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NPC for AM Transformer radios is HERE... Tubed, Solid State, etc. ANY high level TX

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  #9  
Old 01-03-2011, 12:02 PM
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Never took electronics class so I dont know what Im looking @ here on the diagram. Sorry toll...
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  #10  
Old 01-03-2011, 06:00 PM
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If I had a virgin 29, I'd do this and make a video showing how/where to do it. It looks pretty straight forward. Thanks for this!

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Old 01-03-2011, 06:21 PM
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just found this thread. Very cool!

dont have a radio to try it on at the moment, but probably will in a week or so.

i am thinking that using this mod, if someone came in wanting a lower carrier/high PEP swing type mod for a 29LTD, i could just use a resistor in place of JP36, and omit the cap.

then use this circuit instead.

im guessing that since the negative peaks are being controlled by the added circuit, that the AMC can be disabled allowing the positive peaks to run wild?

also, not sure if i missed it, but how many volts do the added diodes need to be able to handle?


great circuit, cant wait to try it.
LC
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  #12  
Old 01-03-2011, 06:33 PM
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Quote:
Originally Posted by Toll_Free View Post
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.



TOLL_FREE -

Your circuit may work (I haven't tried it yet), but I don't understand your explanation. The output from the transformer has a DC (13 volts) and an AC (Audio) component. Since D1 and D2 are reverse biased with respect to the DC, it is unaffected. But BOTH D1 and D2 are forward biased with respect to negative audio peaks and both will conduct on negative peaks. However, I think that D1 will conduct more than D2 because of the voltage drop across the Anti-spike diode. This is confusing to me. Please explain the function of D1 in this circuit. Thanks. 73s.

-399

Last edited by unit_399; 01-03-2011 at 07:03 PM.

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  #13  
Old 01-03-2011, 06:35 PM
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Quote:
Originally Posted by Moleculo View Post
If I had a virgin 29, I'd do this and make a video showing how/where to do it. It looks pretty straight forward. Thanks for this!
Wish I still had my 29, I would send it to ya for a sample. All I got here is a 146GTL which I did a Diode/Res NPC mod to it. Does well @ 1 watt DK/Swinging full 12+ watts. If ya ever come across a 29 post up a vid of the mod.
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  #14  
Old 01-03-2011, 06:58 PM
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Quote:
Originally Posted by Toll_Free View Post
Scope readings? This circuit prevents 'baselining' the audio, or in other words, prevents the carrier from EVER reaching 0 volts, cut off, etc.
--Toll_Free

The amplitude of the carrier in an AM signal is the same with or without modulation. Looking at the familiar "rf envelope" display on a scope is misleading because it can't display the carrier and the sidebands seperately. It only displays the vector sum of the three signals added together. If you look at an am signal on a spectrum analyzer it shows that the carrier amplitude is the same with or without modulation.

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Old 01-04-2011, 10:13 AM
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Quote:
Originally Posted by unit_399 View Post
The amplitude of the carrier in an AM signal is the same with or without modulation. Looking at the familiar "rf envelope" display on a scope is misleading because it can't display the carrier and the sidebands seperately. It only displays the vector sum of the three signals added together. If you look at an am signal on a spectrum analyzer it shows that the carrier amplitude is the same with or without modulation.


I own a spec an as well as an oscope. What you are saying is textbook, and WILL be what is reproduced WHEN YOU DO NOT EXCEED 100 PERCENT MODULATION in a high level modulated system.

When you OVERMODULATE in the NEGATIVE direction, you actually cut voltage off COMPLETELY to the modulated stage(s).... In effect, BASELINING the modulator... This is what actually causes splatter, IMD, etc. You CAN see this effect on a scope, this is the little bar you get between modulated waveforms (the valleys being extended between peaks, causing actual space between the individual RF cycles.... THIS is 0 watts of carrier, THIS is splatter, AND when you look at BOTH the Spec An AND the Oscope you WILL see this effect.)

What the circuit I presented does is PREVENT the modulator from EVER hitting 0 watts, 0 volts, etc.... However you want to PUT it. It does NOTHING to the modulated waveform (at least, that I can detect), it ONLY keeps a couple volts on the finals AT ALL TIMES, whereas the MODULATED waveform, when modulated in EXCESS of 100 percent will actually CUT ALL VOLTAGE off to the final and driver... PLEASE explain to me how you will still have carrier, when you have 0 volts being delivered to the collector / plate of the amplifying device, to put it in other terms.

If your limiters are STILL intact, and act upon the NEGATIVE going modulated voltage, then YOU ARE CORRECT, and you will NEVER baseline the audio.

HENCE the reason I stated THIS circuit will work on any HIGH LEVEL modulated radio. A low level modulated radio should still use the Eitner induced NPC, which was borrowed and modified version of the BDW CBDoc NPC. The low level modulated radios (balanced mods) will actually take the negative going waveform and 'flip' it, causing the carrier to never baseline, but introducing the 'overmodulated' sound that the lincoln, 2510, jackson, etc have, versus the "louder than hell" overmod sound that the high level radios have (which is actually nothing more than IMD....... Baselining the carrier, etc).

THIS is the type of discussion I hoped to implement... In a PROPERLY functioning AM HIGH LEVEL transmitter, that is NOT overmodulated to the NEGATIVE direction, you're right, the carrier won't be cut off... HOWEVER, if you exceed 100 percent modulation, the spec an, the oscope and a fast acting voltmeter with peak and hold detection will show it actually being cut off....

Also, the resistor that feeds this circuit will get noticeably warm under use. If you HAMMER the negative peaks, it will get HOT. Easy way to tell if your overdriving your modulator is to test the heat on the resistor... If you do the math correctly, it will run barely warm...... PROVIDED you're not running a Silver Eagle mic at half throttle, limiters ripped out and dynamic wide open! At that point, you're pretty much baselining EVERY half cycle, and the resistor should be sized in wattage at HALF your total modulator power, or more.

As to LC's question about carrier: You'd want to put MORE diodes in the series diode, ah la the old "dial-a-watt" methods from Secret CB. Forget about the jumper method. The problem with the jumper and then going to the RC circuit is this: You will NEVER get full drive to the final amplifier with the driver at less than peak output! Then, on modulated peaks, you end up with a slight ripple or valley at the CREST of the mod peak. I'd lower the final AND driver Vcc with a string of diodes, use the baseline preventer here, and you're ALWAYS looking at the proper input and output impedances from the modulator to the driver to the final. I've also played with modulating the pre-driver, but didn't get anywhere near the effect I wanted.... Tried hitting the pre-driver with a variable voltage on the collector, and after losing about 2 watts of carrier (8 watts was where it was when I started), the waveform on the output went all to hell. On the scope and analyzer, the best method thus far is dial a watt.... The 3 watts you lose PEP won't be noticed.

The thing to watch about ripping the limiter out is this: You can STILL overdrive the modulator stage (the audio chip) causing a triangle waveform on it's output. The transformer is GIGO, if you hit it with a non-linear waveform on it's input, the transformer will only amplify it and send it to the finals..... Watching the modulating waveform on channel A and the output on Channel B will let you know exactly how much forward power you can have..... Hint, only HALF the modulator power can go forward... If you want more, you require HARDWARE in your audio chain to actually increase the AVERAGE power input to the radio.


With careful tuning of the radios output section, and a 1969, THIS is the way to get 75 PEP out of a 29.

--Toll_Free

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  #16  
Old 01-04-2011, 12:06 PM
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"With careful tuning of the radios output section, and a 1969, THIS is the way to get 75 PEP out of a 29."

Can you really hit 75 watts on a positive modulation peak with a single 2SC1969? (Disclaimer: I don't know the actual specs for a 1969 so I could easily be talking out of my butt, but I was under the impression that since it was originally chosen for standard CB radios designed for SSB operation with 12 watts PEP, the upper limit wasn't that high.)

Obviously, negative peak compression is meant to deal with the case where you have more headroom available for positive peaks than for negative ones (by rounding out the negative peaks instead of letting them turn square when they hit the 0 volt line), but there does come a point where you run out of positive headroom too (presumably when you drive the positive peaks beyond the supply voltage). I thought for sure that a single 2SC1969 would end up clipping its positive peaks well before you hit 75 watts.

I guess a better question would be: even if you can do that, will you still be able to maintain linear operation? Granted, you're introducing some distortion by doing NPC in the first place (you're avoiding the IMD effects of having the waveform flatten out, but still: the audio waveform leaving the radio won't look like the one that went in -- technically that means distortion), but I would think that if the RF amplifier stages don't retain linear operation across the whole output range, you're going to induce some additional distortion near the tops of the positive peaks.

-Bill

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