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Cobra 148 Digital VFO - Frequency Mod

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lydianmode
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Cobra 148 Digital VFO - Frequency Mod

Post by lydianmode » Monday, 19 October 2015, 19:37 PM

Greetings All

This is work in progress (as always) but I can't stand 40 channel switches, switch mods and band selector switches with 40 channels. Finding crystals for older radios can be a pain and the cost is somewhat prohibitive sometimes only garnering a few channels per crystal. In the Cobra 25/29 options are limited to 40 up/down, generally. The 148 needs three switches or a crystal (or two).

This mod is my version of a frequency mod - a digital VFO. Built from a Audrio and AD951 DDS (a programmable digital RF synthesizer) It feels nice to roll through frequencies from top to bottom literately as far as the radio can go. Even as an added benefit, this mod circumvents the entire PLL, VCO and Loop Oscillator (in the 148) bypassing bandwidth robbing transformers, especially L20. Memory presets would be convenient and an RIT control would be just spice. So I guess adding those option will be next after finding a reasonable housing solution. Any suggestions would be greatly appreciated for sure.

Although I don't have one, a Cobra 25/29 would easily do the same thing with no restriction on "bands" and a quick mod in the software . The limiting factor will be the bandwidth of the rest of the circuits. Actually almost any CB radio can utilize this type of VFO.

This makes my radio experience more fun. If you have a beater radio lying around this make a really fun project and I would be happy to help you if you decided to build this. I'll post the code when it's done but you want to "beta" run it Ill send it to you just keep in mind it's not fully complete but it is fully functional as a VFO.

The hardware required is an Arduino Microprocessor, and AD9851 for radios with a high IF and an AD9850 for lower IF radios. These are all available on eBay for dirt cheap even buying them in the USA.

As the code for the menu's / ssb ofsets/ and other function matures, I intend to make it available for free download to whomever may want to use it.

Link to Video - Sorry for the glare in some of the shot.
http://twds.site90.net/Cobra148_DDS_VFO.html" onclick="window.open(this.href);return false;


73's and Happy Modding!!!!
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Re: Cobra 148 Digital VFO - Frequency Mod

Post by silverstreak » Monday, 19 October 2015, 23:49 PM

This is so cool.I would love to do this to my Madison and Cobra 142GTL.How hard is this to do?And say like a 148 whats the frequency range?Any chance of you making like a kit so people could buy it then just hook it up with instructions?


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Re: Cobra 148 Digital VFO - Frequency Mod

Post by lydianmode » Tuesday, 20 October 2015, 8:04 AM

Hello Silverstreak!!

No, sorry, I don't have kits or anything. I just post stuff that I think will be beneficial to others.

Let me answer you question (or try)
"How far does it go?" I can't answer that question in total because I need to know what your definition of "work" is. As an example, I could hear people yacking just below 25 mhz on the 148 but I got less than 1 watt of RF output. I guess that's working but I think the reality is for a 148 2.25mhz or around. I haven't had a chance to any broad banding work on the radio yet so ..I guess we will see... Although I will say, and this is my theory, that because this bypasses so many circuits, simple broad banding tricks will have a greater effect just because of the reduction of transformers.

To hook it up- Load model specific software on to the microcomputer, shut off the PLL, VCO and Loop, the Lock Detector forced high. The VFO signal is then injected at the point where the "old" signal was. That's about it.

I was wondering how to box this up and make it look nice? Have any idea's?

73's and Happy Modding!!!



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Re: Cobra 148 Digital VFO - Frequency Mod

Post by MDYoungblood » Tuesday, 20 October 2015, 9:21 AM

This is very interesting, need to post on how you control the VFO since the original channel selector won't work, up/down momentary switches would be my guess. I have played with a couple of Arduino (peg board) kits, they are fun to learn on, you can usually find the book with loads of plans in the public library.

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Re: Cobra 148 Digital VFO - Frequency Mod

Post by lydianmode » Tuesday, 20 October 2015, 10:11 AM

Greetings

The VFO is controlled by a rotary encoder which also has a push button switch to change the step.

73s and Happy Modding




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Re: Cobra 148 Digital VFO - Frequency Mod

Post by MDYoungblood » Tuesday, 20 October 2015, 10:56 AM

I would like to see some pics of that just to see how I could incorporate it into the radio's case and a close up of the Arduino board if you wouldn't mine. I'm also looking at the D104 mod you posted, it is interesting as well.

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Re: Cobra 148 Digital VFO - Frequency Mod

Post by lydianmode » Tuesday, 20 October 2015, 14:35 PM

Here are some dimensions of the parts http://twds.site90.net/VFO_Dim.html" onclick="window.open(this.href);return false;

The LCD doesn't fit in the 148 face area or at least that I can see. There is however the option of using a smaller LCD display. Unfortunately not every LCD is compatible but I did find a few that would work which were considerably smaller and I'm sure would fit properly. They were a bit more expensive as I recall.

I putting all that stuff in a 2000 which has ample room to spare. The 148 was being used as a test unit for the Precision Diode Detector, crystal IF filter and VFO so I guess mainly receive and freq mods.

73's and Happy Modding



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Re: Cobra 148 Digital VFO - Frequency Mod

Post by MDYoungblood » Tuesday, 20 October 2015, 14:52 PM

I was actually think of using it and installing it all in a gutted DakX chassis which still has the PS and meters in it, I can pick it up cheap, it has plenty of room and can install the USB port, etc.. Will make for a nice winter project.

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Re: Cobra 148 Digital VFO - Frequency Mod

Post by lydianmode » Tuesday, 20 October 2015, 14:57 PM

Sounds cool! I have a few more of the "whole" setup on it way. I just ordered this morning. One is going in my Viking. I can't wait to dump that drifting 1950's VFO.

I'm not familiar with the electronics of the DAK. I've seen one in person and pics but thats about it. What's it's first I.F.?



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Re: Cobra 148 Digital VFO - Frequency Mod

Post by ElTimple » Saturday, 15 April 2017, 14:58 PM

Hi there

First post and unabe to see the video but i am also working on fitting a dds , a 6850 to a cybernet rig. a Major 80 channel with ssb etc.. I have built a few DDS modules and currently i have the DDS squirting ( 27 + if 10.695 around 38 meg )into the first mixer and i can recieve well. ..The dds also has a programed transmit input so that a true tx freq can be generated.. I have not plumbed that in yet .

I removed the vco module to stop it generating signals , i am not sure if that was a wise thing to do as i am not sure if the lock on the pll02 may stop transmit. the first mixer normally recieved 27+ if and i got the impression that one injection into the mixer might be good for both tx and rx. But no sign of tx , so will experiment with adding 27 on tx after the mixer,,

Can anyone give me some help on getting this going...

I have an arduino nano running a dss ad9850 and a small oled screen...the oled screen fits into the space the channel LED



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Re: Cobra 148 Digital VFO - Frequency Mod

Post by ElTimple » Sunday, 16 April 2017, 13:43 PM

lydianmode..............did you ever get your version going?



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Re: Cobra 148 Digital VFO - Frequency Mod

Post by ElTimple » Sunday, 16 April 2017, 14:37 PM

ok found it .................

..after reading up on the PLL02A we see that pin 6 is the pll lock pin......if its high = lock detected , if its low = no lock....looking at diagram it appears that ........pin 6 turns on Q1 and then Q7 in the rf pre amp...( I think this is the reason why tx was not working and i wondered why) will see if thats it tomorrow , fingers crossed

So with vco pulled the pll is flopping about with pin 6 low...that inhibits operation of tx.......soon change that i think!!



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Re: Cobra 148 Digital VFO - Frequency Mod

Post by ElTimple » Monday, 17 April 2017, 16:23 PM

Very quiet in here...yes that did it




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