Mathamatical equation for 14.91mhz,15.81mhz?

[Mark A Bradley]

I have a Cobra 29 LTD CLASSIC I just purchased, another one? The radio has a board with 2 crystals in it, 14.91mhz and 15.81mhz, along with a 10.240mhz and a 10.695 I assume, but I cannot see it between the pots According to the Cobra parts, is is a 10.695 filter.
I would like to know or try to remember how to get the starting alignment First Frequency or first ,IF frequency? I had Lou Franklin's CB repair book, but I cannot find it is my wife's mess,time to clean closets, again.Is there anybody who can help me out, this is a good learning tool for wanna be CB Repair Personal. This will help you when you get those strange CB radios with 2 or 4 different Crystals.
If you don't have Lou Franklin's book, you are one lost puppy, like me right now. I have 2 of his books,but where they are,I haven't the slightest idea, wife looks at me and tells me,your books,your problem.Time to zip it.
OK, does any one remember how to get to the frequency for RX and for TX, that's what we need to find out with 14.91mhz and 15.81mhz, and the usually 2 other crystals on the board from the factory, which I think we may involve these also?? I forgot,it's been a while,about 34 years since I read the book and messed with CB's. OK, let's see what the RX and TX are,someone has to know,how about Handy, he is pretty smart on this subject.Goodnight, talk or text tomorrow.

 

[freecell]

the complete cobra 29ltd service manual.
download and read the section entitled "theory of operation/s."

http://cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29ltd_sm.pdf

it would appear as though someone was attempting to increase the frequency coverage.

 

[nomadradio]

Here's the quick version.

The 14.91 crystal is to obtain 45 channels below channel 1. Should give you the same channels as a Galaxy radio set to the band below the legal 40 channels, one set of "lowers".

The 15.810 crystal gives you channel 41 with the selector set to channel 1. Same channel numbers as a Galaxy/Connex radio gets when set for one band above the legal 40 channels.

A close look will reveal that the two crystals are installed in an oscillator circuit. The output from this circuit is jammed into the radio's PLL, where it overpowers a 15.36 MHz signal that provides the legal channel coverage.

73

 

[Mark A Bradley]

Here's the quick version.

The 14.91 crystal is to obtain 45 channels below channel 1. Should give you the same channels as a Galaxy radio set to the band below the legal 40 channels, one set of "lowers".

The 15.810 crystal gives you channel 41 with the selector set to channel 1. Same channel numbers as a Galaxy/Connex radio gets when set for one band above the legal 40 channels.

A close look will reveal that the two crystals are installed in an oscillator circuit. The output from this circuit is jammed into the radio's PLL, where it overpowers a 15.36 MHz signal that provides the legal channel coverage.

73

I believe it is for the extra channels.? Someone installed a 3 pole switch in the Delta Tune spot for the upper and lower channels. And using the SWR calibration for a Clarifier, as turning this knob has the frequency sliding up or down.
I'm trying to get the TX and RX frequency set for the 3 pole switch for the receive first, then work my way to the TX frequency, but I am not sure which to do first. The center of the switch I believe is for the regular 40, switch to the left for lower,and to the right for the higher channels,maybe up to 27.805 or above this frequency? I received the radio with it not reading the 2 crystals, I installed new crystals, now at least the frequency is on range of the lower and higher channels,so at least now, the frequencies are around where to set the TX and RX, just need to figure where to set each position of the switch.
I shall look into the link I was giving, and try to decifer where to begin with the frequency alignment, just need to get a starting point, so I able to tune the radio through the 40 channels, so I can flip the switch for the lower and upper freq on the switch. This is a first for me, I have never messed with this type of set up before, so with a little help and some reading, I may be able to get the radio going. I do have white noise on all 3 positions on the switch, but they are very low, I did get the regular 40 locked on 27.185mhz, and I believe that is where the regular 40 may be set at, I am not for certain. Need to find my Lou Franklin book to have even more help to figure this out. Now I shall check out the link that was sent ( thank you ) and get a better idea of where for me to start will update when I do get somewhere with this add on. Thank you all in advance for the help. This is going to be,for me, a good brain teaser,but it hopefully will work out, to be continued,thanks. Oh, the link that you gave me is no good,keeps telling me no such link available, or something to that nature. I will look around on the 29's for the theory of the radio,and see what I may be able to accomplish. Will try to do what I can figure out, I thanks again.

 

[nomadradio]

The radio is built with one quartz crystal only. The 10.240 MHz factory crystal determines the PLL circuit's transmit frequency and receive frequency both. It separates them by exactly 455 kHz, determined by that 10.24 MHz crystal.

If you set the radio to transmit on the correct channel center frequency and the receiver still sounds like it's tuned "off" of the channel center, the radio's delta tune circuit is where I would look first.

73