Back swing issue

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244terminatorVA
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Back swing issue

Postby 244terminatorVA » Wed Jan 27, 2016 8:06 pm

I have a Cobra 2000gtl that is direct injected and on average I am seeing back swing. I have heard of transistor that you can use to amplify your audio into you direct injection port. If nobody has ever heard of this mod is there anything I can do to prevent the back swing? I have tried an mic preamp and it does not cure the issue. Thank for any help.
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Re: Back swing issue

Postby Goldfinger » Wed Jan 27, 2016 10:49 pm

Are you injecting at C174?
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Re: Back swing issue

Postby 31 » Thu Jan 28, 2016 6:01 am

Even though you're direct injecting, you're still going through the modulator and have to deal with setting the deadkey via VR10. Might not be the case but also an idea.
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244terminatorVA
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Re: Back swing issue

Postby 244terminatorVA » Thu Jan 28, 2016 5:27 pm

Yes this is where I injected at. Sowhat your saying is I need to tune for forward drive?
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Re: Back swing issue

Postby Goldfinger » Thu Jan 28, 2016 6:06 pm

One problem with injecting at C174 is it takes a few watts of audio to drive the modulator sufficiently. Line level from rack gear or a sound card isn't enough. Because of this you either have to use a small audio amp between the line level source and the injection point or inject before the 2000's on-board audio chip. I have done the latter a few times and it actually works pretty good.
To inject before the audio chip, inject through a 10UF cap to the junction of the negative end of C99 and the collector of TR21, with the + end of the injection cap towards TR21. If you leave C99 intact, the radio can still be used with a regular CB mic as well as with the injection point.
Make sure to make C46 higher in value as well as C174 so the wide audio will be passed.
C46-2.2uF or larger mylar, polyester of non-polarized electrolytic.
C174-10uF polarized electrolytic, observe polarity.


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Re: Back swing issue

Postby 2510gutbuster » Fri Jan 29, 2016 10:22 am

Thanks for the info 31 and Goldfinger, I am gonna try to work on Terms radio this weekend, When you talking about a small audio amp between the sound card, Are you taking about placing a mic-preamp after the soundcard and before the injection. Just trying to figure out the best situation to suit his needs, and if I inject his radio at c99 would that change the quality of his audio by going further back in the mic chain? Never to old to learn lol. Thanks again guys!
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Re: Back swing issue

Postby 31 » Fri Jan 29, 2016 11:04 am

A few ways to look at this, the stock mic plugs in through the front and needs to be amplified to bring it up to line level needed to drive the darlington. I've seen rack gear pump out big signals and some small. I can't remember off hand what the stock signal is crossing c174 but that's the same drive you're trying to replicate from a sound card or external rack. I seen you have a scope, scope the signal from the 2000 preamp and see if the rack is putting the same out. Most darlingtons like a minimum of around 1 volt peak to peak just as a reference going forward. High fidelity is still possible going through the stock preamp with those cap changes and still sound good. Look at the ham guys, their rigs are built for communication quality audio, they connect gear like W2IHY and achieve pretty good sounding audio just on the fringes of the communication audio bandwidth. By swapping caps, you're widening the bandwidth beyond the communication quality audio moving towards broadcast quality. Also the stock preamp is one less piece to have to add to the external stuff already added. The old adage of less is best still applies here as well.more pieces more chances for RFI, ground loop or any other pesky bugs getting into your koolaid.
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Re: Back swing issue

Postby 2510gutbuster » Fri Jan 29, 2016 11:25 am

Thanks 31, that I have surely learned so far, we done away with ALOT of external components and are down to just a presonus interface and it has made a HUGE difference, now just need to ramp injected audio up and get the tunning right, it's been a journey to say the least lol. I have noticed the more of the stock mic chain that we use in injecting the audio, the better the results have been. I am about to use my 2000 as a lab rat and scope the signal and try different injection point and run it through the mic jack its self and see what is working the best. Thanks again! And to any of the admins would it be okay to mention WWPDX in my videos to try to get more members here?
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Re: Back swing issue

Postby Goldfinger » Fri Jan 29, 2016 11:34 am

31: the signal across C174 is speaker level as it is what is coming out of the 5.8 watt audio chip. It's several volts peak to peak. Line level isn't enough to modulate fully.
TR21 is just a transmit switch. Injecting here drives the audio chip directly. All of the mic preamp chain is bypassed. The audio quality is only slightly reduced as compared to driving the modulator directly but it sounds pretty good!
244terminatorVA: The backwards swing could be from under-modulation. Once you get enough drive to the Darlington pair, it may start modulating forward.

Yes 2510, it would be OK to mention the forum.
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Re: Back swing issue

Postby 2510gutbuster » Fri Jan 29, 2016 12:32 pm

Thanks Goldfinger!
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Re: Back swing issue

Postby 31 » Fri Jan 29, 2016 1:17 pm

GF: I'm tracking what you are saying. Early on in my career in the Navy I would catch myself trying to reverse engineer circuits to be able to troubleshoot more effectively. That was a mistake, I'd find myself deep in the tech manuals and pages and pages of schematics only to find myself more confused if anything else. I started to step back and look at the bigger picture and that outlook immediately changed my efficiency for the better. Instead of individual components looked at it as blocks. Anything that we use for RF is basically a superheterodyne transmitter/receiver, whether it be radar, radio or anything in between. Driving the darlington could be the issue here, the demographics are almost the same so I will use that for conversation. The Ranger darlington is being driven by a small signal transistor (945) being driven by the JRC4558 opamp. I can't recall the gain of the 4558 off hand but is sufficient enough to drive darlington. If memory serves me right, signal coming into the 945 is small to the order of around 1 volt pk-pk. With the exception of some of the single package TIP darlingtons with a gain of 1000, the 2000 darlington and the Ranger darlington is almost the same. Why would one pair need more then the other? Less gain on the 2000? This is on my list to experiment with my Ranger board in a 148/2000.
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Re: Back swing issue

Postby Goldfinger » Fri Jan 29, 2016 2:13 pm

31: The Galaxy/Ranger has a 3 transistor Darlington trio. The Cobra modulator is a 2 transistor Darlington pair. The gain is much higher on the Galaxy version and is why it needs so much less signal to fully modulate. One problem that arises in any Darlington stage is distortion. The more transistors that are coupled together the higher the distortion. For voice work it's not that much of a problem because a small amount of distortion can be tolerated. I did both a Galaxy and a 2000 for Prime. He said that with the same pro-tools computer driving , both radio's sound great but the 2000 is slightly cleaner sounding to him. I told him it's most likely because of the extra transistor in the Darlington array adding additional distortion.

Terminator & Gutbuster, check out how my neighbor RPM sounds with a 2000 direct injected at the C99/TR21 junction:

429 and RPM.mp3
(1.82 MiB) Downloaded 97 times


429 (the guy with a little less bass) is on a hi-fi Galaxy Turbo with a widened D-104. RPM is using an SM58/mic pre-amp/Behringer compressor-limiter/Behringer 31 band EQ.
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Re: Back swing issue

Postby 2510gutbuster » Fri Jan 29, 2016 5:09 pm

That is some beautiful audio his has, perfect tone and clear as glass. NICE :D
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Re: Back swing issue

Postby 31 » Fri Jan 29, 2016 5:59 pm

That's understandable , I can see where the extra stage could add distortion. Would a hotter opamp be a better replacement then the 945? Anyhow this has side tracked a little from the OP. Nice gate by the way, never hear much injected radios in action.
Goldfinger wrote:31: The Galaxy/Ranger has a 3 transistor Darlington trio. The Cobra modulator is a 2 transistor Darlington pair. The gain is much higher on the Galaxy version and is why it needs so much less signal to fully modulate. One problem that arises in any Darlington stage is distortion. The more transistors that are coupled together the higher the distortion. For voice work it's not that much of a problem because a small amount of distortion can be tolerated. I did both a Galaxy and a 2000 for Prime. He said that with the same pro-tools computer driving , both radio's sound great but the 2000 is slightly cleaner sounding to him. I told him it's most likely because of the extra transistor in the Darlington array adding additional distortion.

Terminator & Gutbuster, check out how my neighbor RPM sounds with a 2000 direct injected at the C99/TR21 junction:

429 and RPM.mp3


429 (the guy with a little less bass) is on a hi-fi Galaxy Turbo with a widened D-104. RPM is using an SM58/mic pre-amp/Behringer compressor-limiter/Behringer 31 band EQ.
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Re: Back swing issue

Postby 244terminatorVA » Sat Jan 30, 2016 10:56 pm

Thanks Goldfinger and 31 we got it producing better audio and swing. I can talk to my locals that are 20+ miles away with just the radio. Can't wait till dx starts rolling in to try it out. Thanks again
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Re: Back swing issue

Postby Goldfinger » Sun Jan 31, 2016 2:32 pm

Great terminator, glad to hear.
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Re: Back swing issue

Postby 479tx » Thu Aug 11, 2016 12:18 am

RPM sounds pretty good in that gate!

I talked to a guy in DX land the other day that sounded really good and even with just a widened Cobra 29 receiver could tell he was running wider than stock. Turns out he was running a Galaxy radio, direct injected with an Echomax 2000. It wasn't super bassy but he sounded very full and mid to hifi.

I happen to have a Uniden Grant and an Echomax 2000 sitting here. I'm going to try injecting the Grant at C99 with the Echomax 2000 driving it next weekend. I'll post my results.
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Re: Back swing issue

Postby 479tx » Thu Aug 11, 2016 11:51 am

GF,

Am I thinking about this right?

I want to DI my Grant just before the audio chip and drive it with my Echomax 2000 via the standard microphone plug on the side. Would the following work?

- Remove mic audio wire from stock mic jack on radio and replace it with a wire leading to a 10uf a 10UF cap connecting to the junction of the negative end of C99 and the collector of TR21, with the + end of the injection cap towards TR21.
- Negative/Ground pin of stock mic jack on radio is already connected to the proper ground location.
- C46-2.2uF or larger mylar, polyester of non-polarized electrolytic.
- C174-10uF polarized electrolytic, observe polarity.
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Re: Back swing issue

Postby Goldfinger » Fri Aug 12, 2016 10:38 pm

Yes, that sounds right. As long as the mic has enough output that configuration should be fine.
Lemme know how it works.
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Re: Back swing issue

Postby 479tx » Tue Aug 23, 2016 2:48 pm

The Echomax 2000 ended up having enough gain when direct injected just before the audio chip on my Grant, it just didn't sound so great. I wonder now if the guy in DX land that told me that was the mic he was running was kidding now ...

Either way, I have tried both rack gear and digital audio from a USB audio interface and this is a solid method of injecting in a Cobra 2k/148/Grant chassis. Very easy to drive and great wattage... With just a single piece of rack gear (DBX 286s) and a condensor mic I was getting many unsolicited praises for my audio and questions about what I was running. Sounds even better with audio from the USB audio interface...
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Re: Back swing issue

Postby Goldfinger » Tue Aug 23, 2016 3:40 pm

Great!
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Re: Back swing issue

Postby redlght » Wed Nov 16, 2016 12:53 pm

31 wrote:Even though you're direct injecting, you're still going through the modulator and have to deal with setting the deadkey via VR10. Might not be the case but also an idea.


Seems like a nice schematic ...any chance of getting a copy of that one? Thanks
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Re: Back swing issue

Postby 31 » Mon Nov 21, 2016 6:01 am

redlght wrote:
31 wrote:Even though you're direct injecting, you're still going through the modulator and have to deal with setting the deadkey via VR10. Might not be the case but also an idea.


Seems like a nice schematic ...any chance of getting a copy of that one? Thanks

I'm confused? The schematic is in the post. Are we talking about a different radio/schematic?if we're talking about the full schematic, it's over on cbtricks.
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Re: Back swing issue

Postby redlght » Mon Nov 21, 2016 10:16 pm

The one in the post with notes and known faults
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Re: Back swing issue

Postby Goldfinger » Tue Nov 22, 2016 9:35 am

I think he wants the URL to that schematic. The one on the main 2000GTL page is a scanned document, not a redraw.
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Re: Back swing issue

Postby redlght » Tue Nov 22, 2016 10:24 am

Yes the redraw if it is still around
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Re: Back swing issue

Postby a365 » Mon Dec 05, 2016 4:17 pm

This is the link for the redraw

http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148gtl_sch_redraw.pdf

Hope this is what you are looking for
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Re: Back swing issue

Postby redlght » Tue Dec 06, 2016 10:34 pm

Thanks A365 ~Cheers~

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