EXPO Model A Kit schematic

[dino_84]

I Had one of these little boards that I got out of a junked radio someone gave me, but can't find the thing anywhere... Does anyone have a circuit schematic out there? Or a kit of your own that maybe you can trace out the schematic for me? so i can just build another. "A" or "+A" kit... doesn't matter... All the thing is, is a very simple crystal oscillator.... I've searched everywhere on the internet for a schematic, but no avail. I do remember it used a 2n2222A NPN transistor and of course they scraped the frequency off the crystal... a**holes . Too bad I didn't measure the frequency of it as soon as I removed it... Any help would be greatly appreciated guys.

[MDYoungblood]

The only thing I found was CB Tricks
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Sorry it is not much help.

3's

Greg

[dino_84]

The only thing I found was CB Tricks
[Please login or register to view this link]
Sorry it is not much help.

3's

Greg

Well, thanks anyway. I appreciate it. Too bad it doesn't show a picture of the component and trace sides of the board. That would help a great deal. Maybe someone else holds the answer... anybody?

[Night Crawler]

Does anyone have a circuit schematic out there? Or a kit of your own that maybe you can trace out the schematic for me? so i can just build another.

Scrap that thing and build your own here's a few schematics for a basic oscillator and one with a buffer amp that has a frequency adjust.
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[TheCBDoctor]

Hi dino_84,

I have a couple of old Expo "A" kits. I can look up the frequency of the crystal this weekend. The method used for the "A" kit is forced signal injection at the VCO mixer; a simple crystal oscillator will work You will need a 15 Pfd cap and a 51 ohm resistor to match the impedance of the mixer circuit.

I will see what I can do.

Regards,

Rick

[dino_84]

Thanks for the help guys. After I get the right frequency and build another, I can just use the install info from cbtricks, right? Also, what is the proper injection level? And the resistor/ cap for matching should go across the output in parallel right?

I've been a ham and homebrew enthusiast for a few years and have built many transceivers with simple direct conversion receivers operating at VFO frequency, but when it comes to frequency synthesis, I'm a bit of a **Censored**. I understand what's going on, but!!! The closest I've came to synthesizing anything was a mixer vfo utilizing an NE602... so just bare with me here....

73

[Night Crawler]

Here's how you would do it with a crystal oscillator for a Cobra 29
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I'm also a ham and have been homebrewing and also building kits for around 35yrs.

[dino_84]

Here's how you would do it with a crystal oscillator for a Cobra 29
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I'm also a ham and have been homebrewing and also building kits for around 35yrs.

Hey thanks there, Night Crawler, this helps a lot. Actually pretty simple.... you know, I read an article once about "crystal Penning" where a Ham took apart one of those hc/(whatever) style crystal holders and marked on it with a permanent marker (or a graphite pencil) and it actually lowered the frequency of the crystal a bit... I've tried this with 80m and 40m meter crystal with success. It's hard to get an exact frequency this way though. I've gotten 2 or 3 hz to my target before. That was an accident though! I had to literally draw over the whole surface of the 80m crystals to get it to drop only a few kc's, but the higher freqs can be shifted much more. I'd probably have stability problems on those higher freqs. I have some 16mhz fundamental rocks, so if I can't find a crystal i'll give it a try. No harm there.

Ham radio is fun... sometimes. A lot of the operators are a bit up tight, but the local bunch around this area on 80m (the dumb hillbillys: as they are called by the city hams) are an alright bunch. It's fun to build your own stuff, throw a wire up in a tree, and make contacts with it! Nothing is more rewarding. My CW sucks, but it's one of my favorite modes.

[Night Crawler]

I read an article once about "crystal Penning" where a Ham took apart one of those hc/(whatever) style crystal holders and marked on it with a permanent marker (or a graphite pencil) and it actually lowered the frequency of the crystal a bit.

I've done that it can also be done with the smaller crystals if you unsolder the case near the bottom and slde it off the quartz.
When you add the graphite it adds capacitance to the quartz crystal which lowers the frequency I also erased some of it off the side of the quartz and it will raise the frequency.

Another thing that be done to the quartz is to take some # 400 or # 600 sandpaper and run it across the top of the quartz and shave it down which will also raise the frequency of the crystal.

What you can do if you which is a lot easier is to add around a 10pf trimmer either in series or parrallel with the pins of the crystal (which ever works best) which will also pull it down a bit in frequency.

I got my ticket in 1977 and 90% of my contacts at that time were CW I once was able to do 20 to 25 wpm I took one of the online tests a while back and just passed the 5 wpm exam.

80 meters was always the outlaw band there were a bunch up here in the northeast that used to hang around on 3.910mhz back in the late 70's through the 80's they used to call it channel 39.

I haven't heard them in quite a few years I know they had about ten years on me so I presume they're either dead or in an old folks home somewhere by now.

[dino_84]

Well, that was before my time... there's still a good bunch of old timers who get on and chew the rag for hours about absolutely nothing around here on 80m. And it seems there's no technical question I can ask that one of them can't answer. I also like to listen to some of the dirty old codgers up around 75m late at night. 25 WPM would be nice. I can send around 15-20, but copying is a different story. Some practice would probably help me out a a lot.

I couldn't get the 16MHz crystals to shift low enough to work properly. The upper channels were primarily my interest. I did have a 9.810 crystal lying around, so I decided to whip up a circuit and throw it in an old 25 LTD I had lying in the closet. It works pretty good. I didn't like how the original kits just run a simple wire from the oscillator to the mixer circuit, so I used some mini 50 ohm coax for all the rf connections-- I have no idea of the input impedance of the mixer. I also didn't like the idea of having to broadband the front end so much. It would be nice if I could construct another switchable filter for the lower bands, but I figure if I'm gonna go through all that trouble, I may as well by an export radio. The object here is experimentation and fun.

I wonder if it's possible to bring one of these rigs up to 50mhz? I'm sure there would be loads of circuit overhaul...

[Night Crawler]

Here's a place to get crystals that's fairly cheap compared to getting one cut you might find one one close enough that you can pull down in frequency to get it to work.
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The output from that chip is actually in the 5mhz range and then it goes to a tripler (the expander kits output goes to the tripler) so if you can find a crystal 1/3 in frequency of what you need it will work.

I saw a few articles where cb's were put on the 6 meter band I have them in my files here somewhere I"ll take a look it's not a major overhaul there really isn't much involved to do it.

But is it worth the trouble how much AM activity are you doing to get if you plan on doing it to a Cobra 25 a SSB rig would be a different story.

[dino_84]

Here's a place to get crystals that's fairly cheap compared to getting one cut you might find one one close enough that you can pull down in frequency to get it to work.
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The output from that chip is actually in the 5mhz range and then it goes to a tripler (the expander kits output goes to the tripler) so if you can find a crystal 1/3 in frequency of what you need it will work.

I saw a few articles where cb's were put on the 6 meter band I have them in my files here somewhere I"ll take a look it's not a major overhaul there really isn't much involved to do it.

But is it worth the trouble how much AM activity are you doing to get if you plan on doing it to a Cobra 25 a SSB rig would be a different story.

Thanks for the link. This will come in handy.

Well, I don't really have any 6 meter capability-- actually, I've never even used it. I don't really know the mode of choice on the band. But you're right, nowadays everyone is using sideband. I I have a stack of old 29's, 25's, and other junk rotting away... Just a thought that crossed my mind. But maybe, this'll be a project for another day.

[Night Crawler]

Double Post Remove

[Night Crawler]

Well, I don't really have any 6 meter capability-- actually, I've never even used it. I don't really know the mode of choice on the band. But you're right, nowadays everyone is using sideband. I I have a stack of old 29's, 25's, and other junk rotting away... Just a thought that crossed my mind. But maybe, this'll be a project for another day.

If you like to experiment try converting one here's a link that shows what has to be done.
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The most popular modes on that band are SSB and FM simplex and repeaters with AM trailing far behind really not too much activity to speak of.

What you could do is add FM to that AM rig which is fairly simple to do for transmit the VCO will be modulated with the output from the audio amplifier that's used to modulate the driver and final for AM.
Then a limiter/discriminator is added between the last IF stage and the audio amplifier stage to demodulate the FM signal for receive.

It sounds a lot more complicated then it is but it's actually very simple to do I have done it to a number of cb's that I've used on 10 FM.

If your interested I can send you the info for the conversion the limiter/discriminator for receive only requires one chip that's used for a TV's receiver audio detection (uPC 1028H) and for the transmitter only a pre-emphasis circuit is needed which is just a cap/resistor and a potentiometer to set the deviation.