How to perform the 2sc2999 and Schottky diode swap

[wa4gch]

Hi Wa4gch

Your tests show a huge 5dB difference in sensitivity between the two transistors and I think something surely isn't right there.

I have actually built the front end of a 148 receiver on a separate PCB and tested it for noise figure. i.e. I tested the input amplifier stage.
I tried it with the AGC diodes and without and also with and without the input L match and L4.
I always got consistent results showing the transistor in this circuit was giving about a 4dB noise figure.
I also tried putting loads of different transistors in the front end amp in the actual radio and always got about the same sensitivity.

Your 5dB improvement seems a lot. Is there any way your radio has been modified in some way or maybe the original transistor was duff?

You can see by my test results in my video that the standard radio can lose 4dB of gain at the top/bottom channels because of the L8, L9 BPF but the S/N only degrades by about 2dB.

This tells me the gain of the first stage in the centre of the band is already good enough to pretty much overcome the noise contribution of the rest of the radio path. So the only way to get better sensitivity would be to lower the front end NF somehow (rather than just pile on lots more amplifier gain).

There's no way you can get the NF 5dB lower just swapping out the transistor. I'd be amazed if it could be 1dB lower NF with just a transistor swap.
However you CAN get a very big change in NF if the input transformer is not set to the best setting for the before and after comparisons.
i.e. it is possible to get poor signal to noise by doing nothing other than adjusting the input transformer L7 slightly to the noisy side of the peak.

i.e. I can make the same radio look bad even though nothing has changed apart from the L7 setting is now 'noisy'

Can I ask, are you testing in the middle of the band for your 5dB improvement or at the outer band edges?

The radio came right out of the box ........

NOW TRUE it was not tuned up at that time but I did the mod because I use it as a DX monitor and wanted a bit more gain because of the very small antenna . It is a otherwise STOCK radio 40 ch no ch mods ... I lissen to 38 lsb
and use my 29.600 mhz fm radio so that I can be aware of 10 being open and how well it is open.

AFTER he transistor was replaced I tuned the front end ( to ch 19 ) but found no real gain so either my 2sc1674 was as you say a dud or the radio just liked the 2sc2999 better ......

The grant can be seen above the computer monitor

 

[G0HZU]

I can offer some more thoughts about the front end design of the radio if this is of any interest.

If you look beyond the first RF amplifier and BPF coils you arrive at the first mixer.

This is usually an active mixer and will have a certain conversion gain (depends on the type of mixer) and it will have an associated noise contribution.

On a standard radio this mixer gets presented with a wall of amplified thermal noise from the first rf amplifier.

This wall of noise will be above thermal noise by approx the amplifier gain plus its noise figure minus the losses in the bandpass filter after the amplifier..

So ballpark 17 to 20dB above thermal noise perhaps.

If you just look at the receiver at the input of the mixer stage it will have its own gain and noise figure. It also has a lot of lossy circuits after it including filters etc before the first IF amplifier stage (with assoc noise figure)

I think you will find on many older Uniden based CBs that the system noise figure looking into the mixer is quite high. Maybe 16dB?

So even if you could reduce the first RF amplifier noise by 2dB I doubt the sensitivity of the whole radio would go up 2dB because the inherent noise contribution at the mixer will become significant with respect to the lower noise of the front end.

I hope this makes sense. The way to design a receiver for a certain noise figure is to cascade the gains and losses of all stages of a receiver and enter them onto a spreadsheet that calculates the total system noise figure.

This is the figure that matters

Because of this I believe it is going to be quite difficult to make a significant improvement in system noise figure in the centre of the band by just swapping out parts in the RF amplifier. The mixer and the losses after it start to become significant if you reduce the RF amplifier noise figure. So you don't get full benefit unless you increase the amplifier gain as well as reducing its NF.

On a standard radio the significance of this post mixer stage noise is masked to a degree by the wall of noise from the first (noisy?) RF amplifer.

Note that I don't recommend an external preamp although this would certainly be one way to reduce the system NF by several dB.

The reason is that a preamp will degrade the dynamic range of the radio (in terms of detecting small signals in the presence of large ones) so I dont think the benefits of lower NF justify a preamp. CBs already have poor dynamic range so preamps are not a good idea

 

[G0HZU]

AFTER he transistor was replaced I tuned the front end ( to ch 19 ) but found no real gain so either my 2sc1674 was as you say a dud or the radio just liked the 2sc2999 better ......

The grant can be seen above the computer monitor

I've not used a grant but I think there's several versions. The one that is common here in the UK is very similar to my PB010 mk2 148GTL-DX in terms of the receiver circuits

 

[wa4gch]

I've not used a grant but I think there's several versions. The one that is common here in the UK is very similar to my PB010 mk2 148GTL-DX in terms of the receiver circuits

There is no doubt mixer noise and the noise of the following stages detract from the final noise floor however at 11 meters external noise maskes most of it anyway ...The mods I did tried to take up for the loss of signal captuer in a 3 foot " base " antenna and it worked just fine.

This is how my station is set up with peramps for EVERY band from 160- 2 meters ....

I do weak signal work on 2 SSB/FM ....

 

[G0HZU]

That's quite a comprehensive setup!

In terms of improving the Cobra 148GTL-DX for receive sensitivity I think there are only two realistic methods (besides an external preamp)

One is to make the sensitivity more consistent across all 200 channels of an expanded radio. i.e. try and get 8dB noise figure across as much of this as possible.

This should be fairly straightforward.

However, to get the radio to a NF below 6dB would take a lot of work.

Therefore, I would suggest people upgrade their coaxial cable before doing any serious mods for sensitivity.

Eg a 1.5dB reduction in coax cable loss is going to be much simpler to achieve than redesigning the radio front end to get a similar reduction in system noise figure inside the radio


If I get some free time I'll try and do a quick youtube video explaining why it isn't straightforward to get better sensitivity when modding these old radios. It's basically to do with the various stage gains, noise figures and passive losses in the whole receiver chain. This will explain why it is much harder than you think to get even 2dB better than the typical 8dB noise figure that I see after a good alignment on a standard radio.

 

[wa4gch]

That's quite a comprehensive setup!

In terms of improving the Cobra 148GTL-DX for receive sensitivity I think there are only two realistic methods (besides an external preamp)

One is to make the sensitivity more consistent across all 200 channels of an expanded radio. i.e. try and get 8dB noise figure across as much of this as possible.

This should be fairly straightforward.

However, to get the radio to a NF around 6dB would take a lot of work.

Therefore, I would suggest people upgrade their coaxial cable before doing any serious mods for sensitivity.

Eg a 1.5dB reduction in coax cable loss is going to be much simpler to achieve than redesigning the radio front end to get a similar reduction in system noise figure inside the radio

As you know there are many factors in receivers one is bandwidth here is a photo of my TS-2000 on CW you can see backgtound change ... this is why cw works so well .....

 

[mackmobile43]

As you know there are many factors in receivers one is bandwidth here is a photo of my TS-2000 on CW you can see backgtound change ... this is why cw works so well .....

I heard you on 27.385 today and I do not care for your audio.

 

[wa4gch]

I heard you on 27.385 today and I do not care for your audio.

If your talking to me

I don't run on 27.385 sorry it was someone else ......

 

[loosecannon]

G0HZU, just to clarify, i am not Robb.

i found your video to be quite educational.

thank you for taking the time to do that. i feel that we should make the distinction here between the 148GTL and the 148GTL-DX.

being that we are "across the pond" from eachother, when you guys say "cobra 148", you are referring to the DX model with the MC145106 PLL chip in it.

when we say "cobra 148", we are referring to a stock 40 channel CB radio with the MB8719 chip in it.

your 148GTL-DX is quite similar to the Galaxy radios we are used to over here which use the EPT3600 series boards. DX99v is a good example.

just wanted to point that out for clarification, although when it comes to the receiver front end and band pass filter sections, they are all pretty alike.

a couple of questions. first, is there any way for those of us who dont have one of those cool dB meters to do the "proper" alignment of the transformer preceding the RF amp transistor?


second, the mods you are referring to that are meant to widen the bandwidth of the band pass filter for a more even receiver response across the band; do they involve raising the value of the coupling caps between the transformers?

if so, doesnt that mod reduce the selectivity somewhat?

i am very interested to hear about any improvements you have found for the 148GTL-DX chassis, as the Galaxies that are based on that design are very popular over here.

thanks again for taking the time to do all that work.
LC

 

[G0HZU]

Hi LC
Yes, one of the things to do is alter the coupling cap. However, this does reduce the signal level so there is a tradeoff here. To counter this the gain of the first stage can be increased by altering the bias point. However, this needs care to avoid instability and there's other mods required to prevent instability.

is there any way for those of us who dont have one of those cool dB meters to do the "proper" alignment of the transformer preceding the RF amp transistor?

I'll have a think about this but I used to do it with just my ears and a signal generator because you can hear that L7 has a noisy side to its adjustment.

It might be possible to do it with an old analogue AC volmeter even if it doesn't read true rms. I'll see if this is possible.


BTW my 2SC2999 transistors arrived today. They are marked as D versions although this really isn't significant.

I have just done a comparison test between this and a standard radio.

I did these tests on a mk2 SS360FM PB010 which is the same as the mk2 148GTL-DX.

Basically, the amplifier gain as measured on the network analyser and probe was identical (within 0.1dB)

This was measured after optimising to try and match the gain response of the original 2SC1674L. The difference was so tiny between them it wasn't worth measuring.

I then did some sensitivity tests.

Now it's very difficult to get the 'same' results across all 120 channels even with no changes if L7 gets tweaked but I tried very hard with the 2SC2999 to get the lowest noise setting for L7.

Here are the results for SSB sensitivity on the high tone setting


2SC1674L

26.515MHz -119.7dBm for 10dB (S+N)/N
27.185MHz -122dBm for 10dB (S+N)/N
27.855MHz -119dBm for 10dB (S+N)/N


2SC2999D
26.515MHz -119.7dBm for 10dB (S+N)/N
27.185MHz -122.5dBm for 10dB (S+N)/N
27.855MHz -119dBm for 10dB (S+N)/N


You can see than the 2SC2999 was 0.5dB better in the middle of the band.

This is a very small improvement and I could maybe get the same result on the 2SC1674L if I tried as hard to find the sweet spot for NF at 27.185MHz.

However, testing it for NF in the radio isn't really fair on the 2SC2999 and if there is an improvement in transistor noise figure then the best way to quantify this is to test it in isolation with a test circuit and use my other test gear. Not sure when I can do this. The reason I think it is worth doing is because it may show that there is more than a 0.5dB improvement in the amplifier itself.

If so then this transistor may support my other mods slightly better.

I should point out that a 0.5dB change in S/N ratio is so tiny it really isn't going to be perceptible to human ears.

There's probably a few people that can spot it in controlled tests when flicking back and forth but I don't think I can do it...

 

[G0HZU]

Here's the analyser plot of 2SC1674L gain vs 2SC2999D gain.

The scale is 1dB per division.

It isn't really worth explaining which trace is which because they are so close. This really isn't a surprise because the gain of this common base amplifier is defined by the ratio of input to output impedances of the amplifier circuit.

So as long as you select a transistor with high Ft (eg >500MHz) then I would expect all transistors to give the same gain within a small fraction of a dB.

I've tried loads of different transistors here and they all give very similar gain.

Altering the biasing to give a higher collector current WILL increase the gain (because it lowers the input impedance) but this has to be done carefully to avoid instability. Choosing a transistor with a higher Hfe will do very little to increase the collector current because the operating point (and hence collector current) is defined (dominated) by the biasing resistors.

I'd expect the gain would only go up 0.1dB if you went from lowish Hfe to high Hfe.

 

[jazzsinger]

I am not an RF engineer

trouble with that statement is if i'm not mistaken the guy your arguing with IS an RF engineer, (with access to top quality test equipment to boot). if i'm not mistaken a mr 250 from another forum that i'm banned from for exposing copious amounts of bullshit in the past.

bullshit often floats right on by me, but picking up on rf prowess is something i fair a bit better on, even without the mythical 2sc2999e mod and dubious quantities of schottky diodes.

Exactly. That's very well put

I'm actually trying to help people like Robb spot the BS but he really wants to keep up with the beliefs and reject any attempt at scientific analysis. His post #95 sounds a bit like someone defending a religion based on numbers of followers. They can't all be wrong can they?

i love irony,


blind faith, now your talking Robb's strong point, if i've learned anything about Robb in the religious political section, he has exactly the same outlook on his faith, keep up with belief and reject any semblence of fact, like jesus and god DON'T exist, have never been recorded as living despite the area they supposedly inhabited having many record keepers, albeit thats a minor drawback when you have faith.

trouble is more often than not faith and belief are seriously misdirected.

oh and yes they can all be wrong, like the thousands of mugs who bought the A99 believing it was the best antenna ever made with 9.9 dbi gain. pmsl. oh how easily the deluded are parted with cash.

 

[Robb]

trouble with that statement is if i'm not mistaken the guy your arguing with IS an RF engineer, (with access to top quality test equipment to boot). if i'm not mistaken a mr 250 from another forum that i'm banned from for exposing copious amounts of bullshit in the past.

bullshit often floats right on by me, but picking up on rf prowess is something i fair a bit better on, even without the mythical 2sc2999e mod and dubious quantities of schottky diodes.




i love irony,


blind faith, now your talking Robb's strong point, if i've learned anything about Robb in the religious political section, he has exactly the same outlook on his faith, keep up with belief and reject any semblence of fact, like jesus and god DON'T exist, have never been recorded as living despite the area they supposedly inhabited having many record keepers, albeit thats a minor drawback when you have faith.

trouble is more often than not faith and belief are seriously misdirected.

oh and yes they can all be wrong, like the thousands of mugs who bought the A99 believing it was the best antenna ever made with 9.9 dbi gain. pmsl. oh how easily the deluded are parted with cash.

Well George, you are asking for me to post on this thread once again. So I will talk to you.

It has already been demonstrated that:

*G0HZU may well be an RF engineer with uber cool test equipment that I don't frankly have to test the same question. I do trust what I can hear for myself.

*That it has also been demonstrated from another poster here that his results were different. HZU 'fought' to rationalize these findings and attempted to both dismiss them and the person that brought them forward.

*HZU unwavering faith in the supreme and absolute quality of each and every 1674 manufactured/used to produce all radios that are made cheaply and without the benefit or real quality control. After all; these are just CB's - and not Ham radios.

*BTW - 'G0HZU' appears to be a Ham call sign that does not appear anywhere that I can find.

*In addition, his faith in his own ability has led him to camp out here on this forum - on this and only thread - for the past week or longer. Up to 12 hours a day or longer as well. So the term 'faith' is often misused and misunderstood.

*While it has been both informative and interesting, it still doesn't make up for the fact that I am not an engineer, nor have I ever claimed to be one. Neither would I want to be - if the truth be told.

George - Irony is a conflict in the mind that may be used as the core element of humor. Or other avenues as well. Having said that, I find it ironic that HZU must camp out here in order to fulfill some equally ironic personal goal. Obsessive would be the right term here.

Faith is often used as a weapon in many parts of the world fought with bullets instead of God's own words. I know that is what you have seen growing up and for many years. But there really is life beyond what you have experienced.

You haven't got on the radio yet? Think I've talked to most of Ireland and the UK several times over the last two weeks. George - even a mobile will work.

 

[jazzsinger]

Well George, you are asking for me to post on this thread once again. So I will talk to you.

i thought you might, lol.

It has already been demonstrated that:
*G0HZU may well be an RF engineer with uber cool test equipment that I don't frankly have to test the same question. I do trust what I can hear for myself.

*That it has also been demonstrated from another poster here that his results were different. HZU 'fought' to rationalize these findings and attempted to both dismiss them and the person that brought them forward.

i think he was trying to point out the original posters inaccuracies. i'm willing to bet many different results are achieved due to the enormous amount of fake transistors around and few people who can identify non fakes,

would you tune your radio with your ear? i doubt it, why trust it over mighty expensive high quality test gear? just because a radio sounds noisier, don't mean its picking up more useable signal, a point you seem to be missing, the same goes for quieting, could be it just ain't tuned properly.

*HZU unwavering faith in the supreme and absolute quality of each and every 1674 manufactured/used to produce all radios that are made cheaply and without the benefit or real quality control. After all; these are just CB's - and not Ham radios.

despite what you may or may not believe about ham radios. many of those are built as cheaply and without the real quality control you talk of, think yaesu ft100, biggest heap of shite i ever had the misfortune of using.

components used in both aren't decided by the radio industry, infact tv, dvd,hifi, computer manufacturers dictate whats new in electronics, radio is a very small and almost minority part of electronics these days.

*BTW - 'G0HZU' appears to be a Ham call sign that does not appear anywhere that I can find.

really ? i found it on many electronics sites and forums, not necessarily related to radio, but i can tell many peoples writing styles/ knowledge no matter where they appear or what username they use. good radio knowledge is a very rare commodity and stands out like a sore thumb. because someone isn't on qrz.com don't mean they ain't clued up.

to make it easy for you Robb, i believe his other passion is motorcycles, something i know fuck all about : http://www.google.co.uk/#sclient=ps....,cf.osb&fp=583243cef56aa3b5&biw=1280&bih=529

a bit later i quote you talking about faith, war etc, it always pays to know who your arguing with, something i learned at a very early age for survival in this city, research goes a long way, that link should help you.

*In addition, his faith in his own ability has led him to camp out here on this forum - on this and only thread - for the past week or longer. Up to 12 hours a day or longer as well. So the term 'faith' is often misused and misunderstood.

having read many of his posts elsewhere, i think his faith in his own ability is well justified, although i admit often he poses the problem without offering a solution, which i find a bit egotistical myself, but nonetheless the guy is very smart when it comes to radios, of that i have absolutely no doubt whatsoever,

as for this and only thread, i believe the uniden receiver designs, or shortcomings of them to be more precise are a bit of a passion for G0HZU, or more accuaretly improving on them, either that or i've totally misread his style, and haven't got the right person who writes very passionately on this very subject elsewhere in the uk, but my self belief, like his faith in his own ability, doubts that very much.

p.s. no i don't know him nor am i a friend of his, but technical ability stands out as surely as bullshit does, and i've witnessed plenty of that from him.

*While it has been both informative and interesting, it still doesn't make up for the fact that I am not an engineer, nor have I ever claimed to be one. Neither would I want to be - if the truth be told.

neither am i, but i certainly wouldn't argue with someone who is.

George - Irony is a conflict in the mind that may be used as the core element of humor. Or other avenues as well. Having said that, I find it ironic that HZU must camp out here in order to fulfill some equally ironic personal goal. Obsessive would be the right term here.

Faith is often used as a weapon in many parts of the world fought with bullets instead of God's own words. I know that is what you have seen growing up and for many years. But there really is life beyond what you have experienced.

obsessive is a word which just about sums up anyone who believes blindy in something they have NEVER witnessed, you know like God, or have you actually met him?

i've seen many things over the years, my life experience is a lot further ranging than the narrowminded bigoted morons within my city, or elsewhere for that matter.

You haven't got on the radio yet? Think I've talked to most of Ireland and the UK several times over the last two weeks. George - even a mobile will work.

too me, many things are more important than radio, working stateside is no big deal, even from our humble mobile setup,it was exciting when i was 15, but at 44 it doesn't quite have the same appeal, done it got the t shirt springs to mind, i have nothing to prove to me or anyone else.


but time is a rare commodity for me these days, and as i say radio is a very low priority, 20 years ago i would never have said that, but i'm older now, with 2 kids and i hope to hell i'm a heck of a lot smarter than when radio consumed so much of my time.

 

[Mudfoot]

Praise the Lord and pass the ammunition.