Cobra 139XLR to 10 meter voice conversion

Hi guys,

I've searched around and have found a lot of stuff... all not relevant to what I'd like to do. I've got a Cobra 139XLR that I've had for years (has a uPD858C PLL). I want to make it work from 28.3 to 28.5 (going further up band is okay). My two younger kids are Technicians and, with 10 opening up more and more, I'd like to give them their own rig/antenna that gives them "channels" to work in the range they're licensed for. I'm hoping this will get them to pay some attention to amateur radio and get on the air.

Can anyone please steer me to some relevant documentation on how to do this?

Just to be clear - I do not want "freeband" modifications, changes in power, sound effects, or modifications to modulation (the Internet is cluttered with garbage on how to do this, or how to make the radio transmit voice in a sliver of the non-voice subband). I also do not need to retain CB frequencies in the radio, the less they can get into trouble with, the better. Just CB to amateur radio 10 meter band where Technicians have voice privileges. It's okay to go above 28.5 into the general-plus band (their siblings or I can use that range).

Thank you!

Wirelessly posted (BlackBerry8530/5.0.0.973 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/105)

Have you checked www.mods.dk for the mod?

Yes. Nothing obvious. I've also contacted the gentleman who built a number of these for 10 meter beacons. He's given me some great information, but there's nothing turnkey like "oh, change the 10.24 MHz crystal to 11.46 MHz and realign the radio." Most of the PLL mods only go up into the CW portion. He also suggested changing the USB oscillator crystal (and leaving the reference at 10.24). Now to hunt for the right frequency to change it to.

If you figure out what the synthesizer is doing frequency-wise, getting the right frequency to change it to is simple arithmetic. (Figuring out what the divide ratio is in a custom IC isn't possible, but the information of how that specific synthesizer works may be published, or you can change the reference a few times and log the outputs. Then do the math to figure out the configuration.)

W2PMX said:

If you figure out what the synthesizer is doing frequency-wise, getting the right frequency to change it to is simple arithmetic. (Figuring out what the divide ratio is in a custom IC isn't possible, but the information of how that specific synthesizer works may be published, or you can change the reference a few times and log the outputs. Then do the math to figure out the configuration.)

Click to expand...

Looks like that's what I'll have to do.

I was hoping that I wouldn't have to reinvent the wheel is all.

73!

902 said:

Looks like that's what I'll have to do.

I was hoping that I wouldn't have to reinvent the wheel is all.

73!

Click to expand...

Not sure if the pdf doc i have will help you but if you can pm
me with your email address i can send it to you.

It is a CBC International doc on the 23 channel model frequency
crystal synthesis. It might be enough to get you started.

902 said:

Hi guys,

I've searched around and have found a lot of stuff... all not relevant to what I'd like to do. I've got a Cobra 139XLR that I've had for years (has a uPD858C PLL). I want to make it work from 28.3 to 28.5 (going further up band is okay). My two younger kids are Technicians and, with 10 opening up more and more, I'd like to give them their own rig/antenna that gives them "channels" to work in the range they're licensed for. I'm hoping this will get them to pay some attention to amateur radio and get on the air.

Can anyone please steer me to some relevant documentation on how to do this?

Just to be clear - I do not want "freeband" modifications, changes in power, sound effects, or modifications to modulation (the Internet is cluttered with garbage on how to do this, or how to make the radio transmit voice in a sliver of the non-voice subband). I also do not need to retain CB frequencies in the radio, the less they can get into trouble with, the better. Just CB to amateur radio 10 meter band where Technicians have voice privileges. It's okay to go above 28.5 into the general-plus band (their siblings or I can use that range).

Thank you!

Click to expand...

Just a note...Wouldn't it still be illegal due to the fact that the cb radio is not type accepted to be used on amateur radio frequencies?

twjr80 said:

Just a note...Wouldn't it still be illegal due to the fact that the cb radio is not type accepted to be used on amateur radio frequencies?

Click to expand...

Nope, there is no type acceptance for Part 97 radios. Only going the other way is illegal (10m to CB).

KG4INW said:

Nope, there is no type acceptance for Part 97 radios. Only going the other way is illegal (10m to CB).

Click to expand...

Right. As a ham, I can build equipment that conforms to the technical parameters outlined in the Rules. Not so long ago hams used to do just that. This radio is a nice package (almost an heirloom considering its history) and would be a pretty addition for us and would be simple enough for the kids to use (they're both Technicians).

On another note, I did hear back from the gentleman who converted CBs to beacons. He provided me with a formula that he asked me to check, so if it's correct (I'm flattered he asked me, but I'd bet he's much better at math than I am) I'm on my way to ordering a crystal and realigning the radio to 10 meters.

I'll post the results here on RR once I know I did it right. A Novice or Technician class licensee could perform this mod and talk on 10 meters between 28.3 and 28.5 MHz.

I'd love to hear about the final results. This is what amateur radio's all about and it sure is fun making radios do things that they weren't necessarily designed to do (within good engineering practice of course)!

Interesting, let us know how it works, a lot of them are out there for conversion
DOCTOR/795

Ah ok. Wasn't for sure about it all. Cool! Would be nice to hear how it all goes.

902 said:

He also suggested changing the USB oscillator crystal (and leaving the reference at 10.24). Now to hunt for the right frequency to change it to.

Click to expand...

Hopefully that's not the plan now... Or you'll need a new sideband filter to match your new balanced modulator output frequency..Stick with PLL reference frequency change (and realignment)....:wink:

I found a 10 meter conversion for a Realistic TRC 458...since it's the same board, me thinks it should be the same for the Cobra 139 XLR...

Converting the TRC-458 from CB into 10meter rig:


The mod is VERY simple! Replace the crystal "X3" which is the right-most one of three, right inside the little "wall" around the PLL circuitry. You will need an 11.933 Mhz crystal. (That will give you UPPER SIDEBAND. If you want to add LSB & AM, you'll need to calculate those x-tal freq's.) The rig will cover almost all of the 28.000 to 29.999 band, so care must be excercized to remain within band! (28.000 - 29.700)

You will also need to RE-TUNE the PLL circuit, so that it "locks" on the new band. This can pretty much be done by ear... you'll hear a "buzzing" sound in the speaker when it goes out of lock. That buzz will change pitch as you tweak. Tweak it until it stops, then go a little further, until it starts again. The point you want is in the MIDDLE of the range where it doesn't buzz. You'll need to check the stability at both ends of the band you wish to cover. I don't think it will go the whole 2Mhz, only a portion of it... but it's a pretty large portion! The "cans" you need to tweak are the two single ones between the 3 X-tal sockets and the PLL chip. The one on the right (if memory serves) is the main one.



The receiver's front end will need to be retuned. That's pretty simple, and can be done by ear, also. (Having a weak signal on the air, or a stable signal source to test with, is helpful, tho!) There are 2 small RF "cans" right by the modulation transformer, at the left-rear of the board. There is also a "double-barrelled" one to their right. Those 4 adjustments are the ones you want. Don't worry about the "ADMIT NO ADJ" can... that is for the NOISE BLANKER, so you don't need to mess with it.

There is a diode in the clarifier circuit that needs to be removed, to allow the clarifier to affect both the TX and RX. You also will need to change a few resistors, to give it a wider range. (There is one between pot & ground, and one between pot and + voltage. I used 100 ohms for both.)

You *MUST* use a voltage regulator to power the modified clarifier circuit. If you don't, the frequency will move as you transmit. It wants to see about 9 volts on the "top" of the pot. (An LM7809T will be perfect.) That should give you about +6 to -5 Khz range. You can't get more than -5, since that is what you get at 0 volts. You don't want it to see more than 9v, to avoid damaging the varactor diode.

a2phil said:

I found a 10 meter conversion for a Realistic TRC 458...since it's the same board, me thinks it should be the same for the Cobra 139 XLR...

Click to expand...

Beautiful! Thank you, Phil! I've got to get it in gear before the sunspot cycle declines again .

a2phil said:

... snip ... You will need an 11.933 Mhz crystal. (That will give you UPPER SIDEBAND. If you want to add LSB & AM, you'll need to calculate those x-tal freq's.) The rig will cover almost all of the 28.000 to 29.999 band, so care must be excercized to remain within band! (28.000 - 29.700) ... snip ....

Click to expand...

USB is all you will need for 10 meters. There is very little AM most all is USB.

Let me know if it works...I want to try it if/when I get mine working...maybe even a video, since about the only soldering I've done on a radio was rewiring mics....

Easier 138/139xlr mod

Hello,
I know this thread is several months old but there is a no crystal needed 10 meter mod for the 138/139 xlr radios.
Cut pins 19 and 22 of the 858 chip away from ground and add 4.7k or 10k pull down resistors to ground.
remove the channel selector wire going to pin 21, and jumper from pin 12 to pin 22 to pull pin 22 up to 5V.
Since I'm doing this from memory you will need to do a search to find alignment instructions for the radio.
This gives you channels 8-40 on the radio 1 mhz higher (28.055-28.405), and with a switch pulling up pin 19 to 5V you can also get (28.455-28.805).
Hope this helps,
Carl

Re: 458 TRC to 10 meter mod

The only problem I have with modifying the pll is which pin is which?? Knowing MY luck, I'd end up with a pile of burnt up electronic junk...I think I'll stick with changing the crystal...at most, doing some de-soldering and re-soldering...STILL trying to get my POS Cobra 139 XLR working (first time I called it that-must be getting pissed at it...

Note to self: NEVER buy a radio WITHOUT testing it on the air first NO MATTER "how good a deal it is"!!

WD4MIZ said:

Hello,
I know this thread is several months old but there is a no crystal needed 10 meter mod for the 138/139 xlr radios.
Cut pins 19 and 22 of the 858 chip away from ground and add 4.7k or 10k pull down resistors to ground.
remove the channel selector wire going to pin 21, and jumper from pin 12 to pin 22 to pull pin 22 up to 5V.
Since I'm doing this from memory you will need to do a search to find alignment instructions for the radio.
This gives you channels 8-40 on the radio 1 mhz higher (28.055-28.405), and with a switch pulling up pin 19 to 5V you can also get (28.455-28.805).
Hope this helps,
Carl

Click to expand...

I go back into threads from time to time, so I'm very glad you posted this. I'll have to try it out. 6 meters has been pretty active (not tonight, unfortunately), but I can only imagine what 10 is doing. If it works, my younger ones and I will be most grateful!

73!