Getting 40 Channel Coverage on a 23ch Cobra 135?

[fsjonsey]

I've got a mid 70's Cobra 135 base that I'd like to use for SSB Dx. It's a 23 channel radio, and has really good sounding audio and a great receiver section. I'd like to use it on sideband, but most LSB calling happens up on channel 38. This presents a bit of a problem on a 23 channel radio. I read somewhere that there was a crystal and switch mod for the 135 that would give it full 40 channel coverage, but I can't find detailed information on it anywhere. Would anyone happen to know of a channel mod for this rig?

Thanks,
Bryan

 

[N0MSS]

There was already a post on this topic.

http://www.worldwidedx.com/cb-band-cb-radios/18231-extra-channels-old-cobra-135-a.html

Hope this helps.

BTW, one trick i used to do with 23 ch crystal radios was switch the oscillator crystals around for different set of channels. they droped 5kc's but it was still different band. You would have to retune on the new band to optomize. I even put some on a switch to have both bands.

12 crystal setups was switch 2 crystals, 14 crystal setups was switch 4 pairs.

All of my 12 crystal setups dropped below somewhere in this range.26.550 to 26.800.

26.800 used to be used alot, an old timer called it the oscar-meyer band.

The 14's went up above. forget the exact range. I think 19 became 27.500

 

[MisterFatty]

Swapping crystals used to be a good option, now at the cost of crystals, your probably going to find an old Pal VFO/VCO, or the Glenn Controller is a more attractive option. Stay away from the Siltronix stuff it is much less stable then either of the previously mentioned options.

I believe I have a page or two in my collection mentioning the Mixer Crystal mod, its pretty straight forward though.

 

[fsjonsey]

Swapping crystals used to be a good option, now at the cost of crystals, your probably going to find an old Pal VFO/VCO, or the Glenn Controller is a more attractive option. Stay away from the Siltronix stuff it is much less stable then either of the previously mentioned options.

I believe I have a page or two in my collection mentioning the Mixer Crystal mod, its pretty straight forward though.

How hard would it be to use an external VFO with the old cobra 135 or another 23 channel crystal synthesized CB?

 

[MisterFatty]

How hard would it be to use an external VFO with the old cobra 135 or another 23 channel crystal synthesized CB?

not hard at all, just remove a single crystal and attach the correct VFO in its place. Most of the old 23 radios aren't very broad banded though, and without a re-tune would only do maybe 26.7 to 27.6, this is just an average for the 23s I've had over the years. You can do better, but then you have to start wondering if it is worth the time to do it.

 

[mechanic]

I was given this old 135 Cobra from a friend and I thought to myself.... "Oh and old 23 channel .... There is a switch on the bottom! :drool:"
I have this old gem and it was modded years ago by ???.... There is a switched crystal inside that gives the normal channels and the "high frequencies", also the voice lock (slide) is opened up to cover all the RC skips on the higher band. Not sure of how the "slide" is done but covers over five channels on the HF band and about three on the CB band. The "slider" is not at all that touchy!
To use channel 39 LSB, go to channel 12 on HF and move the voice lock to about 12:00, 3:00 is channel 40.
Attached is a pic of the "cheat sheet" that came with the rig and there was a whole package of info from a side band group in the Chicago area from the mid 1970s!

Have fun with yours! mechanic

 

[fsjonsey]

I was given this old 135 Cobra from a friend and I thought to myself.... "Oh and old 23 channel .... There is a switch on the bottom! :drool:"
I have this old gem and it was modded years ago by ???.... There is a switched crystal inside that gives the normal channels and the "high frequencies", also the voice lock (slide) is opened up to cover all the RC skips on the higher band. Not sure of how the "slide" is done but covers over five channels on the HF band and about three on the CB band. The "slider" is not at all that touchy!
To use channel 39 LSB, go to channel 12 on HF and move the voice lock to about 12:00, 3:00 is channel 40.
Attached is a pic of the "cheat sheet" that came with the rig and there was a whole package of info from a side band group in the Chicago area from the mid 1970s!

Have fun with yours! mechanic

Thanks so much for the Info! I really appreciate it!

Would it be possible for you to take a look inside to see how the Crystal and switch was installed? I'd love to get this radio working on the upper 20!
If not, than I've been keeping my eye on a few VFO's on ebay. Hopefully I win one.

 

[mechanic]

Thanks so much for the Info! I really appreciate it!

Would it be possible for you to take a look inside to see how the Crystal and switch was installed? I'd love to get this radio working on the upper 20!
If not, than I've been keeping my eye on a few VFO's on ebay. Hopefully I win one.

I'll take some pics and post them here.... I'll try to figure out how the "slider" is done and any other tricks.

mechanic

 

[Rogerbird1]

Is this a Cobra 135 A or B?
The xtal boards are different.
The A series converts with a single xtal replacement.
My radio had a simple relay that switched out the xtal hot sides, and left them with common grounds. x311. Change to a 13.000mhz xtal gives from ch 26 to CH 55.
x311 should be a 12.xxx Mhz xtal. it is next to the 5 trimmer caps near the center of the xtal board.

the B version does not convert this way x311 is a different xtal in a different part of the ckt.

CB Tricks has the manual online at: Cobra 132, 135

I hope this helps.
Roger

 

[mechanic]

Here are some pics of my Cobra 135..... I will also have some uploads from an old Secret CB manual that may help....


First pic shows the location of the crystal that is removed but reused with the switch....

Second is a broader view....

Third is the location of the switch on my rig with the handle going through the grill...

Last pic shows the switch and crystals out of location and the "padding" capacitor can be seen.... The gray covered xtal is the original...

The upper frequencies crystal is not the recommended frequency so the mod uses a capacitor to get it on frequency...

Next post I'll have more....

mechanic

 

[mechanic]

Is this a Cobra 135 A or B?
The xtal boards are different.
The A series converts with a single xtal replacement.
My radio had a simple relay that switched out the xtal hot sides, and left them with common grounds. x311. Change to a 13.000mhz xtal gives from ch 26 to CH 55.
x311 should be a 12.xxx Mhz xtal. it is next to the 5 trimmer caps near the center of the xtal board.

the B version does not convert this way x311 is a different xtal in a different part of the ckt.

CB Tricks has the manual online at: Cobra 132, 135

I hope this helps.
Roger

Thanks Roger for the additional help on this and the CB Tricks site is great! I did not know my Cobra sold for that much in 1973! :w00t:

I can just imagine what those dollars are worth today!? Maybe $1000 or better? :blush:

 

[mechanic]

This is page 28 from the old Secret CB Vol #1. Shows the location of the mod for the slider. Seems simple and straight forward enough! The diagram shows the control as vertical but in reality it is a cap flat on the crystal board. Photo shows location of the adjustment point.....

More to follow......

mechanic

 

[mechanic]

From page 53 of the Secret CB vol#1, even though this is for the Cobra 135B, this is what is in my Cobra 135A....
The switch used in my rig STDP type, SCB shows a DTDP....
I made a drawing of the switch in mine and they used a 13.120 crystal and a padding capacitor to adjust the crystal to 13.100....

The book said to keep the leads as short as possible but in my rig they are about 6 inches long. Also there is a disk capacitor on the bottom of the crystal board where the original crystal was removed. I do not know if this added after the mod but the value is 15pf...

This is all I can tell you about the Cobra 135 I have. The radio works well on the frequencies it can get to and the voice lock is easy to use! Have fun with the mod and I can answer more questions if needed!

 

[fsjonsey]

I was able to get ahold of a Siltronix Model 90. It's hooked up now, the CB band works fine, but the HF band doesnt seem to work. Was it disabled somehow at the factory?

 

[fsjonsey]

Upon closer inspection, It turns out that the VFO works with the cobra 135 half way through the HF band, with output power decreasing from about three watts to zero from 27.305mhz on up. Looks like I'm going to have to align the cobra to be more broad banded, in order to operate correctly on higher frequencies than the original 23 channels. Does anyone have alignment data for a Cobra 135B?