In a previous post I described the QRP transceiver project found in an old QST magazine. I had bought that issue at a radio store while I was studying for my Novice license, and still had it stashed away in my library even after moving five times.
The prototype as described by W6DMK (sadly, now a SK) was built in a rather unique project box (no longer available) which allowed the top, bottom and side panels to be removed for access to the internal chassis or circuit board. The front and rear panels attached to the internal chassis. In a previous post, I described how to build a similar type of enclosure from printed circuit board material using an old one gallon paint thinner can as the outside. However, I found a ‘used’ 7″x5″x3″ Bud or LMB minibox in the junk box, the same size as the enclosure used in the article.
This mini box is of the type made from pieces of sheet metal, each bent into a U shape such that the two nest into and around each other to form the enclosure. The larger ‘outer’ part would become the bottom chassis part, and the smaller ‘inner’ part the upper ‘lid’ part of the enclosure. Both of these parts had a few holes already drilled into them from its previous use. The bottom had three holes in what would be the back panel. Two of these were drilled out larger to accept a 1/4″ phone jack (for the key), and an SO-239 coax connector for the antenna. The third hole was just the right size for a Phono jack that would be used for the +12 volt power input.
The top of the box had quite a few holes of various sizes. These were filled in with JB Weld metallic epoxy. I first cover the inside of the holes with a piece of metal foil (I used copper foil, but aluminum ‘duct tape’, or heavy duty baking aluminum foil attached with epoxy will also work. The foil is pressed down with a burnishing tool to remove any bubbles. The epoxy is then mixed up and applied to the outside. A putty knife or an old credit card is used to apply a thin layer over the holes, attempting to keep any voids forming in the surface. It takes the JB weld about 24 hours to harden enough to be sanded. This is done using medium grade sandpaper wrapped around a small chunk of 2×4. The surface is sanded down until only the epoxy filling the hole remains. If there are any voids in the epoxy fill, a second layer is applied, and again sanded, this time with a finer grade of sandpaper. If necessary (usually only for very large holes) a third application of JB weld is applied.
After the holes had been filled in, I drilled any additional holes required for my use. Then it was time to paint. The surfaces of the box were rubbed with extra fine grade steel wool, and then wiped clean with vinegar (a mild acid which helps the paint to stick). If the box is to be painted a dark color, I would first spray on a light primer coat. I used white paint for this on the inside top part which was later painted a flat black. The bottom chassis was painted white, two coats were used.
Here is how the chassis box came out (I actually did this several months ago):
The labels are made with a Brother P-touch label printer.
I mentioned in my last post that I was entering the schematics into Kicad. I have made some changes to the original circuit, mostly to use parts I already have, instead of parts that are no longer available. The VFO and BFO circuits were the first to be built and tested. While the QST prototype was constructed on a single large circuit board (the author didn’t provide a PC artwork, but pointed out that his first prototype was built on a vector board), I prefer to divide up the circuitry into smaller modules which can be tested on the workbench and then built into the finished rig. This is how Charley Morris ZL2CTM builds the rigs he features on his YouTube channel. He builds small modules onto strip type vector board, and then solders these modules onto a larger un-etched piece of PC board material. This is what I intend to do.
VFO-BFO (pdf download)
The VFO and BFO circuit schematics are shown above.
Here is the VFO. The two transistor circuit is built onto a small piece of vector board which is soldered to the rest of the parts attached to the back of the VFO tuning capacitor. This 365pf TRF type variable is mounted to the front panel by two threaded spacers. The VFO is quite stable, it only drifted a few tens of Hertz during an hour of being powered up.
The BFO is built on another small bit of vector board. Wiring and parts are on both sides of the board. You can see the small wire ‘feet’ that will be used to solder the board to the main PC ‘chassis’. The wire feet hold the board above the PC ground plane so it won’t short out to it. Sockets were used for the HC-49U crystal and transistor. This will allow me to try several crystals to find one that will match the two in the receiver IF filter.
keyer (pdf download)
The transmitter keyer circuit is shown here. The original used a Germanium PNP transistor to key the +12 volt power to the transmitter. I’ve converted that to use 2N2905 Si transistors, and I’ve added a switch for the RX +12v as well. I intend to use a diode signal switch between the antenna and the receiver front end to provide for QSK. I tried to use the transmitter power switch as a logic inverter for the receiver power switch, but there was too much collector to base feed though from the RX switch that appeared on the TX power line, it would not go below 2 volts on key up! I therefore added a third transistor to the circuit.
Here you can see the construction of the keyer switch. This time I used a pad cutter to create ‘islands’ of copper on a small piece of circuit board. The cutter was purchased from Harbor Freight. It is sold as a spot welder cutter. By overlapping cuts, I can create islands smaller than the size of the cutter.
AF-Amp_Sidetone (pdf download)
Finally, here is the receiver final audio stage, and transmitter sidetone. The original QST version used an unijunction transistor oscillator for the sidetone generator. These devices are now as rare as hen’s teeth, so I substituted a two transistor multi-vibrator circuit. The transistors are old Germanium devices salvaged long ago from some surplus computer circuit boards (maybe from old IBM 360’s, who knows!). I added a volume control to the sidetone, something that was missing from the original.
Again the pad cutter method was used, this time with a home made cutter made from a hex cap head bolt. To make this cutter I drilled out the cap head of the bolt to make the walls thinner, and then cut 4 ‘teeth’ into it with a Dremel cutoff wheel. While this type of pad cutter worked, it doesn’t stay sharp long, and soon starts to rip the copper leaving just large holes instead of islands (you can see two of these failures in the photo). The HF tool works better, but makes larger islands.
The Keyer switch and the audio board will both be mounted with double sticky tape to the rear and front panels respectively.
Next, I’ll get to work on the receiver circuity. With the receiver working, this project will really come to life!