Pixie Ears

The hearing of my Chinese Pixie is much better now.

Pixie is a ultra-minimal ham radio transceiver that started several decades ago. The circuit has been tweaked, modified and improved over the years with the same goal of super small and simple.

The last couple of years, some Chinese electronics companies have started selling them on EBay. As you might expect, they are cheap in both senses of the word. They can be found for less than three US dollars which includes shipping across the Pacific. At that price, you can understand why the components are the lowest grade available, all from foreign manufacturers. Buyers are well-advised to double-check parts during assembly which, by the way, you have to do yourself.

Here’s mine in its original form. The board is two inches in each side and there are about thirty electronic components. (A commercially designed ham radio transceiver today will have several hundreds of components, many of them integrated circuits each containing ten thousand or more individual resistors, capacitors and transistors.)

In the middle of the second column of parts on the right you might be able to see two black semi-circles. Those are the two main transistors. One of them works with the crystal (silver part marked 7.023 nearby) and the other is the “power amplifier” that puts out less than one watt. (Those old lights your grandparents used to put on their Christmas tree were about seven watts each. A whole string might consume 100 watts or more. But this radio uses less than one.)

The two connectors on the left are for headphones and morse code key.

And the two white things on the right are connectors for power such as a simple 9 volt battery and antenna.

When I say ultra-minimal, I’m not kidding.

There’s no microphone. This is a CW-only transmitter.

No volume control. If it’s too loud, take the ear buds out of your ears. If it’s too soft, well, wait for a stronger station.

You can’t change frequency. The aluminum-housed crystal is near the upper right and you can read the frequency, 7.023 megahertz (7,023 kilohertz), that’s stamped on the top. That’s in the Amateur Extra Class portion of the forty meter band which, if you’re not Amateur Extra Class, you can’t use so most Hams have to order and then install a different crystal.

This is where most Hams start making changes.

Mine now has several small modifications. The first was to make the audio quality much better. For the aficionados, I changed the resistor between V+ and pin 6 of the LM386 audio amplifier from 1K ohms down to 47 ohms. This was an easy change because the new part goes right into the same holes vacated by the old one.

The next change, completed yesterday, was to help this tiny little radio deal with a really big transmitter a few miles away. KFNX is a commercial AM broadcast station that pumps out 50,000 watts of power about fourteen miles from my home. And even though that station is on the dramatically different frequency of 1100 kilohertz (KHz) than my little Chinese Pixie (7023 KHz), that huge amount of power just blows right in. That station is all I can hear; it drowns out everything else.

So yesterday, I replaced the Pixie’s low-pass filter with a band-pass design. Now my little radio blocks out the constant blathering on the 100% talk station.

For those interested, here are some pictures.

First, three parts are replaced. (Indicated in color in the lower-right corner of the board.)

Then, the circuit found at the end of this linked article is put it their place but fitting them in is a challenge.

In the schematic (see link), notice the band-pass filter is made up of two tank circuits (a capacitor and a coil in parallel for each) that are then connected by another coil. To keep costs down, I used cheap 10% capacitors and put a 2200 pf (that’s the abbreviation for picofarads for you non-electronics type folks still reading) in parallel with a 470 pf to get the 2700 pf value.

I then ordered some cheap T50-2 toroids off the internet. The “50” in T50-2 is the diameter of the little donut-shaped ring. That’s 0.5 inches. And I used an on-line “toroid winding calculator” to figure out that the 240 nH (nanoHenries) coils need 7 turns and the 1 uH (microHenry) coil needs 14.

To keep things small, the tank circuits are assembled by poking the capacitors into the donut hole of the toroid as you see here.

The top and bottom toroids are the two tank circuits in the schematic. I pre-assembled these by poking the capacitors into the holes and soldering the wires together so each assembly had two legs. Those legs were then pushed through the old holes in the circuit board.

The center toroid is the 1 uH coil. It has 14 turns of 14 AWG enamel-coated wire. You see it in the middle of the other two. (The outer coils, the ones in the tank circuits, have only 7 turns for wire giving them 240 nH of inductance.)

Here’s a straight-down picture of the finished modification.

Comparing this to the original (first picture), you can see it’s a very tight fit!

Electronics experts are probably gnashing their teeth at this point because, to make everything fit, I had to break several rules.

First, toroids should rarely be placed this close to each other. The magnetic fields set up by those coils of wire will be jumping up and down and, packed this tight, they’ll interact with each other.

A better arrangement would be to have the middle toroid at a right angle to the other two.

And my “poke the capacitors into the donut hole” approach subjects the capacitor to those same magnetic fields. Who knows how that will affect them?

But to make it fit on this tiny little board, none of that was possible.

And it works.

That’s the bottom line. It works.

With the original circuit and it’s three component low-pass filter, I could hear nothing except that one broadcast station. Hams call it BCI–BroadCast band Interference. It happens in cheap rigs. Remember, this baby was less than three bucks from China and that included shipping so,yeah, this is a stupendously cheap rig.

But with the new five components wired into a band-pass configuration, while that station can still be heard, it’s now well down in the background noise. Morse code signalling, when the band is active which basically depends on time of day and sun spots, comes through very nicely now.

Hurray!

I now have what I consider to be a usable radio. I measured the power output at 200 milliwatts. When conditions are excellent, I should be able to communicate with other hams hundreds and thousands of miles away. Of course, we’ll all need some pretty good antennas and, when conditions are less than stellar, I’ll be relying on them to have extremely good receivers. But if they are pumping out a strong signal, I’ll be able to hear them.

So, you’ll excuse me while I go and practice my morse code. Way back when I could do fifteen WPM (Words Per Minute) but today I’m hard-pressed to copy ten. Down in the Amateur Extra Class portion of the band where this radio transmits with its cheap Chinese crystal, most hams are cranking out 20-25, 30 and maybe even 40 WPM.

I have a long way to go before I’ll be ditting and dahing with those experts!

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