How to perform the 2sc2999 and Schottky diode swap

[G0HZU]

In case anyone is a bit mystified why the AGC causes the noise it's because of where the AGC takes place and because of the gain distribution in the radio.

Basically the WORST place to put ALL of the AGC would be at the very front of the radio.
This is because the signal would always be tiny and noisy after the AGC and the S/N ratio would stay poor for all signals even if you were 1/2mile apart!
This is why the onset of the AGC diodes at the front of the radio gets delayed until signals are quite big.

However... on a mk2 148GTL-DX on AM/FM the initial range of AGC happens at the second mixer. The way this mixer loses gain with AGC is noisy plus it's still quite close to the front of the radio. So the signals stay quite noisy. Not good.

The D7 mod above will work (to a degree?) because it is deeper into the radio and hopefully does the initial AGC range with minimal loss of S/N (followed by the second mixer AGC and finally the AGC diodes at the front of the radio chime in for big signals)

Hope this makes sense

 

[milkman21218]

The Topic is How to perform the 2sc2999 and Schottky diode swap. Not what G0HZU does, or does not know about CB radio. You Hi-jack a 5 year old thread. You really need to start your own. And many of us truckers here in America have found that this Mod does work in the real world.

 

[wa4gch]

The Topic is How to perform the 2sc2999 and Schottky diode swap. Not what G0HZU does, or does not know about CB radio. You Hi-jack a 5 year old thread. You really need to start your own. And many of us truckers here in America have found that this Mod does work in the real world.

ALSO .....

HERE in the US FM is illegal ......

All I have to go by is how it worked for me on a GRANT LT which I use as a receiver ..... Now I did get a bit fancy and installed a Channel Guard IF filter and remeasured the radio with no real change in how it worked EXCEPT it was about 3 more "S" units down more than one ch away.

It is not a fix all but like most things some wil swear about it some will swear at it .....

 

[workman]

COMPUTER DUMB

JUST CAN'T FIND THE RIGHT PLACE, TO POST A QUESTION? BUT IF BUY CHANCE SOMEONE COMES ACROSS THIS QUESTION ,IF ANYONE CAN E-MAIL THE SOUND OF A BROWNING AND EAGLE SOUND AT ITS KEY UP I'D APPRECATE IT THANKS BOB, I'M NEW AS YOU CAN SEE. (201 NC WORKING MY TRASHY OLD TRAM)

 

[wannabetech]

I guess you now know not to listen to techno 'kids' on forums that don't have any RF design experience.

The best bits for me are their ridiculous technical explanations as to why these mods work.

:laugh::laugh::laugh:

Since you mention that; please explain the NPC-RC mod.
Exactly how does it really work. I don't mean how good it is--I mean what is the reason that it works? I know it looks good on the Oscope but I haven't been able to get anyone to tell me why it does what it does. I've installed about 2 dozen and they all like them but I feel dumb when I can't tell my customer why it works. (lol)

Thank you

 

[wannabetech]

I went back and read a lot of GOHZU's posts. You guys must not be reading them with any interest at all to think what you are saying about the thread being hijacked by his posts.

He is not hijacking anything--he is right on base.................how rediculous can anyone get. He's answering exactly what he should be with his knowledge.

 

[AdvancedCommunicationWest]

He is correct. I must say like it or not, this mod does nothing but bring up the gain in the front end of the radio where you don't want it and it's still a bipolar transistor. For parts changers to feel that they have accomplished something, it is a free country, so go for it. However, for true receiver performance this does nothing "real" at best, and at worst, you have modified a circuit without making other necessary changes and brought up gain once again in a stage that already has too much.

 

[jazzsinger]

The Topic is How to perform the 2sc2999 and Schottky diode swap. Not what G0HZU does, or does not know about CB radio. You Hi-jack a 5 year old thread. You really need to start your own. And many of us truckers here in America have found that this Mod does work in the real world.

PMSL, it must be real easy to pick out approx .5db improvement (at best) in an 18 wheeler.

as for what G0HZU knows about cb radio, i think that is blatently obvious to anyone who has got half a clue about radios, i'll say this much, its not that easy to impress me with knowledge about cb radio, but I can safely say Jeremy has managed that (and i guarantee not just with me either),

i know when i read technical posts from people who are smarter than me instinctively (like many posts on this thread), mainly because like so many others i have blind faith in my own intuition. but i'm smart enough not to have the same blind faith when it comes to my technical expertise, as i often get things wrong, it's one of the 2 ways that i learn, the other is from other peoples mistakes, and boy do i learn when the blind faith bullshitters start posting.

as for hijacking a 5 year old thread, pmsl, what you should have said was,

"its taken 5 years for someone to come along who actually knows what the fuck he is talking about and why this so called improvement mod is just more smoke and mirror bullshit".


oh and just incase you haven't noticed every single topic on every single radio forum is about what one or more members of that forum knows about radio, its how those that want to learn, learn and those that want to bullshit get exposed, so if this thread has turned into a what G0HZU knows about shite mods that don't deliver the goods then all i can say is HALLEFUCKINGLUJAH, it was long overdue.

And if G0HZU has the time, like wannabetech asks it would be real nice to see the npc-rc*Don`t do it* mod blown apart too,op: with hard fact, because all it is, is distortion and splatter,whether the human ear can hear it or not.

there is no shortage of antron 99 style wannabe myths just waiting to be shattered. but then again that might never happen "because my mate who's a rig doctor" (as the experts will tell you) thinks the a99 is shit hot, it may well be shit but its far from hot.

:blushing::blushing:

 

[KG6YGE]

I'm going to put the 2sc2999E npn in my otherwise stock TRC 458 (uniden 858) in the TR5 position. I have measured the noise floor already, with the 2sc1730L in there, and will get a new number for that after the swap. The NF of the 2sc2999E is about 2.2, and what I can find on the 1730 puts it at 4 dB, based on older documentation. The HP spectrum analyzer I'm using has a noise floor of -110 dBm, which is = to 0.5uV into 50 ohms.

NF of the 1st stage of amplification establishes it for the rest of the receive chain (if built right).

Cascaded Noise Figure Calculator

 

[KG6YGE]

Correction, the analyzer noise floor is -130dBm.

At the 1730L collector (TP 13), with antenna connected (or not), I measured a noise voltage of -125 dBm within a 20 KHz band around 27 MHz, and, receving a carrier at full needle, measured -25 dBm. When I get the new transistor, I'll remeasure this, then change it out, and measure again.

 

[G0HZU]

Well it's interesting that people still are interested in doing the 2SC2999 swap...

Hi KG6,
Your test method with the analyser isn't particularly scientific because you will load the circuit at TP13 with the analyser and also the signal you see at TP13 will be affected by AGC action when you apply the big signal. So my guess is you could make this mod look terrible or you could make it look brilliant or anything inbetween because of the poor test method you are using.

Why not measure S/N ratio at the AF out put of the radio using a calibrated signal generator at the aerial port and a true rms meter at the audio jack? That's going to have less measurement uncertainty by a long way

Besides, the easiest way to put this mod into perspective is to look at the typical level of band noise on the CB band. Galactic noise at 27MHz alone is about 20dB above thermal noise.

So if you have 20dB excess band noise and maybe 1.5dB feeder loss and maybe a receiver noise figure of 10dB for a standard radio the system noise will be dominated by the external band noise so the equivalent system noise figure will be about 20.5dB. Even if you could reduce the noise figure of the CB by 3dB with the 2SC2999 (isn't going to happen) then the system noise figure will only be reduced to 20.25dB.

So all that work for a 0.25dB improvement in signal to noise ratio assuming you really do get an improvement in S/N of 3dB at the radio. However, when I tried fitting a 2SC2999 to a Cobra 148GTL-DX it probably improved the receiver noise figure by 0.5dB to 1dB at best. So when you factor in the band noise with the aerial connected the improvement was about 0.1dB in theory.

It's a complete waste of time to do this mod unless the original transistor is damaged/faulty so you have to replace it anyway or the radio was poorly aligned to start with and gets a benefit of realignment...

 

[Robb]

G0HZU:

I've watched your viddy on aligning the receive of the 360XX chassis/Uniden eqivalents radios. Learned some from that, and has made aligning the receive of these radios easier and more accurate. It helped to understand how it works and what to expect. Thank You.

http://www.youtube.com/watch?v=2pRbw3MuYhs

I have found that in some radios the 2999 mod should be removed. Overload was observed in the EPTZ-0696XX chassis that was directly contributed by the 2999 in both the AM and SSB receive when both circuits were subbed with the 2999. The Schottky's are solid in every radio I used them in.

 

[KG6YGE]

Well it's interesting that people still are interested in doing the 2SC2999 swap...

Hi KG6,
Your test method with the analyser isn't particularly scientific because you will load the circuit at TP13 with the analyser and also the signal you see at TP13 will be affected by AGC action when you apply the big signal. So my guess is you could make this mod look terrible or you could make it look brilliant or anything inbetween because of the poor test method you are using.

Why not measure S/N ratio at the AF out put of the radio using a calibrated signal generator at the aerial port and a true rms meter at the audio jack? That's going to have less measurement uncertainty by a long way

Besides, the easiest way to put this mod into perspective is to look at the typical level of band noise on the CB band. Galactic noise at 27MHz alone is about 20dB above thermal noise.

So if you have 20dB excess band noise and maybe 1.5dB feeder loss and maybe a receiver noise figure of 10dB for a standard radio the system noise will be dominated by the external band noise so the equivalent system noise figure will be about 20.5dB. Even if you could reduce the noise figure of the CB by 3dB with the 2SC2999 (isn't going to happen) then the system noise figure will only be reduced to 20.25dB.

So all that work for a 0.25dB improvement in signal to noise ratio assuming you really do get an improvement in S/N of 3dB at the radio. However, when I tried fitting a 2SC2999 to a Cobra 148GTL-DX it probably improved the receiver noise figure by 0.5dB to 1dB at best. So when you factor in the band noise with the aerial connected the improvement was about 0.1dB in theory.

It's a complete waste of time to do this mod unless the original transistor is damaged/faulty so you have to replace it anyway or the radio was poorly aligned to start with and gets a benefit of realignment...

Check out this document:

http://cp.literature.agilent.com/litweb/pdf/5952-1147.pdf

You are confusing recevier NF with antenna noise temperature, which includes the effects of the sky temperature when determining an estimate for the total noise power in the receiver's frequency and BW. As you know, system noise temperature is a function of the ambient (sky) temp, lossless antenna noise temp, LNA noise figure (our variable here), the line loss between the antenna and the LNA, transmission line temp, and the missmatch (VSWR).

Now, the net signal/ noise ratio with 11 meter antenna connected (versus a 50 ohm load) will be impacted as the noise pours in at 11 meters. It may be that in my quiet environment and launch angle, that I'll notice an improvement, regardless of the improvement the analyzer shows in the shop. I'm using a vintage (1975) HP spectrum analyzer.

The Hfe of the replacement transistor is higher, but some have thought that this meant an overdrive condition (higher AC voltage) of the following state. That's not going to occur, but the front end is going to draw less current from the proceding tank circuit. That will cause a small missmatch in the impendance of the 1st amplificaiton stage (with the 2sc2999e) and the stage before it, since the input impedance is higher on it now, but the reduced noise floor that came along could still provide a net MSD improvement. That's what the test is about.

 

[G0HZU]

Thanks for the link to the old HP document but I'm already very familiar with how to use a spectrum analyser to measure noise figure as I've done this many times both at my place of work and here at home

Note that I'm not saying your test method isn't scientific 'because' you are using a spectrum analyser. It is because of the way you appear to be using it. Your test method introduces a whole load of causes of measurement uncertainty and is far from scientific.

It's hard enough to measure NF accurately using an analyser even if you do it with expensive test gear because of the cumulative effect of the various causes of measurement uncertainty. But your method takes this to a whole new level of uncertainty because you seem to be relying on making a gain/noise comparison with a noise level at -125dBm against a signal that is at -25dBm and way into the AGC range of the CB radio. So you get the uncertainty of your ability to measure (by eye within a fraction of 1dB) where the true FSD of the cheap CB meter is, you also get the uncertainty of the gain and AGC accuracy of a CB radio over time plus you get the uncertainty of the band noise with the aerial connected, plus you get the fact you are coupling into the collector of an RF amplfier (highish impedance point) with a low impedance spectrum analyser so you would appear to be loading the amplifier stage in an unnatural way. You also get the uncertainty of the spectrum analyser because it will suffer drift in gain over time and also it's accuracy across such a huge range of signal levels will not be that good.

So this all makes it difficult to decide if a change in noise level on your analyser is caused by a change in gain or by a change in noise figure or some subtle loading change at TP13 (or all three). So your result could be hard to interpret correctly.

What model number HP analyser are you using by the way?

You are confusing recevier NF with antenna noise temperature

Well first up, I'm pretty clued up on RF design and how to make decent test measurements but I'm certainly not an expert on trends in band noise across the radio spectrum

However, anyone can go online and see the test results of various government agencies that measure the various contributors to the noise levels we see on the bands today. Galactic noise is very significant at 27MHz and is typically 18-20dB above thermal noise.

So I don't think I'm confusing anything here? I'm just stating that the band noise on the 11m band is dominated by galactic noise (in the absence of man made noise) and it's hard to avoid receiving this noise with an outdoor CB aerial.

There is an equation that crudely fits the typical line/curve of galactic noise vs frequency when using an omni aerial.

NFm (dB) = 52 − 23 logF

Where F is the frequency in MHz.

So for 27MHz the result would be 19dB excess band noise due to galactic noise. There may be times when this noise gets masked to a degree by the ionosphere but I have no data to suggest how often this happens or the degree of masking. However, in my experience the noise levels on the CB band have always been high with respect to thermal noise level so getting a lower noise figure on test gear is of limited benefit once you connect an aerial.

 

[G0HZU]

I've watched your viddy on aligning the receive of the 360XX chassis/Uniden eqivalents radios. Learned some from that, and has made aligning the receive of these radios easier and more accurate. It helped to understand how it works and what to expect. Thank You.

Glad it was useful

The last time I looked (months ago) it only had a few views so it's nice to get some feedback