Old school Cobra 139 23 CH

[Squints]

Kinda depends on what channel you want. Simplest fix is to remove one crystal you won't use, like the channel 1 to 4 crystal (marked 8.1590) and installing the new one there. No extra wiring needed this way. The new crystal will deliver four new channels. It will skip one 10 kHz step betwen channel 3 and 4 on the dial, no matter which crystal is installed on that part of the channel switch. This is called the "RC" skip, and the radio does this to the fourth of every five-channel step, between 3 and 4, 7 and 8, 11 and 12, 15 and 16, 19 and 20. A 23-channel radio also skips two RC channels between 21 and 23. To fill in this skip pattern will take yet another crystal, a switch and added wiring to fill in this blank channel skip pattern.

To add channels 26, 27, 28 and 30, use 8.4590 MHz
Channels 31,32,33 and 35 will need 8.5090 MHz.
Channels 36-40, skipping 39 is 8.5590 MHz.
41 to 45 skipping 44 use 8.6090 MHz.

And that's the pattern for each 5-channel step above channel 45, add 50 kHz to that crystal frequency.

Consider the the higher (or lower) you go, the less transmit power and receive performance you'll get. The radio is aligned for channels 1 to 23, so making it perform way above 23 or below channel 1 will require alignment. It will still be a narrow-banded radio, so optimizing it for upper channels will reduce performance on channels 1 to 23.

Sure must sound like "Five miles to school in the snow barefoot and uphill both ways." Heck, that's what this mod felt like that radio was new.

73

I have searched the internet 10 times over and asked every cb,er I know across the globe in search of this little bit of crystal mixing information you have provided and I'm just now finding it because I was searching for a Midland 13-893 until I discovered that it was exactly the same as the cobra 134 and 138.
I have also ordered a sams photofacts about my Midland so that I could get it to where I want it performance wise and use it as my primary base station as I have always liked the old school Midland radios.
My 13-893 is possibly one of the best transceivers I've ever had the pleasure of using and this one I have sounds so gosh darn good on SSB it's loud and clear it receives perfect and after an alignment it's all dead on frequency.
This info you provided will help me complete my project that was only lacking the capability to have go to channel 38 LSB that is only 1 of the 3 channels I always run being channels 14 and 19 on AM then channel 38 LSB and now after a year or so of searching for crystal mixing info that I now understand more clearly my Midland 13-893 restoration project is complete.
I'm currently talking on it right now as a base connected to a 13.8 volt 50 amp switch mode power supply along with one of these new plastic astatic desk mic.
There's a power mod in a secret cb manual where one can add a resistor in parallel to an existing resistor will enable this chassis for tuneing output power as high as 30 watts but I don't recommend going that high but it will do it because mine did then I turned it back down.
Great radio can't go wrong with it should last another 20 years or more.
Ole Shoog Nasty 109 in the bluegrass just got down.

 

[Missouri Swamp Rat]

I ran a 13-895 Midland back in the day, Great radio.....with a ten turn Clarifier .
I had a blast with it.

73
Jeff

I had a pair of those , good radios , wish I had kept them. They sure was heavy tho, drop one and make a hole in the floor , lol.