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Here is an easy to do mod for the SMK-1 that will boost your transmitter power to 1 Watt out. Your actual power out will depend on the transistor you use, but I got 850 Milliwatts out with a generic 2N3053 from Radio Shack, and 1.15 Watts out with a 2SC799 that I had in my junk box.
You need six parts to do the mod. Here they are:
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:
I suggest you read all the instructions first, to get an understanding of the mod, before proceeding with the actual work.
First, remove C-22 from the board (I used two soldering irons, quick and easy)(save it)
Cut one lead of the .01 cap so as to leave about 1/4", bend a 90 degree angle about 1/8" from the end of that lead, and solder that lead to the C-22 pad closest to Q-3 on the board. Leave the other lead long for a moment.
Cut one lead of a 100 Ohm resistor so as to leave about 1/4", bend a 90 degree angle at 1/8" from the end of the short lead, and solder that lead to the ground pad of R-14.
The idea is to have the resistor standing just about straight up, maybe leaning just a little bit towards Q-3. By the way, the ground end of R-14 is the pad right at the edge of the board.
Now, using short leads on all three parts, solder the other 100 Ohm resistor between the free lead of the first 100 Ohm resistor and the free lead of the .01 Cap.
What we are doing here is providing a coupling capacitor from the output of T-1 to the base of the Final Amp we are going to install. The two 100 Ohm resistors form a divider network so as to provide about 2 Volts RF on the base of the Final Amp. The 4 Volts or so right from the output of T-1 is way too much drive for the Final Amp and output network we are using, and if we drove it with that much RF, it would overheat and probably self destruct in a short while.
So, at this point you should have a .01 from the C-22 pad closest to Q-3 to a 100 Ohm resistor, which is connected to another 100 Ohm resistor, which is connected to ground.
Cut the Base lead of your RF transistor to about 1/2", and solder it to the junction of the two 100 Ohm resistors.
Cut the Emitter lead of your RF transistor to about 3/8", and solder it to the ground pad of R-13. Again, the ground pad is the end right on the edge of the board.
Wind 5 turns of #28 (you could use small insulated wire for this if you don't like scraping the ends of magnet wire) on the FT37-43 core.
(This is meant to be about a 10 uh RF Choke. If you don't have an FT37-43, you could use any combination of core and number of turns that gives you roughly that figure. Suggestions are: 35 turns # 22 on T68-2, or 43 turns # 28 on T50-2, to name two.)(or, use a 10 uh RF choke)
The RF choke for the new final amp sits right above R-15 and R-19 on the board. It should be oriented so that the toroid is at a 90 degree angle from T-1 in order to minimize coupling between the two. The two leads from the RF choke should be about 3/4" long.
Solder one lead of the of the RF choke to Pad 4 of T-1. This is just a convenient spot to pick up 12 volts. You could solder this lead to the anode of D-10 and maybe get a teeny little bit more output, but run the risk of destroying the new final RF amp if you accidently hook up the power backwards.
Solder the other lead of the RF choke to the Collector of your RF transistor. Check the lead lengths of the two and adjust accordingly before soldering them together.
Now, the last part, the .1 uf Cap. It goes from the Collector of your RF transistor to the C-22 pad closest to the edge of the board. Check for appropriate lead length, cut the leads, and solder it in.
Now all you will need to do is put a heat sink on your new final RF Amp transistor.
That is it.
As mentioned above, I got .85 watts out with a 2N3053, and 1.15 watts out with a 2SC799. Your mileage may vary, but this mod is just too easy to pass up.
Enjoy,
Wayne NB6M
Downloadable article and schematic
Addendum:
Just a little further discussion on why I picked pad 4 of T-1 as the spot to take 12 volts for the new final amp, rather than taking it from the anode of D-10, the 12 volt supply to the rig.
The main reason was for parts count. By taking 12 volts from Pad 4 of T-1, you already have an RF bypass capacitor to ground at that point (C-23), electrically speaking, and the circuit board run to C-23 is quite short from that point.
You also may get some benefit for low frequency bypassing by C-12, the audio bypass cap for the LM-386, which is also electrically connected to the same point.
If you were to take the 12 volts for the new final amp directly from the 12 volt input, on the anode side of D-10, then you would need to add bypass caps at the 12 volt side of your 10 uh RF choke. DeMaw (W1FB) typically used a combination of three bypass caps in similar locations. A .1 uf for HF frequencies, a .001 for any spurious VHF, and a 22 uf electrolytic for low frequency bypassing.
A similar final amp circuit in the SW-40 from NN1G uses a .1 and a 10 uf electrolytic as bypass caps at the 12 volt side of the RF choke.
The choice is yours. As a precautionary measure, or if there is any indication of need, you could add a .001 uf bypass cap from Pad 4 of T-1 to ground to provide VHF bypassing. I have not had any indication so far that this is needed, in my SMK-1.
Signal reports on the air thus far say the signal is clean. See you on the air.
72,
Wayne NB6M
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Page last updated: April
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