NPC RC modification mod

[Happy_Hamer]

OK I seen the other thread that takes up 18 pages and just could not get myself to read that many pages :headbang

What is the NPC/RC mod and why should it be done?

I remember a tech, SS8541, that spoke of it in the past but I never knew exactly what it was.

So, do tell,,,,,,,,,,,,,,,,,,,,,

 

[Robb]

I'm pretty sure Mr Fatty knows how its done.
I asked him if he would mod a radio with the NPC-RC mod and take some pictures of the scope, as well as giving his own analysis.
I'm curious as you are about what he may find.
If I knew the location and the specific parts, I would do it myself - but I don't have a scope

 

[15minigrass]

Yeah I need one for my 146GTL... Love that NPC mod.

 

[Happy_Hamer]

I assume it is a swing mod for driving an amplifier with low wattage?

Maybe I do not understand it correctly?

 

[Happy_Hamer]

Here is all I found;

1. Remove TR24

This deactivates the modulation limiters in all modes. Use the
front panel mic gain (aka dynamic) control to set the modulation
percentage.

2. Add a solder bridge to the solder side of the board that
effectively jumpers out R196. This is a quick way of
replacing R196 with a jumper (reducing its value to zero
ohms).

This increases the range of VR10 (AM dead-key power) so that the
dead-key can be set to 1.5 to 2 watts later on.

3. Add a 10 uF 25 or higher volt electrolytic cap to these
points: the positive leg goes to the trace that connects to
pin 9 of the IC6 (the audio IC), and the negative leg goes
to the R194/D63/R228 junction.

This is the mod that compresses the negative modulation peaks and
allows the average power to increase based on the modulation
percentage (aka the NPC mod).

This is the end of the solder side work. The rest of the work is
done on the parts side of the board.

4. Set the driver bias to 50 mA. (Power up the radio, put it
in LSB or USB, set the mic gain at minimum, remove the wire
from test point 8, insert a milliamp meter in series between
the test point (which is positive) and the wire, key the mic,
and adjust VR9 until the meter reads 50 mA.)

5. Set the final bias to 100 mA. (Same instructions as in step
4 except the test point is test point 7, and the adjustment is
VR8).

On some of the newer radios the final bias can't be set higher than
about 50 mA. The reason is that the value of R179 has been increased
in order to decrease the effective range of VR8. To solve the
problem, replace R179 with a 500 to 1000 ohm resistor.

6. Once the final bias has been set, unplug the DC power cord,
put the final bias wire back on the test point, cut the final
bias wire 1/4 inch above the connector, strip and tin 1/8 inch
of the wire, tin the cathode (banded) leg of D55 (the reverse
polarity diode), and solder the wire to D55. This assumes the
test point connector is at the end of the wire that is furthest
from the final transistor. On some of the newer models the
test point connector is at the end of the wire closest to the
final transistor. On those models, completely unsolder the wire
at the end opposite the test point connector and solder it to
D55.

This is the mod that converts the RF final stage to linear in all
modes.

7. Power up the radio, put it in the AM mode, key the mic, and
set VR10 (AM dead-key power adjustment) for about 1.5 watts.

8. Tune the RF chain coils (L38 and L45 through L48) for maximum
peak (modulated) output power in the center of the band (that
would be Channel 19 on a stock radio and Channel 40 on one that
has the popular expanded frequency range of 26.815 to 28.045).
If you have a favorite channel that is more than 30 channels
from 19 or 40, do your tuning on that channel.

9. Double check the dead-key power. It should be around 2 watts.
If it is higher than 2 watts, use VR10 to cut it back to between
1.5 and 2 watts. Don't overdo it. Keep in mind that the carrier
(aka dead-key) power increases up to 10+ watts with modulation,
so there's absolutely no point in having the dead-key power any
higher than is required to reliably key an amplifier. Most amps
will key reliably with as little as 1/2 watt of dead-key power.

The following numbers are what you should expect. However, since
there is a lot of variation in CB test equipment setups, don't be
alarmed if you don't see these exact numbers. These numbers are
provided as a guideline to make sure you did the mods properly.

The dead-key wattage should be 1.5 to 2 watts. The maximum average
power should be 10 to 12 watts. And the maximum peak power should
be around 25 watts.

 

[9C1Driver]

I had a Mirrage 2950EX and a Delta Force with the NPC mod. Best talking radio's I ever had. Worked great if you run an amplifier.

 

[Happy_Hamer]

FOUND IT!

Written by SS8541 Richard Byrd-

The npc mod is the most used mod in export radios today. it has been around for yrs, but it is set up wrong and actually labeled wrong (depending on form used) by 90% of the techs using it.

The npc where the limiter is removed and the final/s volted -is- an actual npc. but after reading this i hope you all will see that unless you have the right equipment it is unaceptable for a clean signal (which is the reason for using the npc instead of clipping the limiter). even when set right, a 1w carrier with 35-45w (single or dual final)of swing is not good.

For the npc where the limiter is left intact and and the final/s is not volted, a stock radio set at 100% mod by the factory, needs nothing but the npc components. well, the high power carrier needs reducing to keep the pos peaks from flattopping, but the lower power setting needs nothing but the npc components. this mod is also not truly a npc mod, but a ppe (positive peak expansion) mod. there is no compression of the negative peaks, since the neg peaks still hit 100% mod without the amc being adjusted, but the pos peaks will hit 125-300% pos mod (depending on resistor value used).

The reason is because the amc is only triggered on the neg peaks. the amc is turned off on the post peaks, but it tracks on the positive peaks by an rc circuit.

Here is a galaxy 77hml at 100% mod and a 1.25w carrier as an example....

For a 1.25w carrier the collector of tr51 is at 2.50vdc. the emitter of tr49 is at 2.54vdc. when audio is applied for 100% mod, the collector of tr51 swings from 0-5v (5vp-p). the emitter of tr49 swings from 2.49-2.59v (.1vp-p).keep in mind that the emitter of tr49 only swings .1vp-p as this will be revisited later. the amc samples the neg peaks to keep it at 100% mod.

Now when a 1n914 diode/390ohm res npc (ppe)is used, things begin to change. the banded end of the diode goes to the collector of tr51 and the res goes to the emitter of tr49. the neg peaks still hit the 100% mod i had set the radio for in the last step, but the pos peaks go to about 200%. remember how the voltage on the col of tr51 goes to 0 but the voltage on the emit of tr49 only goes to 2.49v. so when the col of tr51 goes .6v lower than the emit of tr49, the diode is turned on and -tries- to pull the col of 51 back up. but the amc is set to -make- the voltage of the collector of 51 hit 0 (100% neg mod). to do this more audio must be dumped into 49. this is evident by remeasuring the voltages at the emit of 49 with a scope. it was .1vp-p with audio, now it is .2vp-p. since the amc only samples the neg peaks, that same .2vp-p is present on the pos peaks. and since only .1vp-p is needed here, the pos peaks go past 100% (about 200% here). so as you can see, there is no compression of the negative peaks (npc). there -can't- be if the neg mod is hitting 100% before and after the mod without amc being adjusted. but there is positive peak expansion (ppe). this is all done by a way of fooling the limiter/amc. and you can add this to any stock radio and it will work properly as long as it was properly set at the factory for 90-100% mod. you also need to keep the carrier at no more than 3.5w for single final radios and 5w for dual finals for the 390/914 npc(ppe). for a 560/914, 5w carrier for single and 7w carrier for dual finals.

As for the npc where the limiter is removed and finals volted, there -is- neg peak compression since there is no limiter present to react and -force- the neg peaks to 100%. BUT you have to have a scope to set it correctly. and how many of you have that??? you may also have to play with resistor values or mic gain to get it to work properly (once again you still need a scope).you might as well just clip the limiter, since if you don't have the right equipment your in the same shape, but with about 5min-30min (depending on your soldering skill level) more work. also, why volt the finals when with both mods you have to reduce the high power carrier for it to work properly? the reg is being "spared" by reducing the carrier power (which is the reason for "volting" the finals).

Anyone who wishes to post this on other forums has my permission. I'm not doing this to call anyone out or for self recognition, but i'd like to see this get around for techs who don't quite understand what is going on with this/these mods (especially since this is the most popular mod out there). this mod was meant to run clean radios with increased pos mod, so it should be understood how to do it properly. I'm not a "mad scientist" saying it works because I say so. I laid down the facts to prove it.

by
SS8541 Richard Byrd

 

[MisterFatty]

Here is all I found;

3. Add a 10 uF 25 or higher volt electrolytic cap to these
points: the positive leg goes to the trace that connects to
pin 9 of the IC6 (the audio IC), and the negative leg goes
to the R194/D63/R228 junction.

This is the mod that compresses the negative modulation peaks and
allows the average power to increase based on the modulation
percentage (aka the NPC mod).

Believe that was Eitners version of the mod..... Essentially you can strip out everything else, and the above is the meat of the mod.....

 

[MisterFatty]

FOUND IT!

Written by SS8541 Richard Byrd- ............................................................
............................................................
............................................................

by
SS8541 Richard Byrd

I'm not the best writer, and do not convey technical points well in my writing.

From what I've been reading, and there actually is not all that much material on the internet regarding this subject. The whole Negative Peak Limiting, Asymmetrical processing thing is not the easiest to execute properly.

It is all sound theory, and can be implemented on a limited basis, however, from what I've read, the resistor and a Diode can only accomplish NPC on a limited basis.

When done incorrectly it does make the radio louder, and gives it a shit load of "swing" at the cost of distortion. From all I've read the human ear hears distortion as making a signal louder.

I do think that when the Negative peaks are PROPERLY limited, greater intelligible weak signal can be realized, however the same thing can be done with a PROPERLY modulated "Old School" radio. that has a more conventional carrier to peak power ratio.

The thing is, just like "clipping the limiter" this mod when not done properly, ends in the same result.

I would be willing to bet the HUGE majority of NPC mods end up as some fellow tossing a few parts in a radio, no scope, nothing to even insure its even an NPC mod.

I'm purposely not going to get to deeply into theory, or even technical side of this yet. mostly because without supporting evidence, it is all just my opinion.

I will see about modding a suitable radio, and getting some pictures of the scope, etc.

 

[Robb]

I'd also be interested in how it works compared to the TopGun modulator.
(Which makes the radio deadkey 1 watt and swing as much as the output transistors will allow)

There is another parallel.

The compressor unit from TopGun ('CP-1') also places its only two leads on the mic op amp IC (much like the capacitor mod to the IC that Richard Byrd described above).
Sounds like they ('TopGun') have accomplished a similar result by using components that can offer consistency in results.
Doubtful that it attains the same negative peak carrier state - tho...

But that isn't the point here.

Is the NPC mod more the state of the tune with a modulation limiter clip than anything else?
It seems that it is different things for different radios.
I'm scratching my head and trying to learn all this.
I couldn't read a scope if it were to print words on the screen - lol!

 

[Happy_Hamer]

I have known Richard, SS8541, for years, since the days of EEI.

I know he has the equipment to check this stuff and I have had several radios modified or repaired by him with GREAT results.

He has his own shop

8541 Electronics

I do not know if he still repairs CB radios, but he will talk to anyone and takes the time to explain.

 

[unit_399]

Negative Peak Clipping is easily applied to ANY radio...IF you know what you are doing. Usually it requires two components... both diodes, but they must be selected to work with the particular setup on your radio.

Selecting the components requires a complete understanding of how Amplitude Modulation works, an RF voltmeter, an oscilliscope, and a thorough understanding of basic electronic theory. Unfortunately... from what I've seen over the past 2 years, this eliminates about 90% of the people on this forum.

My RCI 2950 is setup with 22 db of audio clipping using a Uniden speech clipper PCB from a Superscope CB640. The NPC is a 2-diode device of my own design that allows 130% positive modulation peaks while limiting negative modulation peaks to approximately 95%. This results in a signal that is extremely LOUD and CLEAR without generating any harmonics and splatter. BTW... the mod limiter in this radio is not disconnected.
There is no rocket science to this. There are 2 mods: one for direct-coupled modulators, and another for transformer-coupled modulators. The basic theory is the same, but the diode selection process is different.

When my wife (and our digital camera) gets back from the US, I'll post some O'scope pics of my mod and more particulars on how it works. Like they say... a picture is worth a 1000 words.

- 399

 

[MisterFatty]

Negative Peak Clipping is easily applied to ANY radio...IF you know what you are doing.
- 399

Seems to me you may be referencing my post with your emphasis.... To address your point, Yes you are correct, in the CB sense of the mod. My reference is more toward NPC, and Asymetrical modulation/processing as a package deal. Doing the NPC, or NPC-RC mod is not rocket science, it is relatively easy, and setup with the proper equipment is quite simple.

 

[Moleculo]

I'm not the best writer, and do not convey technical points well in my writing.

From what I've been reading, and there actually is not all that much material on the internet regarding this subject. The whole Negative Peak Limiting, Asymmetrical processing thing is not the easiest to execute properly.

It is all sound theory, and can be implemented on a limited basis, however, from what I've read, the resistor and a Diode can only accomplish NPC on a limited basis.

When done incorrectly it does make the radio louder, and gives it a shit load of "swing" at the cost of distortion. From all I've read the human ear hears distortion as making a signal louder.

I do think that when the Negative peaks are PROPERLY limited, greater intelligible weak signal can be realized, however the same thing can be done with a PROPERLY modulated "Old School" radio. that has a more conventional carrier to peak power ratio.

The thing is, just like "clipping the limiter" this mod when not done properly, ends in the same result.

I would be willing to bet the HUGE majority of NPC mods end up as some fellow tossing a few parts in a radio, no scope, nothing to even insure its even an NPC mod.

I'm purposely not going to get to deeply into theory, or even technical side of this yet. mostly because without supporting evidence, it is all just my opinion.

I will see about modding a suitable radio, and getting some pictures of the scope, etc.

This was basically sums up my whole point in the latter part of the previous thread. When the thread first started years ago, I didn't understand as fully as I do now about how proper negative peak compression should look on a scope, but anyone reading that thread should be able to see that I learned a few things over the years.

Notice this statement by Richard:

you have to have a scope to set it correctly. and how many of you have that??? you may also have to play with resistor values or mic gain to get it to work properly (once again you still need a scope)

This again emphasizes my biggest beef with this mod: A good tech can set this up properly on his bench, but almost NO CBers have the proper audio gear to regulate the input audio to keep this type of mod from turning into a monster. As soon as they get home, they use a different mic than the tech did and most of them crank the gain way too high, resulting in something that is a completely different animal than the tech's scope showed and causing a ridiculous amount of splatter. Reference NU9N's material on what it takes to control the audio input when you start playing this game.

 

[MisterFatty]

BTW... the mod limiter in this radio is not disconnected.

- 399

just curious, at what point do you consider your amc "disconnected"? 131%, 150%, 200%? Or do you consider it not dependent of the point of limitation? I've always considered the AMC defeated anytime you go over 10%, or 110% total. I believe the US FCC defines that point @0%, or 100%.