Building the Small Wonder Lab’s SW40+

This blog will document the construction of the SW40+ qrp kit from Small Wonder Labs, as well as several planned modifications.   The modifications include a digital frequency display, keyer, audio amplifier, RIT, and possibly some kind of internal battery.   The SW40+ kit was started on Feb 17, 2011.    Small Wonder Lab’s web site has several kits, including SW kits to cover different bands.   Dave’s web site is here:

While waiting on the kit to arrive, a trip to the local junk store produced an old Johnson Messenger 123A CB, which would become the donor for the case for the SW40+.    Being “old school”, I have to admit that building an amateur transceiver into a CB case almost seemed sacreligious!  (OK, true confession – I had a couple of CB’s back in the 70′s, and went by the handle “Buckeye.”)   But since my first ham station consisted of a Johnson Viking II transmitter and a Hallicrafters SX-43 receiver, the Johnson Messenger seemed like a nice nostalgic touch! 

My dad (SK) and his "shack" in the late 50's - early 60's. In 1976, he bought a Kenwood TS-120, and this (Johnson Viking II, Hallicrafters SX-43, Heathkit VF-1, Johnson Matchbox) became my novice station.

It took just a few minutes to remove the circuit board from the case.

Johnson Messenger 123A

Since I plan to add a N3ZI Digital Dial to the SW40+, I needed to enlarge the S-Meter opening in the case, and make room inside for the LCD display to mount.   I opted to use a third party LCD display rather than the one that comes with the N3ZI kit, for its smaller size.

Parts of the inside front panel were ground down with a Dremel to allow the LCD to mount.

Part of the original mounting hole for the Messenger's volume control had to be cut away for the LCD to fit.

Here the LCD display is test-fit into the opening:

The S-Meter hole had to be ground and filed at the bottom.

The N3ZI kit is a very nice and simple way to add digital frequency display to a qrp rig.    Doug’s Universal kit is available without a LCD, allowing you to use your own.  The price is only about $20.  By the time I purchased an LCD display from eBay, the total cost to add a digital frequency display was under $30!  If you order the display from China, the total price will be under $25.   The kit can be seen at Doug’s web site here:

Back of display

N3ZI Digital Dial

The display comes up! It is displaying 11mhz because it isn't hooked up to the radio yet.

The N3ZI kit and a third party LCD display. The 9V battery is only for testing purposes!

The SW40+ kit got here a couple of days ago, but I haven’t had much time to work on it yet.   This is about 2 hours worth of work.   The board is laid out very well, and easy to work on.  

About 2 hours worth of kit building. A lot of time is spent double and triple checking the parts!

Better late than never:

 I found that using a yellow highlighter on the parts placement diagram really helped me stay on track.  As soon as I installed each part, I colored it in on the diagram.   Before I started doing this, I spent way too much time looking for where I was on the board.    

Note: It took me about a half-hour to figure this out.    A little while later, I was looking through Chuck Adam’s very fine and comprehensive manual; “Building The Small Wonder Lab’s SW-40+” for a look at the toroids, and there on page 65 was this tip: “I highlight each part as I install it…”    The only difference is, Chuck highlights the schematic, and I highlighted the parts diagram.   (You can find this excellent manual on Chuck’s web page – it is a 20mb pdf file: )

Lesson learned: If you are going to build a kit, read through everything you can find about it first – there’s no sense in re-inventing the wheel! 

Not quite "paint by number" but very helpful to color in the completed areas.

 It has been quite a while since I built anything, and the parts are a lot smaller than they used to be.  I’ve picked stuff out of my teeth bigger than some of these capacitors!    I also found holding the soldering iron steady to be more of a challenge than it used to be.   The soldering job is not pretty – but so far no bridges and no cold solder joints.

About the half-way point

This is after another couple of hours.  I am taking a little extra time with this, trying to go by the “measure twice, cut once” principle.   Plus, it’s fun!

After another 4 hours of work, the board is done!   Next will be the wiring and testing.

The board is finished!

Before cleaning.

I finally got all the parts colored in on the parts diagram.  I never could stay in the lines!


I had a few extra minutes today to hook up the pots and the power.   The battery is 12v 7ah.  There is a 1 watt audio amp (Rainbow Kits) connected to the audio output, as I wanted to be able to listen through a speaker.   The temporary antenna is an indoor 20 meter dipole.  This was connected only for the smoke test.   I will attach the G5RV when I get ready to align the radio.  

Ready for smoke test - fingers crossed!

Here is the 1 watt audio amp – a very simple and inexpensive  ($7) kit.  Here is the url for the kit:

1 Watt Amp

You might have noticed that the main tuning pot is not your usual potentiometer.   It is a 10 turn pot, which spreads out the tuning a lot.   They are expensive, I have seen them for as much as $30.  I bought this one on eBay, for $7.89, which includes shipping from Taiwan!    It also took only 11 days to get to my door – not bad.

Note: If you use one of these 10 turn pots, be sure to check the pin-out.  It is different from your average pot.  On this one, 1 is in the middle.

50K 10 turn pot for $7.89 works great!

With everything hooked up, it was time for the initial smoke test.   It was better than I had hoped.   First, there was no smoke.   Even better,  I immediately heard several signals on the band.   Not bad considering the receiver has not yet been aligned, and it is hooked up to an indoor 20 meter dipole.   

Clicking on the link below will open a 1 minute video of the initial smoke test.   It will also give you an idea of how the 10 turn pot spreads out the tuning.  

SW40+ Smoke Test

2/26/11 - I haven’t had much time to work on the SW40+ lately, but I did take care of a few little fixes. First, I added a 100K resistor to the input of the 1 watt audio amp. The audio from the SW40+ was just a little too hot going into the amp, giving me a very limited range on the volume control – less than 1/8 of a turn before it was way too loud. Adding the resistor cut the signal enough to spread the volume out nicely over the range of the control.

I also replaced the voltage regulator on the N3ZI Digital Dial from a 78L05 to a 7805. This allows for more current going to the lcd backklight by changing the resistor at R3 from 100 ohm to a smaller value. The backlight is much easier to see now.

3-1-11 - The frequency was initially a little too low. So far, to adjust it, I have done the following:

(1) Replaced the stock 100k pot with a 10 Turn 50k pot. The instructions state that a 50k pot may be used. I assume this might affect the frequency range as well. I can’t stress enough how nice the 10 turn pot is in tuning across the signals. It really spreads out the band nicely.

(2) Removed one turn off of L1. I started out with 25 turns, but that put the freq too low on my board. Removing one turn to make it 24 turns raised it too high, but then it could be brought back down with capacitors at C7.

(3) I ended up using a total of 54pf at C7. I did this with a 47pf and a 7pf in parallel (both are NPO capacitors). I tried taking those 2 capacitors out, and replaced them with a single 58pf, but the additional 4pf was too much. Your mileage may vary.

(4) I added a total of 29pf to C8. I did this by soldering a 22pf cap and a 7pf cap to the leads of C8 under the circuit board (again, NPO capacitors were used). C8 (82pf) is still in place on top of the circuit board. This makes the total value of C8 111pf. Again, your mileage may vary, due to differences in construction and tolerances of your parts.

By making these changes, I now have a bandspread of around 50khz, with a tuning range of 6999.4 – 7050.0 khz.

NOTE: All of the above adjustments have changed since installing the RIT. Stay tuned for the final settings!

RIT – I built an RIT circuit for the SW40+, by using the old Small Wonder manual for the RIT kit Dave used to sell. I made a couple of changes, adjusting the resistors for the 50k tuning pot, and adding a couple of resistor trim pots to allow for centering and adjusting the frequency swing. The RIT worked first time (yea!) – but the bench is getting unruly!



Here is a link to the RIT article at Small Wonder Labs: RIT

And here is a photo of an RIT with the trim pots, as posted by Neal, W3CUV on the Yahoo Group SWkits.

Circuit showing use of trim pots. (As posted by Neal, W3CUV on SWkits).


Here is the SW40+, the Digital Dial, the Audio Amp, and the RIT.    It’s just about time to start installing the boards into the Johnson Messenger case.

SW40+, Digital Dial, Audio Amp, And RIT


ASSEMBLY:  I placed the boards on a piece of Lexan I picked up at Lowes.   It has been cut to fit the inside of the Johnson Messenger.  

Boards Mounted To Lexan



Just a few things left to do: Mount RIT pot, and RIT toggle switch, work on sidetone (it is not consistent) and figure out why the Digital Dial wanders during transmit.  The vfo is solid, but on the lcd display, all digits wander during transmit.  

NOTE:  After going back over the wiring, I realized that the braid on the coax running from R17 on the SW40 to the Digital Dial input had not been connected (on the SW side).   After tacking down the braid to ground, the wandering stopped.

Mounted on Lexan inside Johnson Messenger


Front View


Close Up Of Front Panel



Ok, it’s not even together yet, but I had to do a test.   I listened around this morning (3/5/11) and heard a nice strong CQ on 7042.9.  I answered it, and N4LQ (Steve) came right back to me from Charlotte, NC.   He gave my one watt (give or take a few mw) a 559, and commented that the tone was good.    Steve was solid copy on the SW40+, the vfo never had even a hint of any drift, and it was an enjoyable qso.   All except for the part about it being my first CW contact in many years, my fist was not so hot on the straight key, and my ability to copy matched my fist!   But I copied enough to be able to copy what he said 99%, and had a blast!   This rig might be what it takes to light my fire in ham radio again!


Here is a sad but true lesson learned the hard way:   I fiddled around with C7 and C8 and L1 until I got the tuning range exactly where I wanted it.   Everything was perfect. 
 But then as I started working on the power output, the rig started oscillating!   Not the transmitter, but the receiver.  When I turned it on, it was ok at very low volume, but as soon as I turned it up, it oscillated!  So then I listened to the transmitter output again in the TS-440.  My nice perfect tone was gone!  When I keyed the SW40, it went Woooop  Woooop  Wip Woooop….  I thought – those new caps I added (NPO) must be pulling the vfo somehow.   So I started taking them off and undoing what I had done.   It didn’t matter what I did, the SW still oscillated, and the transmitter still sounded terrible.   So I took L1 out again, and rewound it as it had been, and put all the capacitors back as they had been, and the thing STILL oscillated on receive and sounded terrible on transmit!   So then I started looking at the circuit board under a magnifying glass, looking for anything I might have accidentially bridged while putting parts in and out… nothing!   This morning it sounded great – I got the bandspread just where I wanted it – and then all of a sudden it started oscillating and sounded terrible!
Once everything was put back the way it was,  a lightbulb finally went off in my head…   I have been running this radio on the 12V 7ah battery from the very beginning.   This includes all the time in testing and tuning it, as well as tuning around on the bands,  calling CQ a few times, and working 2 guys on the air.   So I measured my battery voltage… 9.75 volts!  The SW is designed to run on 12-15 volts, and  I was feeding it with less than 10 volts!   So I take the battery off and put it on the charger,  then hook up a power supply to the SW, putting out 12.5 volts… Guess what?    The receiver oscillation is gone, the transmitter sounds wonderful again, and all is at peace in the world!    Now I have to go back and fix my bandspread again, rewind L1 again, etc!
LESSON LEARNED:  The SW won’t run without power!   

An hour or so later,  L1, C7 and C8 are back to where they were.   The tuning range is now 7001.4 – 7122.5 and the transmit sounds good.

Next the RIT pot was mounted to the front panel, along with the toggle switch to turn it on and off.    About all that is left is to hook the speaker up to the earphone jack, and install a power jack on the rear panel.   I have decided to build the keyer in its own enclosure, so I can add switches for the memories, and I have given up on the idea of trying to install an internal battery.  

RIT and Toggle Switch Installed


COMPLETED!     I finished the SW40+ today – one month to the day of starting this project.    The power connector and headphone jack were wired, the Lexan was  fastened to the case, and the case was given a bath and a light coat of Armor All.   

A video of the SW40 in action can be seen here: VIDEO  


Completed One Month To the Day!


The next project will be building the K12 Keyer and a homebrew paddle…


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7 Responses to Building the Small Wonder Lab’s SW40+

  1. Allen Woolfrey says:

    Thank you. I am about to start assembly of an SW30+ with an N3ZI digital dial and a K1EL keyer, so your article is quite timely for me. The RIT looks interesting, so I’ll consider that.

    I look forward to your next post.

    72, Al – VA3IAW

  2. Jim says:

    Thanks, Al. The keyer project has changed slightly, and will be built into a Ten Tec single paddle enclosure. I have an SW20 on the way, and my next planned project is to build an SW20 and an SW40 in the same box – an old Pierce-Simpson CB case. That will be the subject of a new blog shortly. These SW’s are great kits!

    Thanks again for stopping by!
    72, Jim

  3. Allen Woolfrey says:

    Interesting. Do you plan to use the same front panel controls between the two rigs? If so, I’m interested to see the details.

    BTW, my plan is to put the N3ZI Frequency Counter and the K1EL keyer into the same box to be able to share them across rigs.

    72, Al – VA3IAW

    • Jim says:

      Yes, the two rigs will share the same tuning, rf, volume, and rit pots, and lcd display. I plan to borrow an idea from N2UHC’s “MultiWonder” ( and use the switch out of an old printer a/b switch to switch the different rigs. The SW20 has arrived, and an order will be placed for another SW40 shortly. The Pierce-Simpson case is a little on the large size, but this project isn’t so much for portability as convenience. The old CB cabinet has a regulated power supply built in, as well as provision for an external battery. I hope to get the project started soon.

      Nice idea on putting the N3ZI Freq Counter and the K1EL keyer in the same box! I originally planned to build the K12 in the SW40+, but after running across the TenTec paddles with that nice big empty case, I decided to put it there instead.

      72, Jim

  4. Allen Woolfrey says:

    I quite like the result, and I’m tempted to do this myself, but the work looks daunting. It would be nice if a DP3T switch would do it. I’m not in a position to know. I wonder what Dave Benson would say.

    Cheers, Al – VA3IAW

  5. Allen Woolfrey says:

    Your page has been a helpful reference and inspiration to me.

    I have finished the board and am installing the connectors (including a 10-turn pot) and getting ready for alignment.

    If you have time, I would appreciate your advice on the following:

    1. Where/how did you connect the N3ZI frequency counter? I have seen more than one answer to this question.

    2. What resistor values (R101, R102, R103) did you use for the RIT to accommodate the 50K pot? (I may add RIT later.)

    3. Did you have to make any other changes to use the 50K tuning pot? The SWkits site states that you shouldn’t need to, but I wonder if that was your experience.

    72, Al – VA3IAW

    • Jim says:

      Hi Allen,

      Sorry it took me a while to answer. I have been “under the weather” for a while. The N3ZI freq counter is connected to the top of R17. I works great there, though I still get a little bit of the display “wandering” during transmit. The actual frequency doesn’t change, the digits just wander. I’ll take a look when I get back in the ham shack to see which resistors I ended up using. I just scaled them according the the manual that Dave had for his original RIT. I didn’t have to do anything different for the 50k linear pot, though I did have to play with the capacitors until I got the bandspread and freq range I wanted.

      Thanks for the kind words, and I hope your project is coming along well! 72, Jim AD4KB

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