In a previous post, I briefly discussed a way to convert an AM-only crystal-controlled CB radio into a 10 meter QRP CW rig with some additional features. Now that I've discussed the theory, I've written this post to track my progress and give you some information should you want to do the same.
To Recap (No not that kind of recap, thankfully!)...
In a nutshell, the CW CB's transmitter is turned on and off via means of shorting the PTT to ground. On many CBs, the PTT pin may also be referred to as the TX pin. When we short the PTT to ground without a modulation (speech) source, we get an unmodulated carrier. For us hams, that's CW!
On the receive side of things, we have to add only one thing - a BFO, or a beat frequency oscillator. This will allow us to hear SSB and CW signals clearly. In practicality, a BFO injects a carrier into the intermediate frequency (IF) stages to add back the missing carrier that SSB lacks. To further clarify CW reception, we can add a low pass filter that filters out frequencies above 800Hz.
We will also need to change the CB's frequency configuration. If you have a crystal-controlled radio, it's easy - just take out the Channel 1 crystal and add in an Si5351 board! If you're using a PLL-controlled radio, usually a 40 channel radio and a few 23 channel ones, you'll want to replace the VCO/PLL crystal with the Si5351. These crystals usually run at 10.240MHz, and using an Si5351 with the radio on Channel 1, you'll need to replace it with a frequency of 10.635MHz. This should set you right at or around 28.000MHz. For the sake of staying on track, we will only focus on a crystal-controlled CB.
***A word about PLL CBs - replacing the 10.240MHz crystal with a common 11.0592MHz crystal will get you in the middle of the 10 meter AM and the start of the FM windows, so if you don't want to make a CW transceiver, don't worry - you can do this mod instead! This mod is easy to do and you'll only be replacing the PLL crystal.
NOTE: I am happy to announce that I did NOT need to recap the entire radio, but if you have one you need recapped, get in contact with me. I have the 13-853's schematic with the caps highlighted and replacement information on a text document.
The Transmitter
AM is as simple as it can get: A carrier wave is modulated by its amplitude, that is, its power is varied by modulating it with something such as a voice, a tone, etc. SSB works in the exact same way except that it is missing the carrier and instead is modulated directly through the tone, voice, or anything else.
So how can we transform an AM radio into one that sends CW? It's easy - just key the PTT. That's it. We send an unmodulated carrier via the on and off keying of the PTT. Usually this happens when the PTT pin is shorted to ground, but I have seen other types of PTT keying, so double check your radio! To keep the mic socket free in case you want to use AM, you can add a 3.5mm jack to the back of the radio with wires soldered to the TX and GND pins.
Below is an easy optocoupler keyer for your computer if you don't want to key manually:
The Receiver
The receiver does not need to be touched at all. In fact, we don't even need to attach anything physically to any of the stages at all! We can create a simple BFO using one of two ways.
To create the BFO using our already-installed Si5351, we can just add code to our sketch that outputs a clock on 455KHz. Make sure you use CLK1 or CLK2 as the BFO's output! You'll want to feed the output into a terminating resistor situated somewhere near the IF stages. It should, more specifically and literally, "float around" near these stages.
Another way is to build a separate 455KHz oscillator. In a previous post, I built a QRP transmitter using a 74HC04. In a nutshell, it's no more than a Pierce oscillator, and a stable one at that. In fact, this circuit can just "lay" next to the IF circuitry and its output can be attenuated by a series resistor on the output, with an unconnected lead wire as an "antenna." This should be simple enough that anyone can build it! You won't really have a clarifier, but tuning around using the VFO on a small frequency step should make up for this, such as in 1 or 10Hz steps. Here's a 455KHz oscillator I found online that will work just fine:
You can add on a trimmer capacitor to adjust the resonator's frequency, should you need it. 120pF is the closest value for size practicality but should work just fine. For a VERY stable and clean BFO, you could try connecting all of the NOT gates together, though it's not really necessary.
For X1, you'll want to use one of these. You can find them on Amazon, eBay, or Aliexpress for no more than pennies a piece. These are known as ceramic resonators, and unlike crystals, can be pulled with a small trimmer cap over a wide range of frequencies:
The key to installing and adjusting this BFO is to inject its signal at a strength that doesn't overload the receiver. Too much input from the BFO and you'll wipe out the receiving IF stages completely, leaving you with a deaf receiver. Try to find a happy medium where the receiver can detect a CW signal clearly. Again, I have found that resistive attenuation works okay, so on the 7404's output, place a 100 ohm or 1K ohm resistor on the output, leaving the other leg as an antenna, placed and pointed somewhere near the IF stages. You can also add a small wire as a lead if you need more length. If you need more attenuation, use a higher value of resistor. I wouldn't go any higher than perhaps 10K, but your situation may be different and you might need 100K or something like that.
For the Si5351 option, run a small length of coax from the clock output and terminate the other end using a resistor of an appropriate value for attenuation if desired. Fasten it down relatively close to the IF section.
This BFO is for IFs that run at 455KHz. If you have a different IF such as 465KHz, you may want to stick with using the Si5351 so you can customize the frequency. Since my radio is a heterodyne design, it uses 455KHz as the IF in the receiver.
CW audio filter
I think it would be more than beneficial to add on an audio filter to filter down the receive audio. This will limit the frequency response so that you can better hear CW tones. Ideally, you need a low pass filter that caps off around 800Hz or so, since CW zero beat is around 600Hz - this is objectionable however, as some operators like higher or lower pitches. 1KHz may also fit well for your liking. Installation is as easy as soldering a capacitor across the speaker terminals and a resistor in series with the positive lead of the speaker. For about 800Hz (723Hz, to be exact), aim for a 22k resistor and a 10nF capacitor. This radio also comes with a pretty decent ANL circuit, so this should help as well if the band is beyond busy.
One other option to consider would be using an ACTIVE CW filter in comparison to a passive. This is easy to build and requires a few capacitors, a few resistors, and an amplifier such as the LM741. This design will create a bandpass filter instead of a lowpass, so the target frequency we are looking for will be either 700 or 800Hz. This would also provide you with just the desired tone you'd like to receive, and will filter out signals above and below the filter's passband. More on that and calculations can be found here: https://k7mem.com/Fil_CW.html.
CW Sidetone
This is an AM-only radio, therefore it has no CW sidetone. However, one can be constructed using a 555 timer and some other components. Peter, VK3YE, has a very clean-sounding circuit that is readily available online:
This sidetone is operated via RF, meaning that it beeps only when it detects RF. It should be placed at the main source of RF, such as close to the antenna output. A wire antenna wrapped a few turns around the coax should do the trick just fine. Another similar circuit is shown below:
Another option you can use is a field strength meter. It can be audible like above or, if you are hearing impaired, it can have an LED indicator. Heck, it could even have both! There are many simple FSM circuits online, so don't be afraid to build one! If you don't use it, you'll have an extra useful test equipment tool!
Detailed Steps
First, we need to remove one of the CB's crystals. I chose to do the 23.290MHz crystal since that's what CircuitSecrets used. It corresponds with Channel 1 on the 23-channel dial. If you remove a different crystal, you'll need to redo the math, depending on the crystal you choose. I did decide to solder in a socket should I decide to remove the VFO and restore the radio back to its original CB functionalities.
Next, I uploaded the sketch that would allocate me the entire 28MHz band. The most current has yet to be uploaded on my GitHub page. I have confirmed that it works on both Arduino and RP2040-based boards. Pushing the encoder's button will change the frequency step in 10Hz, 100Hz, 1KHz, 10KHz, and 100KHz steps, and for ease of access, I have a line on the OLED that shows you how off frequency you are should you choose to operate in small step sizes, so it's kind of like a clarifier!
The Si5351 is fed into a pre-existing capacitor on the bottom of the channel switch board. My best guess is that this crystal was off frequency somewhat so the original owner soldered one in since the trace on the board is cut. It's around 30pF (printed code of 30), but should be good enough for decoupling. I fed the VFO into the radio with 50 ohm RG174 cable, with the SHIELD and CENTER soldered on the Si5351 end and the CENTER ONLY soldered to the radio's crystal side. I did this to prevent ground loops and unnecessary phase noise in the system.
On the receive end, I whipped up a simple BFO using the 74HC04 circuit I mentioned earlier and I will also test one of the 5351's outputs. Its output is fed into a 1K ohm resistor which provides a sufficient level of attenuation that doesn't overload the receiver. In addition, I added a small wire "antenna" to the output. I tied the antenna around one of the IF section's transformers to provide localized indirect injection.
Originally, I found that none of my oscillator circuits worked when prototyping. In fact, I ran into this problem with the PeaNOT too. The Si5351 did, but it seemed to overload my shortwave receivers on AM-only, meaning that attenuation was more than required. The simple answer to this is not to build BFOs or any oscillators on breadboards should you decide to build your own. Some you can, some you can't. I'm not sure why but that's just how electronics work :)
Before putting it on air, make sure to check that the frequencies are on the dot or within a reasonable tolerance. You can use a generic PLJ-8LED frequency counter or any other counter to set this up. Just wrap a bit of pickup wire around the coax and feed the pickup wire into the counter, or build an RF power tap. Remember to also calibrate the Si5351 properly using the Etherkit sample sketch.
For the sidetone, I wanted a non-invasive circuit with a nice tone. I just wired the output to the built-in speaker, but if you use headphones often, you can wire it to the headphone jack. I may do this later.
I 3D printed a project box I found online and modified it to accept the OLED and the rotary encoder(s). The coax and power wires come out of the back of the box for a sleeker design which minimizes interference. To prevent any shorting internally, I placed a piece of electrical tape over the VFO's soldered connection on the crystal switch.
Finally, I installed a key jack. I soldered it straight to the microphone plug on the GND and TX/PTT lines. This accepts a straight key only, so if you were to use an iambic paddle or bug key, you'll need an external keyer. See my older posts for an Arduino-based keyer that's both easy to make and simple to understand from PA3HCM.
Stay tuned for pictures in the next iteration of this project page!
So...What have we learned?
Ham radio for the budget-minded ham is well within reach, regardless of your license privileges. For less than $100, you can get on the air on 10 meters. Sure, this rig won't be as crisp and clean as any Icom, Yaesu, or Kenwood, but it's enough to get someone on the air without having to use a Pixie kit or other appalling kit. It caps out between 2 and 4 watts, but on CW, that's enough to get you across the nation, continent, and the world! Truly QRP and truly portable!
Adding a VFO to a CB radio is also quite easy and it teaches you a lot about coding and computerized hardware. And it also shows that an Si5351 is a viable replacement for crystal oscillators. Our radio candidate, a Midland 13-853 23 channel CB radio, was an easy and cheap find. In fact, 23 channel crystallized radios are probably the best and the easiest to convert to 10 meters or other nearby ham bands (12 or 15 meters too, I think!), though PLL 40 channels will also work too.
Finally, converting any AM-only transceiver to a 10 meter CW transceiver is VERY easy - just short PTT to ground or however your mic activates the carrier without modulation. Then, find a way to synthesize your desired frequencies. Construct a BFO and place it near the IF stages and bam - CW transceiver on a budget!
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