A NorCal Project ...
The SMK-1 on 20 Meters

by Wayne McFee, NB6M

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In the photo above, some of the changes made in order to make the SMK-1 a viable 20 Meter transceiver can be easily seen.

A One Watt Mod has been added above the PA section of the main board. A 10 DB RF preamp for the receiver is built on the inside of the left panel of the PC board case. The circuit on the underside of the lid is an active audio filter. Difficult to see in this photo is the surface mount Tick keyer chip that is epoxied to the underside of the stereo paddle jack.

In addition to the RF Gain, Rx Tune and Tx Tune pots on the front panel are the toggle switch on the left, for switching the audio filter in and out of the receiver, and a push-button switch on the right for programming the keyer.

A pair of holes was drilled in the right side panel, even with the bottom of the new PA transistor, and two holes drilled in the lid, to allow for airflow around the 1 Watt PA.

I had wanted for some time to give the little SMK-1 a try on 20 Meters, and having experimented with and implemented several "mods" that have improved the usefulness of the little rig, I decided it was way past time to see what it would do on the "Big Boys’" band.

Sure, the SMK-1 has long been "out of print", so to speak, as the kit has been unavailable for some time. And, its simple transmitter and receiver circuitry cannot compare to a full-featured rig. But, it is very surprising how well one can do in making contacts with stations both near and far with this little rig, and doing that with something as simple as this is much more exciting and rewarding to all of us in QRP.

Since the SMK-1 is nothing more than a combo of the Tuna Tin II transmitter and MRX40 receiver, and those seem to be timeless in terms of their ability to provide a lot of fun for very little effort and cost, and since there are a lot of SMK-1s out there that may not have been on the air in a while, perhaps a band change or update to a more satisfying frequency range, better receiver sensitivity, or higher transmitter power output would make operating that little rig even more exciting than when it was first offered as a kit.

My objectives in putting the SMK-1 on 20 Meters were to achieve a useful frequency range in both the transmitter and receiver, hopefully the same or near the same in each, that covered the 14.060 Mhz QRP calling frequency; as stable a transmitter signal as possible with enough output to guarantee many contacts; enough sensitivity in the receiver to hear QRP signals well; and to make the band change by using as many of the parts that were shipped with the kit as possible. These goals were met.

Constructed with those guidelines, the 20 Meter SMK-1, using single crystals in the transmitter and receiver VXOs, covers approximately 14053.5 Khz to 14061.5 Khz. Signal reports on the air have been very favorable in terms of stability, and the transmitter, with a hot 2SC799 in the "easy one Watt Mod", puts out almost two Watts.

To help stabilize both the transmitter and receiver oscillators, and maintain the low parts count, 6 Volts regulated was routed from the on-board 78L06 IC to the tuning circuits. In order to achieve a transmitter tuning range equal to the receiver's, the transmitter oscillator was changed to the configuration outlined in my previous work on extending the transmitter's tuning range. It is necessary to cut two PC board traces in order to accomplish the change.

A 10 DB RF preamp was incorporated into the receiver in order to boost its sensitivity to an acceptable level on 20 Meters, while still retaining the same tuned input circuit and diode T/R switching so as to keep the operation of the little rig smooth and simple.

An added benefit of using the already available 6 Volts Regulated to supply the tuning circuits is that only one additional PC board trace needs to be cut, and one extra wire is run from the top of C15 to pin 1 of the 78L06 IC. A comparison of the tuning range in my unit between using 6 volts or 9 volts on the tuning circuits gave surprising results. Very little difference was noted.

A wider tuning range can probably be realized by using larger value inductors in the two oscillator circuits than the 4.7 uH units that I used in this first conversion. I had two surface mount inductors of this value available, even though there is only one in each kit, because of my modifications to other SMK-1s. Further increases in the inductance values would certainly increase the tuning range, but may pull the upper frequency limit below the desired coverage of the 14060 Khz QRP calling frequency.

Although I used all Silver Mica capacitors in the transmitter's output filter, a variety of high quality, 100 Volt rated capacitors can be used with good success. A prime example of why good quality capacitors must be used in the output filter is that in my unit, when I was using lesser quality caps in the output filter while experimenting with it, and while using the 6 Volts regulated to supply the tuning circuits, I experienced a small amount of transmitter chirp, and at first thought that I was going to be forced to use a better regulator than the 78L06, because when I switched to a 9 volt battery to supply the tuning circuits, the chirp disappeared. Additionally, the transmitter output was lower than normal. However, when I went to all Silver Mica caps in the output filter, not only did the chirp disappear but the output came up to where it should be.

This seems to be a reminder that tuning circuit voltage stability and transmitter PA impedance matching and stability work hand in hand in this rig in affecting the transmitted signal note. I am sure that PA heating also has a negative effect on stability.

As mentioned in my prior work on putting the SMK-1 on 80 or 160 meters, converting the rig to another band requires just a few parts changes in the transmitter's output filter network, the receivers input tuning circuits, the crystals and perhaps the feedback capacitor values in both oscillator circuits. Because of the fact that I did not have on hand any surface mount capacitors of the required values to replace the capacitors in the transmitter's output circuit, and wanted to put a toroid coil in place of L5 in order to support the higher output of later modifications, I used all full sized parts in the transmitter output network.

Full sized parts were also used wherever needed in making the band conversion, extended VXO mod, R14 in the PA emitter circuit, now 2.2 Ohms, and in the entire receiver RF preamp, which I built on the inner copper surface of the left side of the PC board case.

 

The eight parts needed for the RF preamp are:

20 Meter SMK-1 Receiver RF Preamp and connections

Here is a picture of the RF Preamp and the receiver mixer portion of the board:

A stage-by-stage description of the conversion procedure follows:

Make the following changes in the Receiver input circuit:

  • L1, remove and save for later use.

  • Build the receiver RF preamp circuit next. It can be built on any available surface of a PC board case, or on a small piece of PC board attached anywhere, if another type of case is used. "Manhattan" style is quick and easy.

  • Solder a short piece of insulated hookup wire between the L1 pad closest to R20 and the top of the input coil of the Receiver RF Preamp.

  • Solder a short piece of insulated hookup wire between the top of the output link of the Receiver RF Preamp and the L1 pad closest to the edge of the board.

  • C1, remove, save for future use, and replace with a 120 pf NP0 cap.

  • C2, remove, save for future use. Remove C24, 390 pf, and install it at C2.

  • L2, remove and save for future use. Remove L5, 1 uH, and install it at L2.

  •  

    Make the following change in the mute circuit:

    20 Meter SMK-1 Receiver Mixer and Local Oscillator changes

     

    Make the following changes in the Receiver LO circuit:

     

    20 Meter SMK-1 Transmitter Output Network changes

    Make the following changes in the Transmitter output filter:

    VXO Changes:

    20 Meter Transmitter VXO with extended VXO mod

    Make the following changes in the Transmitter VXO circuit:

    Make the following changes to the tuning circuitry:

    Here is the circuit for the One Watt Mod:

    If you care to add the easy One Watt Mod, ...

    Gather the following parts, plus the 13T T37-6 and the 470 pf cap mentioned earlier in the changes to the TX output circuit:

    Here is a picture of the One Watt PA and output filter. Some of the changes in the VXO section can be seen as well:

    I did the mod "ugly" style with all leaded parts, soldering the necessary leads to the pads on the board. 

    Here is what you do:

    Tick Keyer

    A surface mount Tick Keyer IC was epoxied to the underside of the paddle jack. Small gauge stranded hookup wire was soldered directly to the IC pins in order to connect it to the paddle jack and supply voltage circuit. The keying transistor was mounted right in the PC board holes for the "key" connections and the rest of the keyer’s circuitry was built, Manhattan style, on the inside surface of the rear of the case.

    A 1 K resistor and .1 cap in series route the sidetone audio from the Tick keyer to the earphone jack.

    The Tick installation can be partially seen in this photo: