“Those who hear not the music, think the dancer mad”
Softrock 40/80m Kit
Softrock 40/80M Kit

Softrock 40/80M SDR

HF on a budget?

That was my brothers (M3NQG) challenge and at only £16 when ordered direct from the States - Softrock might be the answer..


T1 Toroid - FT30-7 34 turns with 17 turns bifilar on top, all with red wire. Having very carefully installed it I remeasured everything and found the inductance across the secondary windings had dropped… Oh well at least the measurement confirmed it was in the wrong way round!

PTT Switching

Well the PTT was not… Plus the antenna did not seem to be connected… Hmm, so I spent quite a while looking through the circuit diagram but was not making much headway until I decided to simulate the PTT switch section in LT Spice. Interesting exercise and it certainly increased my understanding of various other parts of the circuit as well. Using this model I reproduced the problem; then off to Maplins for another transistor.
I tested the old transistor on the Atlas – “What transistor” it said. But before I could replace it my soldering iron went on the blink and it is only 2 months old!

Time for a large glass of red wine..

“Cheap Easy Performance - Pick Two..”

Father in-law rang and we had a pleasant chat. Back to the wine, then back to the soldering iron which had now started working again. Replaced the transistor and at least now the first part of the PTT worked. Hurrah!

Used the PC IQ TX testing program to test the TX, idle PA voltages were correct and the current draw went up – which seemed OK. However no RF emerged from the antenna socket. That must be why the PA got so hot!


Very weak signals on 80M, looks promising. But for best performance the SoftRock does need a "proper" 50ohm antenna not 15ft of hookup wire...


Nice shaped carrier into L3, the last toroid of the TX LP filter, but no carrier on the output. I disconnected the toroid from the board and output is OK, something is dragging it down but there is only a cap to the ant output and no “obvious” shorts.

Turns out the TX signal is very small (500mv) and is gradually getting squashed by successive stages of the bandpass filter.

“Once more unto the bench dear friends..”

PA Bias

Using the I/Q TX test program (Input 2.01 PTP) I followed the signal through the 2n2222 and across T2 to the PA (Input 4.5 PTP), but only got 500millivolts PTP at the antenna!

The PA gets hot. PA Bias is 3.25 volts (measured at the Q3 side of D2) Is there a problem with T3, or somewhere else?

The SoftRock was put to one side, awaiting inspiration..


As Simon (M3NQG) built his SoftRock I assisted sorting components and advised on Toroids! It went together much quicker than mine, demonstrating the advantages of having already built one and of having two people checking components.

Did a quick test of Simon's rig and found the PTT switching was working but it had no PA output. Recalling the outstanding problems with mine, I decided to setup both rigs on the bench and step through the TX circuit diagram comparing and noting values, hopefully finding the issues with both rigs on the way.

Testing Session 1

Stepping through the PA.

Faults Found
M3NQG - Poor / shorted solder joint on output of T1.

Session 1 Final Measurements Table

A good start as the TX sinewave is now getting further, but still no useful output from the PA of either rig!

Testing Session 2

Stepping through the PA.

Faults Found
M3NQG - C57 shorted to earth.

Session 2 Final Measurements Table

Still no PA output on either rig, both may now have the same problem?

Testing Session 3

Lifted the output wire of T3 and measured a healthy looking 20volt 7Mhz sinewave. Progress! Something between T3 and the 50ohm output load was dragging things down. Performed some DC checks on the now isolated output stage, but as expected it just indicated a 50ohm load. Removed C24 to isolate the PTT switching. Reconnected T3 but still no output. Removed the first low pass filter capacitor C21. The low pass TX filter capacitors C21 and C23 should be 470pf but are 47nf.. They are marked as 473. I can only assume they must have escaped measurement prior to installation!

Once C21 and C23 were removed a 7Mhz sinewave appeared on the output. Both rigs had the same fault.

Looking again at the documentation it does not use any 47nf caps. But does use .047uf ones.. for C38 and C39 as part of the coupling from the RX to the PC line input. This may explain why the RX seemed rather deaf.

Faults Found
M3NQG - Low pass TX filter capacitors C21 and C23 should be 470PF but are 47nf
M1HOG - Low pass TX filter capacitors C21 and C23 should be 470PF but are 47nf
M3NQG - RX capacitors C38 and C39 should be .047uf but are 470pf
M1HOG - RX capacitors C38 and C39 should be .047uf but are 470pf

TX Test - I/Q TX test program input 1.9volts PTP. PA output into a 50ohm dummy load 20volts PTP.

RX Test - Using a 15ft hookup wire antenna and Rocky PC software, CW signals are received at similar levels on both rigs.

Sound cards

After considerable research I decided to buy a Creative Labs USB sound card and following a little fiddling with sound card options to make best use of the USB 1.1 bandwidth, it worked a treat on RX – 96Khz of 40m in one go. Germany and Italy on SSB then Germany, Romania, Finland, Russia and Italy on PSK31 were heard. Works way better than the onboard card. Rocky recognises both sound cards which is a good sign for getting the TX going.

RX/TX Indicator

Softrock RX TX LED

Wired up the basics of the serial PTT lead. An LED indication of RX/TX would be very handy. But I only had common anode "tricolor" LED's to hand. The simplest option would have been to purchase a common cathode LED from Maplin (89p each!). The PTT could then be connected via a 1K resistor to the Red side and 12v power via another 1K to the Green side.

However it would be a much more interesting exercise to use my common anode LED's by adding a transistor switch to the red side of the LED for the PTT.

LED / 3n904 Configuration

PTT 12v -> 10K -> Base

Emitter -> Ground

Collector -> Cathode of LED Red Side

Anode of LED -> 1K -> 12v supply

Cathode of LED Green Side -> Ground

Doubtless there are better ways of doing this, but it works.

PTT now works from Rocky - Green On / Red TX.


The surest way to bring the gremlins out of any radio project is to carefully box it up. Sadly this project was no exception. Having carefully wired up the line and outs, PTT, power and TX/RX LED, testing showed the boards antenna connection to be grounded. Removing the board from the box and brief checking around the antenna revealed nothing but lots of ground connections. Checked again later and resolved the grounding issue. Installed the RX/TX LED in its Maplin LED mounting clip.

Simon wired up his Softrock. We then configured the software and USB soundcard on his laptop, tested both rigs RX with a longer wire - receiving even more stations and tested the PTT functions.


Softrock T1 Toroid
T1 Toroid


Carefully check and sort all components, some of the resistors appear very similar.

Check to ensure no component pads are accidently soldered to earth, as the space around some of the pads is very limited.

Ensure the SMD caps are flat before final soldering.

Binocular toroids - Be very careful to smooth the toroids holes and clean them up with a needle file otherwise the insulation will scrap off the wire as it is threaded, resulting in shorts.

Use a cocktail stick to compact the windings. Triple check orientation before soldering.

Careful with the orientation of the windings. Get it wrong and the circuit board holes will not line up with the wires - You have been warned.

Blutack - Very useful for holding SMD chips in position allowing easy soldering of the first few of pins.

Tip and ring connects to Line in socket on the PC, L & R connects to Line out socket on the PC!

When a laptop battery gets low the IQ TX testing program cannot output nice neat sine waves. I suspect this is because the processor speed drops!

Centre Frequencys

28.06 Mhz crystal

7.015 /4 - Covers CW band segment
3.5075 /8

28.224 Mhz crystal

7.056 /4 - Covers digimode band segment 7.035 - 7.038 and SSB band segment 7.043 - 7.078
3.528 /8