had a 10 ohm resistor from the factory in that spot. We would put an electrolytic in parallel with it to improve the 'swing'. No joke.
Making the resistance value higher to reduce carrier level just kinda logically followed that.
We use a darlington NPN transistor as a sort of variable resistor in line with the driver's collector circuit. It will 'follow' the audio level and maintain a decent audio waveform with a reduced carrier. Not incredibly unique. Added a trimpot to adjust minimum carrier, a diode and resistor to improve peaks at low carrier settings. It lines up with and mounts directly over the two holes in the pc board where a jumper wire feeds into the driver in a Cobra 29GTL/LTD. It uses an adhesive marketed to sign shops called "VHB" that doesn't degrade with heat. Hence the "peel and stick" nickname.
It is "universal" in the sense that we have used it in a variety of AM-only CBs. In some radios, you'll have to cut a foil trace to interrupt the driver transistor's collector supply. The "+" and "-" wires from the module go to either side of the cut. Just have to get the polarity right. One advantage of using a darlington transistor is that it has a built-in diode in parallel with the collector-to-emitter circuit. If it's hooked up with polarity reversed, it just gives the radio full carrier no matter the control setting. It has six connections. Three of them go to the front-panel control. One is ground, one marked "+" goes to modulated B+ and the one marked "-" goes to the driver's collector choke.
One caution concerning 23-channel radios. Reducing carrier will cause a lot of them to have an audio feedback squeal when the carrier is turned down. Really can't say why, but there it is. If you rock one of the transmitter's tuning slugs in an upstream circuit you can find the peak without a wattmeter. Just set it halfway between the right and left extremes where it starts to squeal. This is almost never a problem on a 40-channel radio unless it has filter caps going bad. Before installing this trick on any 23-channel radio I would make sure that reducing carrier doesn't cause the audio to squeal. Simple enough, just twist one of those three bandpass slugs. They tend to have a narrow, sharp peak.
One thing we learned would also be necessary on some radios is to add a decoupling (bypass) capacitor on the cold side of the driver's collector choke. Some radios use just one disc cap to bypass both the driver and final collector chokes where the cold side of each are connected together.
If you separate them, you'll need a .01uf capacitor added to the driver collector choke where your variable circuit feeds into it.
One handicap of hand-made production of small circuit-board products is high overhead. The labor cost is real. And a comparatively-high price of 35 bucks just isn't competitive. It saves us money by reducing the labor to put variable in a customer's radio. Pays for itself that way, even if nobody buys it as a kit.
But if I wanted to market it as a product at a profit? Pretty sure that would start with a production quantity of 1000 units or so.
A bit rich for my blood.
No other way to get the per-unit production cost low enough to sell it cheap.
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