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I have performed the negative peak compression for a Cobra 2x family, or any other transformer coupled radio.

Total cost, about a dollar. Switchable. I will be drawing schematics up later for a switched one (just set and forget), and a variable NPC that will allow you to dial your carrier level to whatever you want to set it at, for NEGATIVE peaks... Positive peaks will remain unchanged.

Have it working in a Cobra 29 Nightwatch. VERY simple install. Completing the first circuit took about 4 hours. I made a second circuit in 5 minutes.

Incidentally, this IS a circuit that can be built in 'spare time'. It WILL work in ANY radio, but I DESIGNED it for transformer coupled AM transmitters (meaning, you can use this in a tubed Tram, Browning, etc... ANY high level modulated radio this circuit will work in.

Simple? Oh yeah. The first one was a single switch (and you don't need THAT, unless you want a switchable bleedover circuit lol) and 4 wires. Solder tack to the power switch, one to ground, and two to the PC board. THAT'S IT.

When I get schematics drawn up, I'll post them.

My Christmas gift to the CB community.


--Toll_Free

This will be cool to see, Thanks TF

Ain't no thing... REMARKABLY simple.

Now to figure out how to do MS Paint hahahahaha


--Toll_Free

A true friend to the AM community you are, looking forward to this.

OK, here's the 'final' circuit. This is for the 'standard' model, no variable to mess with.

The unlabeled diode is the existing 'anti spike' diode all radios have coming from the modulation transformer. You wrap the two other diodes around it.

This is the way the circuit works.

The diodes and resistors form a divider that allows the D2 to ONLY conduct during NEGATIVE mod peaks.... The HARDER the negative mod peak, the HARDER the diode conducts. During this conduction, it allows B+ (tube speak... Same as "the hot lead" in 12 volt stuff) from the resistor and capacitor to conduct through D2. By varying the resistance of the approximately 50 ohm resistor, you can vary the amount of negative peak limiting that the circuit provides.

I did NOT invent this... This is an adaptation of different peak limiters.

This circuit does NOT work 'just like npc' on the 148 or other class A series limiter radios.... IE, it DOES work at an audio rate, but it will NOT prevent negative 'peaks'... It prevents the carrier from being COMPLETELY pinched off. THIS is what causes the MAJORITY of splatter and garbage. This circuit will NOT clean up the waveform... I can add a splatter filter (to clean up the pos and neg peaks), but those add distortion of their own.

Also, the approximate 50 ohm resistor. It will run COLD if you don't 'bang on the limiter', so to speak. The MORE negative peaks you add (ie, the HOTTER you run the audio gain / power mic / etc), the HARDER this resistor has to conduct..... With a stock mic, a pair of 1/2 watt resistors in parallel (100 ohms) can be held in the hand. With the 'typical' levels a power mic produces, they will get WARM....






FOR THOSE THAT WISH TO MODIFY THE CIRCUIT!!!

If you want variable negative peak limiting, then take the "approximate" 50 ohm resistor and replace it with a simple LM317 or other 'variable' regulator. Copy any typical schematic for the voltage regulator.....

The 5.6 ohm resistor I made with a pair of 10 ohm half watts. The total package needs to be able to pass 1/20th of the total AUDIO power fed to the final... In a typical CB, this means it needs to pass ABOUT a half watt, average. The diodes need to be able to stand 4 times the unmodulated input voltage (13.8 on a transistor rig).

IF YOU WANT TO DO THIS TO A DIFFERENT 'STYLE' CHASSIS, here is the math:

The diodes are 1/2 the final plate impedance.... IE, (Voltage x voltage) / (2 X Power out) and take that answer and divide it by two. For a 50 watt MOSFET, it would work like this:

13X13 = 169. 169 / 100 = 1.69 ohms. Half that. .8 ohms. This is the MATHEMATICAL way to find the resistor value. once it is found, you can place a RHEOSTAT or pot there to vary it to get the level you want. Be careful, in tube stuff, full B+ can be across this resistor, depending on mod index. You can use the same LM317 to feed the circuit..... Tubed or transistor, you only need a few volts to ensure the carrier is NEVER pinched off.

Want to put this in a series modulated radio? Like the 148 / etc? Do the standard NPC volt the final mod, and the wire that goes to the driver, you cut. Insert a "anti spike diode" in series, and otherwise wire up the circuit above. Is it easier than a cap? No. BUT, if you have a buttload of these made up, it makes it easier, and if you put a variable power supply on it, you have a LOT better method of keepalive than NPC...

Incidentally, the power supply that feeds the keepalive circuit MUST be low impedance. Therefore, even though most other schematics that are similiar to this show it with the cap being unnecessary, I keep mine in circuit ALL the time.... Low ESR cap means you get a low R power supply.

Enjoy. Any questions, please feel free to ask. Simplest circuit I've come up with , and this will do all that is needed.

Merry Christmas.

--Toll_Free

Attached Images

Cobra NPC.JPG (17.1 KB, 554 views)

No feedback / questions / this is what happened when I tried it from anyone thus far? lol. It's been a week for one of the most wanted mods and nobody posted any feedback from it....

Nobody tried it, could figure out how it works, etc?


Anywho......

--Toll_Free

How bout a video of it?

__________________
Mobile:
Prez Grant Export/Stock Mic
Black Widow 350V
Two 9" Predator 10K
RS SWR/Watt Meter

Base:
P/T COBRA 146GTL(Spare)
President Jackson
Turner +3 Hand Mic
Pyramid 25 Amp P.S.
Home Made Ground Plane Base ant.

A video of what, exactly? I can see making a 'how to' if it was some esoteric or hard to source parts, but a pair of diodes and resistors, as well as a cap is about as simplistic as can be....

If need be, and I knew what people WANTED a video of, I could do one... But this is fairly self explanatory: I'm betting people are going WTF, this doesn't hook up like NPC on the electronically regulated chassis..... And blowing it off.

Circuits like this seperate screwdriver wizards from technicians.... Unfortunately for America, the math involved will stump some.....

Scope readings? This circuit prevents 'baselining' the audio, or in other words, prevents the carrier from EVER reaching 0 volts, cut off, etc.

Anywho.. Not against making a video, just don't know what people want a video of?????

--Toll_Free