Cobra 148- Much Higher Performance Receive Setup Than Shottky Diode Receive Mod

[loosecannon]

ok, after reading through this thread, i have to tell you CBphreaker that this is the best thread i've run across in a long time.
so much to discuss!

To start with, on the low distortion AM detector circuit (the coolest part of this to me), i'm not seeing where any test equipment is necessary.
from the description given in the link, it sounds like you just lift a component lead to disable the AGC (not sure which one yet, as i haven't looked at a 148GTL schematic yet to find it)
Then tune the pot on the detector board by ear and using the radio's S-meter.
from the way he describes it, there should be a definite "sweet spot" with that pot and should be attainable by ear.
Please let me know if i am mistaken about this.

I would think that a TL082 could be used here, as the TL072 is probably getting harder to find as time goes on. (just a guess, haven't looked yet). im hoping this is true since i already have a few TL082's around here.
I also plan on using 1N6263 schottky diodes in mine. (yes, they will be matched up)


now, as for the 7.8mhz IF filter, this is the same concept used in Lou Franklin's "channel guard" filters he used to sell.
As a matter of fact, the schematic you posted is the same design as he used to sell.
I've seen it around the net for a while, and it does work well.
The thing is, yours looks different!
six resistors instead of four, and electrolytic on the board, and an LED?
what the heck is the LED for?

I have one of these and have put it in a couple of different radios, and i did like the performance.
CBphreaker, since you are all about the hi fi audio; have you noticed any adverse effects as far as audio bandwidth is concerned by having the narrower skirts in the IF?

another question i have is that since you replaced C47 with the crystal filter, is the radio more narrowbanded with regards to receive gain over say 128 channels?

I ask this because one of the most popular mods for broadbanding the receive in this chassis is to replace C47 with a 3-5pF cap. im sure you can see what im getting at.

again, i see no need for special test equipment here beyond what one uses to align a radio.


When it comes to breadboarding up circuits from schematics, the truth is that either you are up for the challenge or you are not, and aside from selling a kit with silkscreened part designators on the PC board, those that can build a circuit off of a schematic will be interested, and those that can't, won't.
I don't think it's something you can learn how to do from just a youtube video.
you have to jump in, buy some parts, and get to soldering.
then, when the light doesn't come on, find out why.
eventually, with enough trial and error, you learn certain concepts with regard to circuit layout that will help you along the way. you also learn about a product called stripboard that will change you life! LOL

CBphreaker, I see that you also run a FET in your D104!
I use a JFET (MPF102) to get the ultra high input impedance, and don't bother using a second stage. I didn't design the circuit, it's something i found online, i just figured out a way to use the PC board in my silver eagle to accommodate this circuit.
I agree that it really helps the low freq response of the MC320 element.

lastly, i wanted to ask you about your line out circuit, as i built one for my madison(8719).
Mine just taps off the high side of the volume control, and uses a TL082.
I am no circuit designer, and got this circuit from a friend who does not want me to post the schematic. I will say that it is online, but that's all i can say.

so, if i wanted to run your AM detector circuit, can you see any problems from letting it feed the stock volume control like usual, and my line out being tapped off of that?

Im sure there is more i want to ask you, but i'd better stop there. LOL
This kind of stuff is what i live for, so please know that your responses will be actually utilized to build this.
LC

PS- just wanted to add that it sure looks like there are a lot more components on your AM detector board than there are on the schematic.
can you draw a schematic of what you have built, or give some details about what you changed and why?
it also looks like you used a 10K pot instead of a 5K.

 

[cbphreaker]

@loosecannon

As it pertains to the Precision Diode Detector

Not only will you get lower distortion but this circuit removes the forward voltage drop. Which is why you want to match your diodes. You can use shottky, germanium, or silicon. It makes no difference in performance. I've use 1N4148, 1N60P and 1N6263 and have noticed no difference at all. Actually, I like the 1N4148, no technical reason I can think of.

I use TLO82 in all the detectors I build. The secret here is to use an opamp with a fast slew rate. . I can say with authenticity that the TLO82 performs flawlessly, it's cheap and totally available. If I recall the 82 is a bit faster than the 72, I may be wrong though. After building more than several of these, the though escapes me as to why I use and 82 vs 72. Someday, I'll remember.

The 7.8Mhz IF filter is very close to the Channel Guard. Although I have never seen one in person. The extra resistors and cap. I use a 10ohm in series with the B+ and a cap for RF decoupling. The final "extra" resistor is for the LED.

Why the LED? I don't know. I like LED's and I think it looks cool. It makes the unit look "On". I have no technical reason but since I make the PCB it was no matter to add it. Oh and the "6th" resistor is for the LED. Many who meet me find that I am white and nerdy, and I do nerdy things. I do have a pocket protector, I do carry a calculator, (even a slide rule) and I like lights.

Pertaining to C47. I hate that capacitor. I have seen the mods on the internet where, as you have described, claims of increased bandwidth. Malarky. That cap's main function is to shape the bottom of the skirt it really doesn't determine the insertion point bandwidth. Without to much technical jargon. A typical CB radio only has about -18db attenuation at +/-8khz. That is way "not enough". -18db to me means lots of bleedover. Just listen to channel 6 and some of those boys bleed up to channel 11 on my stock 25. The stock 148 isn't as bad but still not that good either. With the filter channel 7 and 5 are usable (still has some Ch 6 artifacts) but the bleedover is way down. Channel 8 and 4 are totally usable and after that there is no more. The filter can not fix bad transmitted audio. I think there are some who talk to their meter, those operators are usually the usual suspects of splatter.

The IF filter will sharpen the bottom of the skirt - not the top. It still has the typical 16 Khz (+/- 8khz) but much more attenuation at the bottom. You will still hear the HIFI stations just without all the noise and bleed over.

You also have to contend with the 455khz filter (in reference to the 148/2000) and it's audio response as well.

I have toyed with idea of using a 4066 to switch between "wide and narrow". The 4066 is really neato as you only need a voltage to run the switch. Which means you don't have to pipe the 7.8Mhz signal all over the place. A simple switch will suffice. One position for the capacitor and the other position for the filter. Perhaps some day.

As it pertains to audio, I exclusively run the crystal filter I have found it to be more than adequate for even HIFI stations. I enjoy it's performance.

All these numbers don't really mean much. I've included screen shot of a log sweep of 20hz - 20khz . Transmitted from my 2000 (configured for Low-Level AM) and listened to on the RX modified 148.

In regards to bandwidth for channels, it makes no difference as there is only one frequency, 7.8Mhz, regardless if the rig is on channel 1 or 40 above 40 - still 7.8mhz. Incidentally, the PLL coil is one of the biggest culprits of usable bandwidth. But there are other areas where broadening techniques can be used and should be performed. The riq looses it performance as the VCO signal gets weaker and weaker, not the 7.8Mhz signal. There are other areas that loose it as well, but this one is a biggie.

In conclusion of the filter, I'll leave you with a rule of thumb. Filters in the high IF generally establish selectivity. Filters downstream closer to the detector remove noise, increasing the s/n ratio. You could use two of the 455khz filters with the same circuit placed where the single 455khz filter was originally. But both high IF and low If filtering carry properties of the the other. The real world performance isn't this cut and dry and describing a filters operation with two points is, most likely, presumptuous.

Line Out - I have used the 2nd amp in the TLO82 as a boost to line level. And Ill tell you the reason why is to not let my precious recovered audio become colored with audio bandwidth robbing capacitors. Even with the "Goldmember" mod it still sucks and I personally can't accept the TA7222. Have you ever taken one out, wired it up to a good speaker and cd player? You don't have to take my word for it but I find the quality of audio REVOLTING. It's cheap sounding, and my ear can't handle class B audio. Unless it's a really well designed Class B audio amp, the zero crossing sound is like fingernails on a chalkboard - it makes my teeth hurt. The TA7222 is another pet peeve of mine regarding the 148/2000. Actually, it's a double peeve. Wait, no it peeves even more than that. As you can tell, I am duly unimpressed with that amplifiers performance. So much so, that I don't use it. The rx audio goes to a much nicer amp and decent speakers, and I have my 2000 configured for Low-Level AM so bye-bye TA7222 - I like to say "Nice to know ya, Yoshinoya" as I wave like a beauty queen on a float passing by at the Rose Bowl Parade. The only difference is I'm not waving a friendly five, but only have one finger up.

So to answer your question about the taping of line out - yes you can, I can't see why that wouldn't work. If you just take and install the precision diode the radio would behave "normally". The only anomaly I found is the ANL. I have always had an intense dislike for ANL as it can destroy the audio from bigstrappers. ANL in a CB is timed for 300-3000 and not large bandwidth audio. I almost never run the ANL and with the precision diode, it's really not needed anymore. One could modify the ANL, but why. It's like using Shottky diodes in the AGC. It make no sense, to me, to speed it up or have it kick in early so I put that in the "why bother" category as well. If you utilize the second amp in the TL082 and grab line-out from there, the ANL will get bypassed but the NB will still be operational. Thats how I run it, essentially, all the time.

Ah, the "magic" mod for the audio opamp. I'll go out on a limb and bet your tech shoved that little guy into running Class "A". It's the only "mod" I can think of that pertains to audio opamps.

Regarding your MPF-102. How do you have it configured? Common Source or Common Drain. I personally use the MPF102 as a triode (Common Source) but that introduces a 180 degree phase shift. Which is okay if you have more pronounced negative peaks in your voice. If you don't, that little amp will invert your positives to negatives - perhaps not so good for AM. That is one reason I use the inverting input of a NE5532. Simply to fix the phase. I have a positive modulating voice so just the MPF102 in Common Source makes my voice very negative. Using the opamp also provides a bit better s/n ratio than just using the single FET.

A final thought on the sweet spot of the precision detector. It is totally obvious where the adjustment should be. You can go though the procedure of shutting off the AGC and finding the most noise, but that's overkill. Turn it on, twist the adjustment, when it sounds good, your done.

,
Thanks for your kind words and responding to my post.


PS The extra stuff is the power supply decoupling, and the "stuff" to use the 2nd amp in the TL082 as my line out. Please follow the schematic that I have linked to. I used the 10k because I didn't have a 5k.

 

[Klondike Mike]

I listened to your audio sample and do notice the improvement on the low end. As noted in your frequency analysis chart, the signal level at 4khz is way too low for my liking and my ears confirm what my eyes are seeing. I'm sure you have already given it some thought on how to improve this end of the audio spectrum and I look forward to your findings.

 

[cbphreaker]

Don't put to much faith in that. It's best to hear it. Also the screen is super squashed so the sides look very close together. Also my transmitter my not be able to reproduce the tones perfectly and the detector is showing its flaws.

Ill haven't really had a chance to go through my TX audio chain for perfection. I really just needed quick bandwidth to play with the detector. There are still questions I have to answer in my own setup.

In reality, I'm never done.

Here's what that sounded like.

PS: If your referring to the audio clip that I posted, that was just some dude I head out there. He actually had the best audio that day. I took lots of recordings, most were to tinny, to loud, or not loud enough.

In perspective the "precision-ness" is also largely a function of the transmitted audio - garbage in garbage out.

 

[Klondike Mike]

... In perspective the "precision-ness" is also largely a function of the transmitted audio - garbage in garbage out.

Duly noted. As we are not aware of the all the parameters of the TX portion of your experiment, opening up the rx/tx audio chain for better frequency response on the Cobra 2000GTL (PC497AB) is well noted already. You may want to implement the following changes to get some better results;

Credit given to "Marc" on CBTricks for the following information;

Cobra 148/2000/Grant XL

TX circuit audio improvements:

C46 Change to 2.2uf mylar/polyester film or non-polarized electrolytic
C101 Change to 1.5uf tantalum 16 to 50 volt
C102 Change to a 910pf ceramic disc
C104 Change to 2.2uf tantalum 16 to 50 volt
C108 Change to 1uf to 2.2 uf tantalum 16 to 50 volt
I found that if you have a very deep voice start with 1uf on C108
and work up if need be.
If you need more highs on tx Change C 105 to .001 to .0001uf mylar/polyester film The smaller you go the more highs you get.

RX circuit audio improvements:

C71 Change to 1uf mylar/polyester film or non-polarized electrolytic
C75 Change to .001 to .0001uf this will give you more highs on rx
optional
C76 Change to 2.2uf mylar/polyester film or non-polarized electrolytic
C182 Change to .001 to .0001 for more highs on rx and tx
optional
C183 Change to 2.2uf mylar/polyester film or non-polarized electrolytic
C187 Change to 2.2uf mylar/polyester film or non-polarized electrolytic
With C75 and C182 you can start with no Change to them, then if you feel you want more highs then change them out

​

 

[cbphreaker]

Actually, I was just playing around and found that configuration was wrong on the 2000.

The mods you posted are for high level radios using the stock preamp and TA7222. I don't use the stock preamp or the TA7222. I'm simply unbalancing the balanced modulator and feeding that with audio. Well there is a bit more to it but not much. There is also some housekeeping that needs tending to. Namely it appears I needed better isolation from the SSB filter.

Here's an audio clip of the band Big Bad Vodoo Daddy. One clip is with the filter in and the other capacitive coupling in.

I have never been able to get this kind of audio resolution with the mods you have listed above. Referring to the audio clips.

Thank You for responding to my post .

Back to the 2000

 

[loosecannon]

CBphreaker, it looks to me like i would lift one side of C64 in tie that to the input of the new detector circuit, and lift the anode of D22 and tie the output to the empty hole.

is that what you did?
please let me know if im overlooking something.
LC

 

[loosecannon]

ok, on to the other stuff.

Thanks for the info and tips on the channel guard circuit.
I am also a big fan of LED's and love to add indicator lights to any function switch i add.
you must operate your radio with the covers off a lot. LOL

as for the FET mod for the D104, i am feeding the gate of the FET directly from the mic element. It is a common drain circuit.
here is the circuit i used:
http://www.qsl.net/hcara/Improving_the_D-104_N9WB_rev2.pdf


line out- my line level converter is already in place in my Madison, and works very well so i don't think i want to change that.
Im thinking that i can just add the detector in series and keep the line out circuit the same.

I will also want to bypass the ANL, and it looks like i can just remove C78 and D23, and put C78 in place of D23.


something you said has me a bit confused:
"Ah, the "magic" mod for the audio opamp. I'll go out on a limb and bet your tech shoved that little guy into running Class "A". It's the only "mod" I can think of that pertains to audio opamps. "

im not sure what you are referring to here.

lastly, you have got to be the very first CB enthusiast i have ever heard of that purposely converted a CB radio for low level modulation.
im sure you know how much that sounds like sacrilege to most of us. LOL

i mean, High level AM is the only thing we have going for us over a ham transceiver!
LC

 

[cbphreaker]

@loosecannon

You don't need C64, D21 or D22. Install the input to the precision detector where the input to C64 was and the output goes to where the output of D22 was. That's it.

Yes, it will work with your already installed line out.

ANL- you don't have to bother with all that, just don't turn it on.

Don't get me wrong, I love high level modulation. I thought it might be fun to see what the 2000 will do in terms of bandwidth and have a cost of no more that 25 dollars. This has been really fun.

I've worked on that Low Level all day and here is a pic of nice increase in bandwidth.

 

[loosecannon]

all i can say is bravo.

our hobby needs more guys like you who love CB radio, but still want to take it in new directions.

the only reason i mentioned the workaround of D23 was because in the madison i will be installing this in, the ANL/NB is one switch.
Im going to change D23 out and put C78 in it's place so that this switch will just turn the NB on and off.

great stuff, can't wait to get to solderin'!
LC

 

[Klondike Mike]

I've worked on that Low Level all day and here is a pic of nice increase in bandwidth.

Bravo! The new bandwidth chart is now definitely in Hi-Fi territory. Now please provide us with the details as to how you got there. A summary of what needs to be done to achieve your same results would be desirable.

 

[cbphreaker]

@Klondike Mike

Thanks for the kind words. The exact problem was the my lack of isolating the ssb filter properly and an incorrect input capacitance to the balanced modulator. The combination of the two was enough the foul my tests.

I will have to investigate but it seem my computer looses bandwidth above 10k. It is pure speculation based on watching the output levels.

I'm not sure what you mean by duplicate the same result. Are you referring to the Precision Diode or Low Level AM?

 

[711]

@cbphreaker
I too appreciate your efforts in both the receive and transmit improvements you've done. I finishing up the detector board , just thought I'd run the second opamp at unity gain as buffer out. I'd like to also try your low level circuit. Thanks again.

 

[cbphreaker]

@711

Cool, I'm excited for you. Good on ya'

I found, on my setup, I needed just a bit of gain. Right now, I have a gain of 5.
So, not to much and it has plenty of volume. Just to give you a reference point.

 

[loosecannon]

well i got mine built, but not installed yet.

that darn daylight savings time got me and i had to go to bed before i could install it in the radio.

Im going to do a couple of quick tests with my signal generator so i will have something to compare it to besides my eyes and ears.

i'll post back probably on Thurs. with the results.

Thanks again to CBphreaker for bringing this mod to the forum.

i vote that this thread be made a sticky!
LC