Grant LT receive circuit

[poboy309]

Years ago I got a grant lt off a driver at the local TA. Traded him a brand new cobra export for it and some cash. I put in the schottky diode mod for a better receive and this thing works great!
My question is, Do AM and SSB use the same noise canceling circuits? I can dig through the diagrams but I figured I could start up a conversation and pick someone's brain that knows more than me.
I'm asking because the noise floor on am is low but when I switch to SSB, I have to lower rf gain quite a bit.

-- Tue, Oct 07 2014, 17:17 --

It could be just the tone of the noise. I also like to use rf gain instead of squelch.

[TheCBDoctor]

Years ago I got a grant lt off a driver at the local TA. Traded him a brand new cobra export for it and some cash. I put in the schottky diode mod for a better receive and this thing works great!
My question is, Do AM and SSB use the same noise canceling circuits? I can dig through the diagrams but I figured I could start up a conversation and pick someone's brain that knows more than me.
I'm asking because the noise floor on am is low but when I switch to SSB, I have to lower rf gain quite a bit.

-- Tue, Oct 07 2014, 17:17 --

It could be just the tone of the noise. I also like to use rf gain instead of squelch.

Good morning poboy309

As far as I can remember AM and SSB do use the same noise cancelling circuitry. You may be hearing more noise on SSB because the AGC voltage on SSB is increasing the gain on the receiver. Normally, SSB has half the noise because it is only half the band width. If you were to put back the factory components you will notice SSB will be quieter.

With the Schottky diode the 1st RF transistor may not have the proper AGC voltage (Automatic Gain Control) on SSB. By adjusting the RF gain you are manually adjusting the RF gain and overriding the AVG. Schottky diodes, unlike all other diodes, have gain.

Respectfully,

[poboy309]

Thanks cbdoctor! I've been thinking about restoring back to stock anyway. After reading some more on the schottky mod, there are arguments that it doesn't really help. As I've read from people with knowledge far surpassing mine, it does change receive but if you adjust the radio to spec after the mod, it doesn't make a difference.
I think I'll stick to stock radios and skip the gimmicks from now on.

[TheCBDoctor]

Thanks cbdoctor! I've been thinking about restoring back to stock anyway. After reading some more on the schottky mod, there are arguments that it doesn't really help. As I've read from people with knowledge far surpassing mine, it does change receive but if you adjust the radio to spec after the mod, it doesn't make a difference.
I think I'll stick to stock radios and skip the gimmicks from now on.

Good morning poboy309,

The Schottky mod is just another mod to separate an operator from his or her money. If the engineers thought it would improve the radio's performance they would have used the Schottly diode. When you increase the over-all gain of the receiver you also increase the noise level as well.

You are correct when you posted that you can eliminate the noise by making some adjustments, but you end up where you started anyway. Video log amplifiers, used in radar systems, use Shottly diodes, but the diodes also cost about $600 each. Since Schottly diodes have gain they are used to increase receiver sensitivity, but diodes are non-linear and only the "Knee" of the curve between zero volts to .6 volts (Point 6) is used. This is the only linear part of the diode's operation. The increase in noise is the portion outside this operating range used to increase a signal.

Good luck with your mint Cobra 2000 GTL. You probably have one of the last ones made. I am jealous If you ever need parts for your Cobra 2000 GTL many of them can be found in a Cobra 148 GTL made in Taiwan or the Philippines. The Cobra 2000 GTL is the same radio as the Cobra 148 GTL except the 148 GTL uses a modifiable MB-8719 and an 11.3258 MHz mixing crystal, which allows you to center USB on the Voice-Lock if it is unlocked.

L-59 must be bottomed out to get close to centering the Voice-Lock in the USB mode. Even then it will never center with L-59 bottomed out using the 11.3250 MHz mixing crystal found in the 2000 GTL. Collect any scrap Cobra 148 GTLs made in Taiwan or the Philippines while they are still out there. It never hurts to have a scrap Cobra 2000 GTL for parts that are not found in the 148 GTL.

Click on a photo for more info on the Cobra 2000 GTL or Cobra 142 GTL using this link to my educational Website.

[Please login or register to view this link]

Respectfully as always

-- Tuesday, 21 October 2014, 9:52 AM --

I'm sorry I just realized you were talking about the Grant LT, which is similar to the Cobra 148 GTL. The Grant LT is closer to the Cobra 142 GTL.

It's early I should have paid more attention. The info I posted about the Schottly diode does still apply. The Grant LT made in Malaysia is a different animal altogether. I have never been able to open the Clarifier and have it track on the newer version of the Grant LT between Tx and Rx.

This link will take you to my page with a list of different radios I have worked on over the past 6 years. It needs updating, but here it is anyway.

[Please login or register to view this link]

Respectfully and not fully awake

[poboy309]

Thanks cbdoctor! I've been thinking about restoring back to stock anyway. After reading some more on the schottky mod, there are arguments that it doesn't really help. As I've read from people with knowledge far surpassing mine, it does change receive but if you adjust the radio to spec after the mod, it doesn't make a difference.
I think I'll stick to stock radios and skip the gimmicks from now on.

Good morning poboy309,

The Schottky mod is just another mod to separate an operator from his or her money. If the engineers thought it would improve the radio's performance they would have used the Schottly diode. When you increase the over-all gain of the receiver you also increase the noise level as well.

You are correct when you posted that you can eliminate the noise by making some adjustments, but you end up where you started anyway. Video log amplifiers, used in radar systems, use Shottly diodes, but the diodes also cost about $600 each. Since Schottly diodes have gain they are used to increase receiver sensitivity, but diodes are non-linear and only the "Knee" of the curve between zero volts to .6 volts (Point 6) is used. This is the only linear part of the diode's operation. The increase in noise is the portion outside this operating range used to increase a signal.

Good luck with your mint Cobra 2000 GTL. You probably have one of the last ones made. I am jealous If you ever need parts for your Cobra 2000 GTL many of them can be found in a Cobra 148 GTL made in Taiwan or the Philippines. The Cobra 2000 GTL is the same radio as the Cobra 148 GTL except the 148 GTL uses a modifiable MB-8719 and an 11.3258 MHz mixing crystal, which allows you to center USB on the Voice-Lock if it is unlocked.

L-59 must be bottomed out to get close to centering the Voice-Lock in the USB mode. Even then it will never center with L-59 bottomed out using the 11.3250 MHz mixing crystal found in the 2000 GTL. Collect any scrap Cobra 148 GTLs made in Taiwan or the Philippines while they are still out there. It never hurts to have a scrap Cobra 2000 GTL for parts that are not found in the 148 GTL.

Click on a photo for more info on the Cobra 2000 GTL or Cobra 142 GTL using this link to my educational Website.

[Please login or register to view this link]

Respectfully as always

-- Tuesday, 21 October 2014, 9:52 AM --

I'm sorry I just realized you were talking about the Grant LT, which is similar to the Cobra 148 GTL. The Grant LT is closer to the Cobra 142 GTL.

It's early I should have paid more attention. The info I posted about the Schottly diode does still apply. The Grant LT made in Malaysia is a different animal altogether. I have never been able to open the Clarifier and have it track on the newer version of the Grant LT between Tx and Rx.

This link will take you to my page with a list of different radios I have worked on over the past 6 years. It needs updating, but here it is anyway.

[Please login or register to view this link]

Respectfully and not fully awake

I do appreciate all the information and the help! I don't want to do any more mods to my classic grant. The one I have is a philly version. The only mod I think I'll keep is the frequency counter. After I get the rest back to stock (only other mod is the schottky mod), I plan to have it aligned and I'll leave it alone.
I just bought a president lincoln (hr2510) so I'll be happy with that as far as channels go. Much of the mods done to my radios were done myself when I first got into the hobby. Trying to squeeze out every watt I could get and trying out a bunch of mods. My approach has changed towards stability and clean signals. My preference has also changed from AM to SSB. I'm not knocking the mods but my goals have changed. I'll check out your page as I've learned a lot from reading through mods and others experiences. I wish I was as far along in my radio knowledge as many of you but it's always nice to have knowledgeable people such as yourself to help.

[lydianmode]

Greetings!!

I, also, was disappointed with the performance with a schottky diode swap. It didn't seem to make that much difference. Also, it seems that diodes in general have a not so good dynamic range, produce distorted audio with any incoming signal that is more than 120% in modulation, have a tendency to be noisy, and they have that ugly "Knee" as CBDoctor. pointed out.

While investigating alternative demodulation techniques, I happened to learn of the "Super Diode". It's an active setup up using a hi-speed opamp and 3 diodes plus a few supporting components. Two of the three diodes are used as feedback and the final one is used to to the actual demodulation. This setup offers a superior A.M. demod section all while over coming the issues with conventional diode detection.

I built and installed this circuit in a beat up Cobra 148 (I'm just getting back into radio after some time off so this is all I got), but it should work with practically any CB radio. The parts are cheap and it does what schottky diodes only dream of. When I initially got the unit installed I was instantly astounded at the lowering of the the noise on AM. Upon further testing, I found that it's ability to receive weak signals was greatly enhanced. Using a stock Cobra 25 for comparisons, it was totally apparent there are conversations happening which are completely readable on the 148 where the same signal on the 25 was not even perceptible. The increased dynamic range was further enhanced with the usual cap mod's in the RX area and this beater 148 now has a beautiful tone and clarity. According to the circuit description, this setup has the capability of cleanly demodulating signals of up to 200%.

I have no way to test the 200% claim but the Superbowl big-strappers sure sound really good - way better than before the mod. The only change I did to the circuit was use a dual op amp and configure the 2nd as an adjustable gain audio buffer using a harvested potentiometer solely so I could mess with it.

I tried to post the link but I am unable to, however, if one searches google for low distortion detector the result from amwindow (2nd one down) with the document named lowdisdet.pdf is the schematic and description or PM me and I can give you more information if your interested in building this.



73's

[TheCBDoctor]

Good morning lydianmode,

I love op-amps. Your post was very informative and right on the money. I am looking at the schematic right now.

[Please login or register to view this link]

As a sponsor I can post the link. I recommend anyone that is interested in increasing his or her radio's receive performance to try this circuit instead of using a Schottky Diode method.

Thanks for the post

Respectfully as always

[lydianmode]

Hey, thanks for posting the link!!!

I was just playing with the circuit and I realized the output from the opamp is large enough to drive a HiFi audio amp, such as a home stereo or make really nice watergates on a computer. Its amazing how CB sounds when not mangled by all the crappy/too small caps and other circuits that are unacceptable for good sounding RX audio.

@CBDoctor - I'm making some watergates and I am curious to know if you would be interested in hearing one or two and in exchange for your opinion on the circuit performance?

73's

[a365]

I will be interested in listening to those watergates as well

Also I been trying to pm you for details about the circuit but can not do ?

Regards

[Bombero]

I will be interested in listening to those watergates as well
Also I been trying to pm you for details about the circuit but can not do ?
Regards

Hey a365...

As soon as you have a few legit posts in the forums you'll be able to post pics, add an avatar and PM other members.

I've noticed that you have been a member for over a year...might want to start off by posting an intro here: [Please login or register to view this link]

Bombero