Crystal addition on 23 channel AM/SSB CB radio

[3pinplug]

Hi all

Just purchased a Siltronix SSB-23A Cb Radio which has a number of crystals in it, i want to mod the radio to fit some more crystals to get me the high channels (27.415Mhz up to 27.705Mhz) whilst keeping the xistingin channels which I believe can be acheived by selecting another in place of the one fitted. To do this I found a diagram in the 'Secret CB Book' that said to use a DPDT switch with a crystal (exisitng and new) soldered each side of the switch, wires from the switch (keeping them short) then go back to the original crystal socket position.

I have 2 questions I hoped you would be able to help me with:-

1. I need a 12.255mhz crystal, I have a 12mhz crystal, can I pull this enough to get to 12.255mhz?

2. If yes -  how would i do that? when using the original crystal the trimmer next to the crystal socket will be set for that crystals frequency, when selecting the new crystal -  can i fit a trimmer in line with one leg and trim it, whats the correct way to add a trimmer for the new crystal?

Many Thanks

Mark

 

[cbrown]

Off hand I don't think it will be worth the effort. First off, because of the way they set up the channels for 23 channel radios you'll be skipping the 'A' channels when you convert to the upper frequency crystal. So let's assume you start with channel 1 now being channel 24:

Old channel 1 - now channel 24 - 27.235
Old channel 2 - now channel 25 - 27.245
Old channel 3 - now channel 23 - 27.255
Old channel 4 - now channel 27 - 27.275
Old channel 5 - now channel 28 - 27.285
Old channel 6 - now channel 29 - 27.295
Old channel 7 - now channel 30 - 27.305
Old channel 8 - now channel 32 - 27.325
Old channel 9 - now channel 33 - 27.335
Old channel 10 - now channel 34 - 27.345
Old channel 11 - now channel 35 - 27.355
Old channel 12 - now channel 37 - 27.375
Old channel 13 - now channel 38 - 27.385
Old channel 14 - now channel 39 - 27.395
Old channel 15 - now channel 40 - 27.405
Old channel 16 - now 'channel' 42 - 27.425
Old channel 17 - now 'channel' 43 - 27.435
Old channel 18 - now 'channel' 44 - 27.445
Old channel 19 - now 'channel' 45 - 27.455
Old channel 20 - now 'channel' 47 - 27.475
Old channel 21 - now 'channel' 48 - 27.485
Old channel 22 - now 'channel' 49 - 27.495
Old channel 23 - now 'channel' 52 - 27.525

You can see you do miss frequencies because of the way the original 23 channel band was set up (the missing frequencies are R/C channels, the only R/C that is 'shared' between both CB and R/C is channel 23 (27.255).

But in the end it's up to you. If you DO want to get the other frequencies on the radio, I'd suggest using a VFO instead.

[3pinplug]

Hi CB Legend

Sorry you have misread my post, I am looking to add some of the "High" channels which would be from 27.415mhz - 27.705mhz (Ch1-23)as mentioned in my initial post, not the remaining channels of 23-40 in the mid band as you mention. Unfortunately a VFO is not an option for this as they do not exist over here in the UK and shipping from the US to UK now makes this a non-starter and unsure if they would cover the frequency I am after.

Not worried about the Alphas at present, if i need those i can possibly add them in the future if i can get the new crystals up and running, wanting to use the SSB capability of the radio and 27.55mhz is the dx calling channel hence why I am looking to get the highs.

If I can get a 12.255 crystal or how to pull a 12mhz to that frequency then I am off and running, from your experience would you know how to do this at all?

many thanks for answering

Mark

[2600]

Crystal oscillator circuits come in wide variety. The frequency marked on a crystal will only be valid in the circuit the crystal is "correlated" to match up with.

That being said, putting a capacitor in parallel with the crystal will cause the frequency to drop. But if you pile on enough capacitance, the crystal will stop oscillating. Too much signal current goes through the capacitor, and not enough through the crystal. So there is a limit to how much you can "pull", as they say, the crystal's frequency. Likewise placing an inductance in series with the crystal will lower the frequency. Putting a capacitor in series with the crystal raises the frequency. But these two tricks will also stop the crystal in its tracks, if the inductance is too high, or the capacitor too small. Either one will reduce the feedback that makes the crystal oscillate to begin with.

But there is a bonus to all this trickery. It usually makes temperature drift worse. The reputation that Galaxy radios have for drifting frequency on sideband has a lot to do with a clarifier that travels more or less one full channel.

Ken's Electronics has some 12 MHz crystals listed for sale on this page: www.kenselectronics.com/lists/cbxtal.htm

He has 12.255 listed, along with a few close to that.

73

[3pinplug]

Hi Nomad

Many thanks for the link, that looks a very interesting seller will definitely have a good look through the site and see what else he has. He ships to the UK for $20 + $15 for the crystal so will see what else i can slip in the package to spread the cost a bit.

thanks again

Mark

[2600]

Back in the 70s there was a channel-expansion kit for 23-channel crystal-synthesizer radios that had six crystal sockets and a single push button switch in it. And 13 wires. It would duplicate the factory-built six crystals and double the coverage. At the time 46 channels seemed like a lot. The wiring for it was a headache. This also demonstrated how the stray capacitance and inductance of the added wire length would pull down the frequency of a crystal. As a rule, the location of the crystals inside the radio was never close by a convenient place to mount a switch. Long wires to and from the switch would pull down the crystal's frequency more than any trimmer capacitor could bring it back up.

Keeping the wires short would reduce this risk. Since the factory crystals were routed through 2 wires, and the 'new' channels only one wire, it would throw the original 23-channel crystals off frequency worse than the "extra" channels.

Sure wish I could remember what that kit was called.

73

[cbrown]

This was a popular item in my area for awhile, it's called the VJ Xpander I and would give you an additional 23 channels.

For this model, the green & white wires replace the 11.730 MHz (receive) crystal and the red & black wires replace the 11.275 (transmit) crystal.



I have a bunch of other kits, like the Expo kits. I believe I also have that crystal box that 2600 mentioned. I'll have to dig through my old stuff and see what I can find.

[2600]

The V-J was built to match the 23-channel Cobra 29. The long wires from the box to the radio's circuit board could cause trouble, especially if you used an amplifier.

73

[3pinplug]

The kits you guys mention sound very interesting, i wonder if i could replicate one of those to help with my issue. Would it be posdible to post a pic of the inside of the vj expander?
Thanks
Mark

[cbrown]

Jan 23, 2023 12:29:48 GMT -5 3pinplug said:

Would it be posdible to post a pic of the inside of the vj expander?
Thanks
Mark

Unfortunately the maker filled the insides with epoxy to prevent anyone copying his crystals (which anyone with basic math skills can figure out) so there isn't any insides to show. Let me see if I can grab an expo kit, if I remember they didn't bother covering anything.

[2600]

For the Cobra 29 putting channel 26 onto the dial's channel 1 calls for a 12.03 MHz crystal for receive, and 11.575 to transmit. The schematic is just a three-position switch with one wire for the factory crystal and the two new crystals at the position either side. Nothing tricky.

73

[3pinplug]

Hi Cb brown and 2600,
The switch idea i saw before, i have a 12.500mhz crystal and need to make it 12.255mhz. I have some yellow and green trimmers so would i add this in series between one leg and the switch contact, or across both legs to lower the frequency.
Want to get it between 27.415 - 27.625mhz hi band.
Thanks for your help with this, much appreciated.