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MIDLAND CB MODEL 79-265 MOBILE EXTRA CHANNELS/CLARIFER MODS
- Thread starter Retro CB Guy
- Start date
Hello Mr. Retro CB Guy,
This may be something worth looking at - may be of some help to you may be not
https://youtu.be/qqhpb2w2RKg
This may be something worth looking at - may be of some help to you may be not
https://youtu.be/qqhpb2w2RKg
Last edited:
Dear Mr. Retro CB guy
I think the point in the video is separating the 10.240 reference oscillator and injecting 10.695mhz into the TX oscillator which makes the radio run on 1 VCO frequency not 2 but I'm not an expert. I just happened across your question and was "following" this persons progress. Thought I would share it with you - this youtube poster may be BSing..watch with caution. Therefore I am unable to to answer your question of "what is this person getting at?".
Wish I could be of more help..
Best Regards
I think the point in the video is separating the 10.240 reference oscillator and injecting 10.695mhz into the TX oscillator which makes the radio run on 1 VCO frequency not 2 but I'm not an expert. I just happened across your question and was "following" this persons progress. Thought I would share it with you - this youtube poster may be BSing..watch with caution. Therefore I am unable to to answer your question of "what is this person getting at?".
Wish I could be of more help..
Best Regards
Those videos by T T are of a Cobra 29, which is an AM radio, not an SSB like the Original Post Midland 79-265. He is "reinventing the wheel", where a standard oscillator injection circuit like the Expo "A" kit is used to over ride the 5.12 MHz tripled to 15.36 MHz to offset the VCO to go up band or down band.
This would not work with the Midland SSB as the 10.240 crystal is offset for the Upper and Lower Sidebands, and the VCO is offset accordingly, to match the offset used when the TX is generated by the balanced mixer.
LC7131 reads the VCO frequency direct, so there is no 5.12/5.0MHz mixing that can be done to fool the PLL.
The TX/RX line, Pin 20, can be isolated and grounded, this would give an upward shift of 455 KHz, which would be from 27.420 - 27.860 MHz, so you would have to open the Clarifier to shift down at least 5 KHz to get back to the 27.415 MHz band.
This would not work with the Midland SSB as the 10.240 crystal is offset for the Upper and Lower Sidebands, and the VCO is offset accordingly, to match the offset used when the TX is generated by the balanced mixer.
LC7131 reads the VCO frequency direct, so there is no 5.12/5.0MHz mixing that can be done to fool the PLL.
The TX/RX line, Pin 20, can be isolated and grounded, this would give an upward shift of 455 KHz, which would be from 27.420 - 27.860 MHz, so you would have to open the Clarifier to shift down at least 5 KHz to get back to the 27.415 MHz band.
Okay thanks for the clarification. I guess I posted this in the wrong thread. My apologies.
In all honesty I had to look up what an Expo A kit was and I did notice there are two crystals with 14.91 and 15.81 megahertz not a single 5 megahertz. So my question, if allowed to ask, is with the Expo kit can I use a frequency counter? I have a stock Cobra 29 LTD Classic which has no modifications.
Thank you again for your insight.
In all honesty I had to look up what an Expo A kit was and I did notice there are two crystals with 14.91 and 15.81 megahertz not a single 5 megahertz. So my question, if allowed to ask, is with the Expo kit can I use a frequency counter? I have a stock Cobra 29 LTD Classic which has no modifications.
Thank you again for your insight.
The Expo 100 "A" kit works by "drowning out" the 15.36 MHz output from a slug-tuned coil that takes that 5.12 MHz square wave from the PLL chip and selects the third harmonic of 15.36. That now feeds into pin 4 of a TA7310 mixer chip that is part of the Cobra 29 PLL circuit. The stock 15.36 Mhz signal is still there, but the one coming out of the Expo kit has a higher level. The radio's PLL locks onto the stronger one when the Expo kit is set for upper or lower channels. Turn off the Expo kit and it now locks to the radio's internal 15.36 signal as before.
The signal level from the Expo kit is higher than the 'stock' 15.36 MHz from the radio's original circuit. Feeding 14.91 MHz into this pin causes the PLL to run 45 channels lower than stock. Feeding the 15.81 MHz moves the PLL's coverage 45 channels above.
The guy in the video is moving upstream in the circuit, to the frequency that feeds into the 15 MHz coil.
Substituting 5 MHz for the 5.12 that originally comes out of the PLL chip will move the radio down 36 channels.
Cute, but not so useful to Joe Trucker, who wants his Cobra 29 on the same channel "19 lower" as his buddies' Galaxy/Connex 10-meter radios.
The Expo kits originally used a crystal that only moved the radio up and down 44 channels. This caused channel 1 to appear at "40" on the selector when the Expo is switched to lowers. And CB channel 40 was what you would get with the selector on channel 1 of the upper band.
But this was before the multi-band radios became popular. Eventually they modified the Expo 100 "A" kit's circuit to 'pull' the lower-band crystal down one channel, and the upper band crystal up one channel.
This makes the modified Cobra 29's channel selector agree with a Galaxy on upper and lower channels.
But it can cause performance issues. Not gonna go there today.
73
The signal level from the Expo kit is higher than the 'stock' 15.36 MHz from the radio's original circuit. Feeding 14.91 MHz into this pin causes the PLL to run 45 channels lower than stock. Feeding the 15.81 MHz moves the PLL's coverage 45 channels above.
The guy in the video is moving upstream in the circuit, to the frequency that feeds into the 15 MHz coil.
Substituting 5 MHz for the 5.12 that originally comes out of the PLL chip will move the radio down 36 channels.
Cute, but not so useful to Joe Trucker, who wants his Cobra 29 on the same channel "19 lower" as his buddies' Galaxy/Connex 10-meter radios.
The Expo kits originally used a crystal that only moved the radio up and down 44 channels. This caused channel 1 to appear at "40" on the selector when the Expo is switched to lowers. And CB channel 40 was what you would get with the selector on channel 1 of the upper band.
But this was before the multi-band radios became popular. Eventually they modified the Expo 100 "A" kit's circuit to 'pull' the lower-band crystal down one channel, and the upper band crystal up one channel.
This makes the modified Cobra 29's channel selector agree with a Galaxy on upper and lower channels.
But it can cause performance issues. Not gonna go there today.
73
Maybe this is a viable option. Need to translate site to English from German.
https://www.on-radio-shop.de/shop/Z...ung/PLL-Module/-Platinen/PLL-LC7131::449.html
- 399
https://www.on-radio-shop.de/shop/Z...ung/PLL-Module/-Platinen/PLL-LC7131::449.html
- 399
Thanks very much for the history lesson! Its always nice to hear from someone who has been there and done that.
The video shows also frequencies above channel 40, now Im really puzzling over this.
That looks pretty neat. Would I be able to use a frequency counter with this? That is what I would like to have on my Cobra 29 oh and a few extra channels too.
Forgive my lack of understanding but isn't that what the frequency counter is for?
I appreciate y'alls answering of my noob questions and rest assured it does not fall on deaf ears.
If it is a VCO frequency direct reading Frequency Counter being used on an AM only radio such as a Cobra 29 or 25 (GTL/LTD/LX etc...), it will show the correct frequency when in Receive mode, but it will also show an offset frequency 455 KHz higher than the receive frequency when in Transmit, because they inject 10.240 from the PLL reference oscillator into the Transmit Mixer, and the PLL shifts the VCO accordingly. So an additional 10.695 MHz oscillator would be needed to go into the TX Mixer, and the TX/RX line on the PLL and possibly the VCO too (some AM radios do), then the PLL is tricked into staying on the RX frequency even in Transmit.
Now, all that being said, I came up with an idea a while back, for a Universal PLL cracker board, that will take ANY PLL from ANY radio, AM and AM/SSB too, and give you the correct banded Upper and Lower s like a Galaxy type radio, and even give a shift of -5KHz, +5KHz and +10KHz, which will not affect the PLL frequencies being generated, the circuit I have in mind goes between the PLL and the TX and RX mixers.
If the TX VCO Frequency is shifted from 16MHz down to 13MHz (then doubled to 27MHz), or shifted direct to 27 MHz, then the TX offset in the VCO would have to be disabled, and the PLL TX/RX pin would also have to be disabled, and locked into RX only mode. This circuit does have provisions for a TX Oscillator to be injected into the TX Mixer.
It acts as a buffer between the PLL and the TX and RX mixers, which the PLL continues to "think" it is on frequency (it is), but offsets the RX and TX signals going into the mixers.
Any SSB offset in the PLL follows through, and any Clarifier/RIT mods also follow through.
This does not involve replacing the PLL with a tricked out PLL board (there would be too many variations), my design works with the existing PLL, unaffected (mostly).
I have the board design mostly completed, I need to make a few tweeks, then send the Gerbers to my board maker.
Now, all that being said, I came up with an idea a while back, for a Universal PLL cracker board, that will take ANY PLL from ANY radio, AM and AM/SSB too, and give you the correct banded Upper and Lower s like a Galaxy type radio, and even give a shift of -5KHz, +5KHz and +10KHz, which will not affect the PLL frequencies being generated, the circuit I have in mind goes between the PLL and the TX and RX mixers.
If the TX VCO Frequency is shifted from 16MHz down to 13MHz (then doubled to 27MHz), or shifted direct to 27 MHz, then the TX offset in the VCO would have to be disabled, and the PLL TX/RX pin would also have to be disabled, and locked into RX only mode. This circuit does have provisions for a TX Oscillator to be injected into the TX Mixer.
It acts as a buffer between the PLL and the TX and RX mixers, which the PLL continues to "think" it is on frequency (it is), but offsets the RX and TX signals going into the mixers.
Any SSB offset in the PLL follows through, and any Clarifier/RIT mods also follow through.
This does not involve replacing the PLL with a tricked out PLL board (there would be too many variations), my design works with the existing PLL, unaffected (mostly).
I have the board design mostly completed, I need to make a few tweeks, then send the Gerbers to my board maker.
The frequency counter on the video does track properly on transmit and receive and it is read directly off the VCO + loop signal . Actually I didn't understand at first but things are starting to make more sense to me because I spent a good portion of yesterday studying the PLL and how its used in a cb radio.
In the video there appears to be an add-on 10.695 transmit oscillator board which replaces the 10.240 transmit mixing signal for just the reasons you have stated. If your interested the video is here .
It took me a bit of time to realize that this mod uses both sets of VCO frequencies which explains why the switch gives upper channels and lower channels.
I read the slider runs on the 5 megahertz signal to adjust out the landing on 0s instead of 5s when using the transmit set of VCO frequencies and then slide back when using the receive VCO frequencies.
Isn't this the way the cobra 25 works? It would be cool to see a frequency counter on a cobra 25. In your mod, where would the frequency counter be hooked up to? The VCO output? Would you be willing to share a block diagram? I await your board and modifications with eagerness and any other insight you may want to share. If I'm asking to many questions then I apologize in advance.
I wouldn't want a frequency counter that only works on receive which is one reason I've become interested in learning more about this modification process and your PLL modification.
Thank you for your response, information , sharing your ideas on PLL modification and helping me understand this modification.
No, the Cobra 25 works the same way as the Cobra 29, they both use the same RX and TX VCO frequencies. So a 25 would need the same 10.695 oscillator to inject into the TX mixer, while disabling the RX/TX line of the PLL, unless you wanted to go to the upper 0 frequencies like the previous messages. But, the PLL in a Cobra 25 does not use an offset mixer like the 29 does, and can read the VCO directly, so it is harder to trick into being on non CB frequencies. And the idea behind the Expo "S" kit or Lester's clones of the "S" kit are very flawed.
It is in radios that shift the VCO down to 13 MHz then doubled to 27 MHz in Transmit, like the Pro510XL, or the Ranger RCI TR-121 that I was referring to.
The circuit I have on the "drawing table" has 3 buffered outputs, one for the RX mixer, one for the TX mixer, and a third for a frequency counter, if so needed. And, it can be used on radios that the owner didn't want traces cut on the PLL lines, just pull a few components, and put my board in place of the components.
There are 2 different possible variations on my board, whether it is being used in a "low frequency PLL offset (lower than 27 MHz), 16-17 MHz VCO used in Cobra 25/29 et al, or high frequency PLL offset (higher than 27 MHz), like the 34 MHz VCO used in a 148 or Washington or Madison, and 37 MHz VCO like in some of these newer President US models. Different filtering (High Pass vs Low Pass) for the 2 different variations.
Joe Truck Driver would want it so his radio to have the same channels as his friends with Galaxy type radios, not some odd 36 channel overlap or off frequency by 5 KHz.