|KC8AON's QRP PROJECTS
MODS FOR THE SW SERIES QRP RIGS FROM SMALL WONDER LABS
OUTPUT POWER AND TUNING RANGE INCREASE FOR THE SW SERIES QRP RIGS
POWER OUTPUT INCREASE MOD
Some SW series rigs suffer from instability when the output power is adjusted up high with R24 (refer to picture above). The instability shows up in mine as a harsh sidetone, but I have heard of the sidetone disapearing altogether in some rigs. To cure the instability (as posted to QRP-L by Steve Weber, KD1JV) "To cure the instability problem, change R27 to 32 ohms and R25 to 4.7 K
ohms. Now you should be able to turn the drive pretty much up all the way
without any problem. Also, adjusting the spacing on the turns of L4 so that
they are close together instead of evenly spaced around the core increases
power output. You can get 3 - 3.5 watts out with these changes on 20M.
(with 13.8 volts supply)"
I did the above mods to my SW-20+, SW-30+, and SW-40+ and had good success with them and I am now able to turn R24 up much higher with no problems. But I decided to set them all at 2.5 watts out to help conserve batteries in the field. The 2.5 watts resulted with 13.8 volts applied, and with a 12 volt 2.2aH gel cell battery, I get a little less than 2 watts out.
TUNING RANGE MOD FOR THE SW SERIES RIGS
The stock tuning range of the SW series QRP rigs is about 35 khz. This is fairly good coverage and allows one to tune around a bit to find a quiet frequency to call CQ. But the tuning range can be increased by a very simple mod of changing the value of C8 (refer to picture above for location) to a higher value. The mod is even mentioned in the assembly manual that comes with the kit. The manual states that the upper usable limit for C8 is around 1000 pf, but you will not need anywhere near that much C to get a good useful tuning range. The manual also states to use NPO/C0G type caps for the mod for good temperature stability to help prevent drift.
I modified both my SW-20+ and my SW-40+ to cover more than the original 35 khz. On the SW-20+, the stock value of C8 was 27 pf, and I paralleled a 47 pf cap soldered to C8's pads on the bottom side of the board to increase the total value to 74 pf. This gave me a total tuning range of 128 khz on the 20 meter band.
On the SW-40+, the stock value of C8 was 82 pf, and I paralleled a 150 pf cap again to the bottom of the board at C8's location to increase the total value to 232 pf for a coverage of around 100 khz.
I might add that you may not get the same results in tuning range with the values that I used due to variances in parts values from rig to rig. So, it will pay you to just tack different values in place and try them to see if you get the desired result before doing a permanent mod.
I did not modify the tuning range of my SW-30+ since the band is only 50 khz wide to begin with, but it should be a simple matter to expand it to cover the entire 30 meter band by adding just a small amount of C in parallel with C8 on it too.
Finally, when you do the tuning range mod on the SW rigs, the tuning will now be way too sharp and touchy for the original single turn tuning pot. To cure this problem, change the pot to a good quality 10 turn pot and use a counter knob to help keep track of where you are on the band.