Building The Small Wonder Labs SW40+

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

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