so.. I tried the npc

Tell us: any other mods done to radio: Limiter intact and working?
Just curious .

Don't think you are ever going to get those 'pretty waveforms' with the NPC when you keep trying to doing other mods to the radio to boost modulation at the same time.

The best thing about a scope is that it is honest and just telling you where you are at with the radio . . .

9Lives said:

The thought of npc seemed like a great idea to me. So I did the quick diode mod ppb my 959. I took it out immediately after seeing what it produced on the scope. What I noticed was a large carrier that I could only get so low with high peaks and very fast transition. The carrier was almost flat on the negative side. This is like distortion on top off distortion correct?
What it needs its a smoother transition on the neg side. Could the resistor value be raised and 2 diodes in series for more drop? Also the positive peaks were skinny.. a way to fatten them up some would be needed.

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The guy that did the NPC on two of my radio's many years ago used a scope and everything looked great. There must have been something up with what was done. Both radio's were outstanding talkers.

Yea, I bet they can hear you just as well on the next channel up or down from the one your tx'ing on?

9Lives said:

What adjustments exactly change the shape of the wave form. Output chain coils?

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No. They match the output osc to the impedance of the TX mixer IC.

9Lives said:

What I'm confused by is, during all of this I noticed some flat topping before 100% even at 3 watt key.. it's ok bc I don't use that high of a carrier but these radios are designed for 4 watts right?

I tried the 1k resistor on the emitter of the limiter. Took it off and it hasn't been quite the same looking. I can't post a pic right now but in the middle of the pos peak it bumps out slightly.. why is this? During the mod I changed nothing else. But the display on the scope the lobes are a lil crooked.

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Consider putting it all back to stock. Turn down the mic gain all the way, then give it a 1khz audio signal into the mic, and then adjust the mic gain upward slowly until you see symmetrical lobes. You should see NO lobes when the mic gain is at 0.

You gave up too soon, you can make the NPC diode look really good on a Galaxy radio. If done right. What location did you install the diode? Did you volt the final? Is your mod limiter defeated?

You waveform will not be perfect running a tone or audio through the mic jack. The factory audio chain adds distortion.

Take some pictures of your scope!

I install the a 1n4001 in the R266 location. You want the diode in series with the factory resistor. If the waveform is modulating the negatives to much or not enough you can buy a 15 turn 5K variable resistor from radio shack and install it in place of R266. This will help you dial in a decent waveform.

Now since the radio is going to be putting out a few more watts you need to upgrade the AM regulator and/or volt the final. I recommend volting the final because it takes the load off of Q54.

I also recommend installing a 1k resistor in series with the emitter of Q39, sometimes you can get a few more clean watts out before flat topping.

Opps..... The cathode will be to the back of the radio

Some of this stuff is silly when you consider the objective of NPC is to increase the amount of volume without increasing the amount of distortion. The last thing you want to do is "volt the final". These radios modulate the final and the driver simultaneously for a reason. You cannot fully modulate any transmitter in the hi level mode (the type these radios use) unless you apply modulated DC to more than one RF amplifier stage. As soon as you disconnect the final from the modulated 6 volt line and tie it to the main 12 volt line, there is no modulation applied to the final power amp. You just went backwards to make a watt meter look impressive.

Effective NPC is much more than just a diode. Broadcast stations use very complex multi band compressors in conjunction with negative peak clipping that is always followed by many poles of filtering to move the harsh transitions any useable NPC is going to create. Most of these CB mods have been developed by people who think the goal is to stop the negative peak from reaching RF cutoff and if you look at the results, they sometimes do that. Problem is that has nothing to do with the unwanted distortion and actually removes headroom required to avoid it. Again, going backwards.

Most of us know when we see flat topping on the positive peak, we have distortion. The same is true of the negative peak in that we have to pay more attention to the shape of the waveform than the aspect of RF cutoff. An AM carrier perfectly modulated to 100% with a 1Khz. tone will hit RF cutoff 1000 times per second but not cause any objectionable IMD.

This is because at 100% modulation there are no flat portions in the waveform at the top or bottom. If nothing is flat, there is no DC component. If there is no DC component, there are no sharp transitions in the positive or negative peak. That all adds up to a clean signal with a carrier that is constantly going in and out of RF cutoff but the shape of the original audio is never cutoff on the top or bottom.

Now if you do some mod to your radio that clips off the negative peak at say 95% modulation, you just took out 5% of your headroom before the negative peak could become flat. Since that diode is not followed by a filter network anytime its driven over 95%, its output will be clipped, flat and have sharp transitions. Exactly what you want to eliminate. Most people would get more out of a good compressor before even thinking about how to compress the negative peak independently.