Small Wonder Lab’s DSW

20m DSW

20m DSW

All the following is used with permission of the author. Thanks guys.

Drive level adjustment
(Mike KK5F)

Relocate C116 from above the PCB to the same holes, but below the PCB. Don’t let any of the the C116 lead or solder buildup project much above the top side of the PCB for the hole that is closest to R25. Otherwise, it will interfere with adjustment wheel of the trimmer pot to be installed.

Remove the 51 ohm resistor R25. Replace it with a 100 ohm trimmer pot. The wiper and end connection of a Bourns BP3352T 100 ohm will fit exactly into the existing holes for R25. Make the adjustment dial side face the crystal filter.

Adjusting this new drive control for 300 mA total unit current consumption will usually result in the best trade-off for power output vs current consumption. On my DSW-30, going from 300 mA to 430 mA only changed power output from 2.0 to 2.3 watts. The increase in power output for high current draw was even less for my DSW-20 and -40.

NOTE: If you do not wish to install a pot, then just measure the current draw on your DSW before modification. If it is less than 300 mA, try replacing R25 with a fixed value resistor of LESS than 51 ohms. If it is more than 300 mA, try values of R25 that are MORE than 51 ohms.

NOTE: A 100 ohm trim pot will allow adjustment of power output from a little less than 1 watt to above 2 watts.

Changing the final transistor for more power output
(Mike KK5F)

If you have a DSW with a 2SC799 as the final transistor (Q7), you may be able to get more power output by going to the other transistor that Dave at SWL now supplies (2SC1947). This is a relatively expensive part, so Dave will need about $5 each for one.

NOTE: Most DSWs originally included the new transistor 2SC1947 in the kit.

Maximizing applied voltage to the DSW
(Mike KK5F)

The reverse polarity protection diode is in series with the supply voltage. It is a Schottky diode with only about 0.3 volts lost through forward voltage drop. If you are battery powered, and wish to tweak every last bit of power out of your DSW, then wire a short across the Schottky diode D8. Install a shunt 1N4001 or similar diode from some convenient positive supply voltage lead to ground, such that the diode will conduct if power polarity is reversed. Install a fuse of about 1 amp in the supply cable, so that it will blow if the shunt diode conducts on reverse polarity.

NOTE: This method is less fool-proof than the original design, and only results in an effective battery voltage increase of 0.3 volts.

Final transistor heat sink
(Mike KK5F)

The final transistor (Q7) will get quite hot. I recommend some sort of TO-5 type heat sink. I had to scout around to find one that was small enough to fit in the cramped location of Q7. The clip on types like MFJ is supplying with their MFJ Cubs would be ideal. I don’t know if MFJ will sell you any or not. Use heat sink coumpound, and be careful not to let the sink contact local components like D9.

Standard DSW enclosure mods
(Mike KK5F)

The blue anodization that coats most of the surfaces of the standard DSW case is a good insulator. I used a Dremel tool wire brush to remove some of the coating at the mating edges of the case, the PCB standoff mounting holes, and the control/connector mounting holes. This will create much better shielding of the DSW internals, and prevent any spurious signal pickup on the IF frequency (4.000, 5.182, or 5.068 MHz). The early DSW-20 had an IF of 5.068 MHz, which Dave changed to 5.182 due to signals from the SW broadcaster WWCR at 5.070 MHz. I managed to completely eliminate WWCR signals (I’m only 90 miles from their transmitter) without changing to the new IF frequency, by improving the shielding as described. I figure that even if you don’t have a problem with WWCR, it can’t hurt to keep out other potential unwanted signals.

The standard plastic PCB stand-offs can pop back into the case from external pressure rubbing against the outside protrusion. I replaced them with 4-40 x ½” screws, using a lock washer and nut as a standoff bewteen the PCB and the case bottom. This also more directly connects the PCB to the case as a ground plane.

NOTE: 4-40 screws and the associated nut are about the largest size that can be used. Otherwise there won’t be enough clearance between the top nut and components on the PCB.
DSW sideband change

The DSW-20 and -30 receive on the lower sideband. The DSW-40 and -80 receive on the upper sideband.
The difference is due to whether the local oscillator injection frequency is above (DSW-40, -80) or below (DSW-30, -30) the received frequency. It is possible to get an intelligible signal on voice sideband with the DSW. Unfortunately, the DSW-20, -40 and -80 receive the wrong sideband for voice on the associated band. It is possible to reverse the sideband received by switching the BFO injection frequency from ABOVE the IF frequency to BELOW the IF frequency. Due to non-symmetrical crystal filter bandpass, making this change can theoretically degrade the opposite sideband rejection performance of the receiver. Thus, I have not made such a mod on my DSWs. Bruce Prior, N7RR, has developed a detailed modification for those who are interested in making this mod:
I stated mistakenly in my rave review (CQ Amateur Radio, December, 1999, p. 40) of the Small Wonder Labs DSW rigs, “The offset of all DSW rigs is set for receiving on lower sideband.” In fact, only the DSW-20 and the DSW-30 receive on the lower sideband. The 80, 40-meter versions receive on upper sideband. On the DSW-30, this feature makes no difference, since it is not a phone band. However, the design sideband is the reverse of customary sideband usage for the 80, 40 and 20 meter DSW rigs.
After some experimentation and consultation with the DSW designer, Dave Benson, NN1G, I offer the following very simple modification which reverses the sideband on the DSW rigs. This could potentially be very important to a DSW owner in a remote area who needs to contact an SSB station in an emergency. As I noted in my review, the rigs cover all of their respective amateur bands and more, but if you can t understand the SSB stations on 80, 40 and 20 meters, the extra coverage is not very useful. What is needed is to convert the DSW-80 and DSW-40 to receive LSB and to convert the DSW-20 to receive USB. These conversions can be accomplished inexpensively by adding a single fixed inductor. Since the DSW-30 and DSW-40 both have an IF of 4 MHz, the modification is the same. Similarly, the DSW-20 and DSW-80 both have a 5182-kHz IF.

The identical added inductor reverses the sideband of those rigs in opposite directions, however.
The only reason I can imagine that a DSW-40 owner might want to give this modification a miss is that the Canadian time-signal station CHU on 7335 kHz transmits its French and English voice signals on USB. WWV and WWVH on 10 and 15 MHz transmit on AM, so the modification will not change your ability to copy its signals on the DSW-30 and DSW-20. In addition, the modification broadens the audio bandpass a bit, so CW reception is slightly less selective. With the modification, SSB audio reception sounds somewhat pinched, but it is quite understandable. In each case, all you have to do is to place an inductor in parallel with the trimmer capacitor C14.

All DSW-40 and DSW-30 kits include a grey 70-pF trimmer for C14. Early model DSW-20 and DSW-80 kits have the same grey 70-pF trimmer. In current model DSW-20 and DSW-80 kits, however, C14 is a black 90-pF C14 trimmer.
Check the color before choosing your inductor. Here are the appropriate inductor values:
DSW-40: 15-uH inductor
DSW-30: 15-uH inductor
(Note: since 30-m is not a phone band, there is no point in modifying the DSW-30 for USB reception.)
DSW-80 with a grey 70-pF trimmer capacitor C14: 8.2-uH inductor
DSW-80 with a black 90-pF trimmer capacitor C14: 6.8-uH inductor
DSW-20 with a grey 70-pF trimmer capacitor C14: 8.2-uH inductor
DSW-20 with a black 90-pF trimmer capacitor C14: 6.8-uH inductor

To install the inductor in parallel with C14, solder it to the two C14 pads on the bottom of the circuit board. The neatest way is to use a surface-mount inductor. I had a tubular inductor with axial leads available which I was able to bend around and tack onto the circuit board. After soldering on the inductor, re-align the offset pitch by adjusting C14 to match the sidetone. You obtain the offset pitch through your earphones by applying power while holding down the front-panel keyer button. Then depress your key or keyer paddle to give you the comparison sidetone pitch.
This modification will change the 80 and 40-m rigs to receive LSB and the 30 and 20-m rigs to receive USB.

For the surface-mount option, the 70-UMC1812 inductors for the proper values from Mouser would do fine. Dave Benson recommends the following Delevan surface-mount inductors, which are available from Digi-Key:

15 uH: DN12153JCT
8.2 uH: DN12822JCT
6.8 uH: DN12682JCT

The tubular axial-lead 70-IM2 inductors available from Mouser are bulkier and a bit trickier to solder beneath the circuit-board. Now you can use your DSW rig for cross-mode QSOs.

Off-frequency boot up
(Mike KK5F)

This is not a mod, just information.

The DSWs will sometimes boot up slightly off-frequency for transmit (by about 0.5 kHz). The receive frequency is OK. ANY turn of the frequency encoder, or even flipping on and off the RIT will cause the transmit frequency to correct itself. This effect is thus usually not noticed unless one begins transmitting after power up without making ANY frequency adjustments. HOWEVER, there is a consequence when in CALIBRATION mode (power up with the keyer mode button depressed). When you are trying to adjust the receiver offset, if the unit booted up with the slightly incorrect transmit frequency, then your receiver offset adjustment will be off by however much the transmit frequency is off. Powering up, then momentarily pulling and re-inserting the power plug to quickly cycle the power (all the while pushing the keyer mode button) will get the unit to initialize the transmit frequency correctly. The offset adjustment can then proceed.

Inability to adjust the offset to match tones in CALIBRATION mode
(Mike KK5F)

First, refer to the info in item above.

If the offset still can NOT be adjusted to match tones, then you probably have an early DSW-20 with a grey 70 pF offset adjustment capacitor (C14). Dave Benson will supply you with a black 90 pF replacement.

(Arne SM4INV)


I found the passband of the DSW-20 a little too wide when the band is crowded so I decided to make a simple filter to narrow it a bit. This is what I came up with:

The inductor is a Schaffner current compensated choke on 2 x 39 mH but due to the mutual inductance and the close coupling of the toroid core the inductance is almost quadrupled if the two windings are properly connected in series. There are two ways of connecting them in series – if you did it the wrong way the inductance will be close to zero. You will notice if you have done that!

You can of course use another inductor/capacitor/resistor combination as long as the circuit istuned to about 800 Hz whitch is the center of the DSW-20 passband. If the resistor value is tohigh (or omitted) the filter will be very narrow and start to ring. If you on the other handmake the resistor value too small you will also lower the gain of the rx.

Try it, I think its a nice addition to a fine rig! 72/73 de SM4INV, Arne.