Extend the SMK-1's TX tuning range

Extending the SMK-1 TX Tuning Range

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Extend the SMK-1's TX tuning range (and clean up the TX note)

Never having been satisfied with the very limited tuning range of the SMK-1's transmitter, and wanting to clean up the transmitted signal's keying note a bit at the same time, I decided to change the oscillator circuit in the transmitter to a different configuration which would allow the stock tuning elements to provide for about a 4.5 Khz tuning range, and provide for voltage regulation of the supply for the TX oscillator itself and both the TX and RX tuning pots.

This involved moving some of the existing parts in the oscillator circuit to other locations, cutting a few traces to accommodate the new configuration, and adding a few parts. The result, to me, is more than worth the effort involved. After modification, the tuning range of the transmitter, from zero beat to zero beat, is from 7036.4 to 7041.0 Khz. And, as a bonus, the note is cleaner, throughout the tuning range. On the air contacts have reported no chirp.

Now, the tuning range of the transmitter equals and nearly matches the tuning range of the receiver, and makes the little rig that much more useful in terms of dodging QRM and finding a clear frequency for a CQ, or answering other calls.

Reducing the voltage applied to the receiver's tuning pot has not reduced its tuning range, and now the receiver's LO does not drop out of oscillation at the high end of the tuning range. As modified, the receiver's tuning range, from zero beat to zero beat, is from 7036.3 to 7040.9 Khz, only one tenth of a Khz different than the transmitter. In addition, since the receiver's LO frequency is more stable during keying of the transmitter, there is no false indication of instability in the transmitter.

Here is the schematic of the "Mod", including the changes to the TX oscillator circuit and the added voltage regulation both for the TX oscillator and the tuning pots for the receiver and transmitter.

Two, low-wattage soldering irons are a must for this project. Solder wick will also be needed, and thin solder with silver content is recommended. In order to modify the transmitter's oscillator circuit to this configuration, it is first necessary to remove the following parts:

R8
R9
R10
R11
C16
C17

Have a moistened cloth pad ready to wipe the removed part off of whichever soldering iron it sticks to. You may want to tape the parts to a sheet of paper, labeling them as you do so, as they may well be usable for another project.

Clean the pads for C16 and C17 with solder wick, move L4 to C16's spot and move C18 to C17's spot.

You will need the following new parts to add to the circuit:

2 9.1 Volt Zener diodes, added

2 220 Ohm, 1/4 w resistors, added

1 100 K Ohm, 1/4 w resistor, to replace R8

1 82 Ohm, 1/4 w resistor, to replace R11

1 1 K Ohm, 1/4 w resistor, to replace R10

1 120 pf NP0 capacitor, added

1 .1 uf bypass capacitor, added from top of VR2 to ground

In the following descriptions of solder pad locations, the three 10 K Ohm pots are the front of the board, the wire connections are the rear.

In order to accommodate the changed oscillator configuration, cut these traces:

The 12 volt trace between the printed copyright C and C15.
The 12 volt trace between where it branches off from the one just cut above and VR2. Cut this trace close to the nearest leg of VR2, so that the main trace is still good to VR3.
The trace between the rear pad for the removed R9 and X2, right at the R9 lettering.

The trace between X2 and C20, between the junction of the trace that goes to L4's pad and C20. We want the trace to still connect from X2 to L4's pad, but not to C20 or the collector of Q2.

Move C18 so that it connects between the rear pad for C18 and the left pad (closest to Q2) for C17. This is necessary because the output of the oscillator is now taken from the emitter instead of the collector.

Add the 120 pf NP0 cap between the base of Q2 and the left pad (closest to Q2) for R10. The base of Q2 is the contact of Q2 closest to X2, on the side of the transistor that has two contacts. This capacitor, along with the 100 pf cap that was moved from C18's spot to C17's old spot, provide the feedback necessary for Q2 to oscillate.

Solder the 1 K Ohm resistor in R10's location, with the leads cut as short as possible.

Solder a short piece of insulated hookup wire from the rear pad for R9 and the right pad for L4 (the L4 pad closest to the edge of the board). This connects X2 in series with the 22 uh choke that is now in C16's old spot.

Cut the anode leads for the Zener diodes, one lead for the .1 uf capacitor and one lead of each of the remaining resistors to about 3/16" length.

Solder the anode lead of one 9.1 volt Zener diode to the front pad for C19 (ground).

Solder the short lead of the 82 Ohm resistor to the front pad for R11.

Solder the short lead of one 220 Ohm resistor to the rear pad for R11.

Solder the short lead of the 100 K Ohm resistor to the front pad for R8.

Bring the loose leads of that Zener diode, the 220 Ohm resistor, the 82 Ohm resistor, and the 100 K Ohm resistor together, to form a junction as close to the bodies of the parts as is practical. Cut the free leads of all four parts so that there is just enough length for a good solder joint between the four. Solder the leads together.

In my rig, the four parts just described lean over towards U3, leaving a low profile of parts, but also leaving enough room between the soldered joint between the four parts and the circuit board so as to ensure that there are no shorts.

Solder a short piece of insulated hookup wire between the front pad for R11 (where the 82 Ohm resistor is attached), and the front pad for C20 (now unoccupied). This supplies operating voltage from the 82 Ohm resistor to the collector of Q2.

Solder the short lead of the .1 uf capacitor to the front pad of C15 (ground). The other lead, left long for the moment, is soldered, close to the capacitor's body, to the nearest leg of VR2, leaving the remainder of the long lead free.

Solder the short, anode lead of the remaining 9.1 Volt Zener diode to the front pad of C15 (ground). The cathode lead attaches to the junction of the nearest leg of VR2 and the .1 cap just attached there. Bend the free leg of the capacitor out at a 90 degree angle from the leg of VR2 and cut the cathode lead of the 9.1 Zener so that it just overlaps the lead of the capacitor. Trim the capacitor lead.

The short lead of the remaining 220 Ohm resistor connects to the junction of the cathode of the 9.1 Volt Zener diode and .1 cap just installed. Position the short lead of the 220 Ohm resistor at that junction and solder it in place.

Now, in order to supply 12 volts to the regulating circuit for the two tuning pots, the remaining lead of the 220 Ohm resistor must be connected to an "always on" 12 Volt source. In my rig, I ran it to pin 8 of U3 and soldered it there.

You will need to be sure the bare lead is not close enough to any other contact to short out, and cut it so that it is just long enough to form a good solder joint on top of pin 8 of U3.

If you do the same, just use a small amount of solder and check carefully for a solder bridge from pin 8 to pin 7 of U3 before applying power. If you are concerned about attaching the lead there, you can run it to any other 12 volt trace on the circuit board, or even to the power jack's center connector, if you like.

Solder a short piece of insulated hookup wire between the rear pad of C15 and the junction of the cathode of the 9.1 volt Zener, .1 uf capacitor, and 220 Ohm resistor at the near leg of VR2. This supplies 9 Volts regulated to the transmitter's tuning pot.

That completes the mod. Check your connections, again, especially checking between pins 8 and 7 of U3 for any solder bridges.

In my modified 40 Meter SMK-1, it was necessary to solder a short piece of cut off resistor lead across R14 in the PA circuit, so as to increase the output slightly, as the lower voltage output of the oscillator in this configuration meant that the output from the PA was below the threshold of drive necessary for the IRF510 in the 5 Watt Mod I had added. Bypassing R14 increased the available drive from 3.3 Volts RMS to 4.5 Volts RMS on the gate of the Mosfet, which was sufficient to produce 5 Watts of output.

You may need to bypass R14 as well, if you do this VXO Mod, so that your power output level will be what it was with the higher drive of the original oscillator configuration. I would suggest starting with a 2.2 Ohm or slightly higher value resistor in place of R14, so as to help ensure that there are no overheating problems with Q3.

Although this mod sounds a little complicated when it is described on paper, it took far less time to actually do the mod that it did to write it up. I completed the mod from start to finish in about an hour and a half, even with having to remove the 5 Watt Mod parts to get to the board itself, and then replace them after the mod was done.

The results are well worth the effort.

In addition, this oscillator configuration can be used in any of the Tuna Tin II type transmitter circuits, with similar results.

I would welcome any input as to improvements on this "Mod" or any of the others I have developed, as I am sure everyone else who enjoys either the SMK-1 or any Tuna Tin II based transmitter would as well. Please share your results and/or suggestions on QRP-L and to myself via email.

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These pages are designed and maintained by George Heron, N2APB
Page last updated: April 15, 2004