CB to CW Part 3: The Final Installation

 I graduated college!!! Now what am I going to do?

10 meter CB revisited

First things first, I think I'll resume my 10 meter CB conversion. 

If you recall, I have the bright idea to convert an old 23-channel AM-only CB radio to run CW on 10 meters by keying the PTT on and off and receiving SSB/CW using a BFO on the IF section. I won't use any audio input or modulation since we're only looking for carrier generation only. In order to do this, the TX pin needs grounded when in transmit (key down) and then the RX pin will need to be grounded when in receive (key up). Simple, right?

Actually, yes! 

The CW Relay or Logic

A relay is no more than a SPDT switch that is automatically switched over when a current passes through a coil that moves a toggle. On one side of the relay lies the coil, the other side contains three pins - NC, Common, and NO. NC means normally closed, and this is always switched to common when the relay is off. When the relay turns on, it switches within milliseconds to normally open. By using this principle, we can see that NC will be connected to RX, NO will be connected to TX, and Common will be connected to ground. In the case of the Midland 853, we will only need to use Pins 2 (ground), 3 (RX), and 4 (TX) on the mic connector

Any standard relay will work, but I will be using the inexpensive ones you can find on Amazon or Aliexpress. If you're building a similar rig, you need one that is rated at 12V with a minimum current rating of 1 or 2 amps. The SRD-12VDC-SL-A relay is the perfect candidate for our need. It's cheap, mass-produced, and is widely used in other DIY projects needing relays.

The coil side is rather simple. One side will connect to +12V and the other side to ground. In between +12V and the coil terminal, we will place our Morse Code key to turn on and off the relay, or key between TX and RX.

Of course, the use of transistors and logic would be better since solid state devices tend to wear out less. At high keying speeds, relays may become fatigued and wear out within months of use. Here's a circuit I came up with that uses 2 NPNs and a 74HC04 NOT gate:

I have also added LED status lights, but these are strictly optional. The collectors of the transistors are connected to the TX and RX pins on the microphone connector. The transistors in this configuration act as switches, which are activated via a voltage signal fed into the base of the NPNs and switch on and off depending upon the logic condition. Problem solved - and with things that I already have too!

BFO

I still would like to implement a BFO using the Si5351. I think I will inductively couple it first by winding some thin wire around the IF transformer. In viewing the PCB, the IF transformer seems relatively easy to access, and wrapping it with a thin wire such as 24AWG should suffice and work well. A resistor divider added to the Si5351 output pin should help attenuate the output should you find that the output is overloading the IF stage.

Of course, if this doesn't work (I firmly believe it should work first try actually), other injection stages are certainly possible. For example, some hams have found that inductively coupling the BFO (wrapping wire along the coax line) just before the PL-259 plug works just fine. Others have found that direct capacitive injection using a small capacitor soldered into the IF stage works well, especially if a potentiometer is put between the generator and the IF section (more or less a makeshift attenuator). However you choose, make sure to experiment!

VFO Mounting

I decided to mount my VFO inside of an enclosure that will mount on top of the CB radio. This will make it easier to service and will require minimal invasion and modification to the original board. The VFO coax and BFO signal line will be fed into one of the mounting holes and will be tack soldered and wrapped respectively. 

The VFO will not need a large board, and in fact I will more than likely use one of my solderable breadboards. A standard breadboard proved to be ideal for the prototype layout, so there should be little to no issue when converting it to a permanent breadboard.

Testing

Because this is a complex project, testing will be done in separate phases. First, I plan to test the transmit side of things - check that the VFO is on the right frequency, the TX/RX relay circuitry works, and that power is reaching the antenna. Next, we'll move on to the receive side - test and align the BFO, install and test the active filter, and check that incoming CW signals are properly beat against the BFO. It's not difficult, but if you install everything in one fell swoop, it will be harder and harder to diagnose problems should they arise.

Again, this design and implementation is solely for reference purposes. You don't need to stay the course with this exact design, and in fact I welcome any and all experimentation - with some limits. You know...stay in frequency limits, transmit on frequencies you can use, etc.

Legality Of This Conversion

So is this conversion legal?

Yes.

Here's why. We hams can convert any radio transmitter or transceiver to operate on frequencies that we can use. It's why you'll see several hams buy up Part 90 radios, CBs, etc. because they're so easy to modify. These radios, namely Part 90s, pass through very rigorous tests to ensure spurious emissions are null and void, frequencies are rock solid and stable, and much more. So converting a CB to 10 meters is legal.

BUT...

The other way around, 10 meters to CB, is ILLEGAL. I know there are export radios that can do this but just don't! 4 watts of CW is more than plenty if the band is wide open. 4 watts of CW is equivalent to around 15 to 20 watts on SSB - more than enough juice to make a continental contact, maybe even a transoceanic on a good day. To put it in perspective, I was heard in NYC with my MFJ Cub on 20m just putting out a little under 1.5W!