Walford Sutton – DSB Transceiver Construction Project

Walford Sutton – DSB Transceiver Construction Project

As a teenager I built an 80m direct conversion receiver based on a design in the RSGB Radio Communication Handbook of the time. It surprised me by working first time but it never worked particularly well.

I designed the boards and built it pretty much from the circuit diagram onto varoboard. The buffered oscillator was quite stable and I hoped to one day turn the whole project into a transceiver. However, when I started considering this idea seriously it struck me that a kit might prove more likely to be a success

Last updated: Sept 2011


click to zoom in

Richard Francis G4EIE advised me to look at Walford Electronics kits and I decided to have a go at the Sutton – a modular design Double Sideband Transceiver based around an 80m direct-conversion receiver.

In the summer of 2005, I started building and quickly assembled the receiver, transmitter, 10w linear amp, swr-bridge andATU. It has plug-in band cards holding xtal and filters (LO, RF-RX, RF-TX) and I built these for 15, 20, 40, 80 and 160m.   There were two versions of the Sutton design – one for Double Sideband (Sutton Montis) and one for CW (Sutton Mallet) – I built the Sutton Montis but then added CW facilities to it. Walford kits are all named after Somerset place names. 

click to zoom inOver the next three years, I experimented with a number of modifications/add-ons and the ‘final’ design is shown to the right. I decided to take out the ATU/swr-meter and in it’s place is now a loudspeaker, notch filter and LED audio derived relative s-meter.

Early Construction
Here are a few early pics, starting with the basic receiver, then with the tx and linear boards:-

The receiver is sensitive and generally delivers less background noise than my other receivers! Selectivity can sometimes be a problem from two counts – with Double-Sideband you’re listening to 2 SSB frequencies simultaneously. The receiver can also get swamped by out of band broadcast stations, the ATU effectively provides a band-pass filter of course which usually eliminates the problem.

The audio amplifier will easily drive a low impedance speaker. The transmitter uses the receiver’s local oscillator and generates up to 1.5w of DSB. My first QSO with Richard, G4EIE, was very promising giving me 59+ on 80m and good audio, 40m was lower at 56.

The 10w linear increases power considerably on all bands. Maximum power varies between the bands and is typically 5 to 10w. 10w can go a long way and, using the Sutton, I have worked Southern and Central Europe on 40m, 20m and 15m. So I’m waiting for good conditions to try to swim the pond!

Tim Walford’s approach is very cost-effective, using low-cost components such as Polyvaricon variable capacitors instead of expensive air-spaced variables and ceramic resonators instead of crystals. The design takes you step by step through the build, testing as you go and overall is very well thought through.

I designed the front panel using Powerpoint on the Computer which I then printed and laminated.  Being something of a novice in electronic construction and case construction, I built the case out of wood. Not quite to Tim’s taste but you have to start somewhere!  Here are some pics with the original configuration having the T-match ATU installed.

Ceramic resonators are pretty cheap (eg.40p), quite stable but with a much wider tolerance compared to crystals when it comes to their resonant frequency. My initial Sutton build covered 3.48-3.68 MHz, instead of 3.5-3.7 (and I rather hoped for 3.725!). Ultimately the solution was to use same-frequency resonators in parallel as described in Jack Ponton’s article. I experimented with panel switches and various numbers of resonators in parallel but found the best solution was to have a plug in board with two 3.69 MHz resonators on it in parallel which I could then connect across the original ceramic resonator. Using a plug in arrangement avoids adding capacitance that would result from wiring and switches. Using 3 resonators in parallel, my HF band end got me to 3.750 MHz quite nicely!

SWR meter
The Walford SWR bridge uses a LED to indicate forward and reflected power. I decided I wanted to see finer movements whilst adjusting the matching. The challenge was to find a small enough meter that I could use; most stocked meters are roughly 2″ square and I wanted one much smaller. G4EIE had suggested a small signal meter from an old cassette recorder but I found at the Wakefield Rally a battery tester for �2 which has a 1″ square meter! I stripped it out and put a 10k pot in series and found FSD was reached at about 10v which was just what I needed. The LED was disconnected and the meter put in its place.

15m Modification
Output on 15m was extremely low to start with. Ultimately after trying all sorts of things I had to ask Tim Walford to have a look. His conclusion was that 15m was perhaps a band too far on the original design. However, he managed to raise the output from about 30mW to 1w (barefoot)# and he explained his modifications:-
1. R303 reduced to 82R to increase current in TR302
2. Extra 2N3819 buffer stage interposed between R310 and TR304, it has a 1K5 in the 3819 source to deck and a decoupling 150R/10 nF from the VP point to the drain. The source feeds the BS170 gate direct. This reduces the high frequency loading on the 612 output.
3. I have also shorted out the protective diode D303 to increase the supply voltage as quite a small change in volts produces an increase in output.

CW Addition
As a means of pushing me to learn cw, I have modified the Sutton for cw using another of Tim Walford’s kits. The cw kit works by keying an injected 750Hz tone into the DSB modulator. The result is what’s best described as twin-frequency cw transmission, one signal -750Hz and the other +750Hz from the suppressed carrier. Receive is as normal for DSB so both frequencies are heard simultaneously. The kit also provides a narrow 750Hz audio filter and injects the sidetone for monitoring the keyed cw too. There is even a facility to net onto the other station, particularly useful when like me you are relying on a very picky MFJ CW Reader to decode the morse.

Drive and SWR
Sutton at Halifax and District Amateur Radio Society For a while I found I couldn’t match 20 meters and believed that the problem was to do with the low-pass filter or the relay that routes the signal to the LPF. Eventually I realised that the poor swr matching was due to harmonics being present. Assuming the LPF is working, this would usually indicate something was being overdriven. It didn’t appear to be the driver section of the transmitter or the modulator. It was eventually tracked down to RF getting into the cw circuit. A complete clean-up was made of the linking between the boards, more earths, more shielding and moving the cw board further away from the transmit board…this worked!

Instability Problems…Coax!
Around January/February 2007, I did ‘something’ which introduced instability to the rig. The initial symptoms were high swr when transmitting into a reactive load that should have given a good match (eg. through an ATU). After extensive testing, with great thanks to my friend Maurice Firth G3MMK, we were able to prove that the rig only maintains healthy signals with spurs and harmonics well down when the drive is relatively high. The rig visited the Halifax and District Amateur Radio Society, where I demonstrated the problems to the assembled. In the photo, from the right you will see me, Tony G0DLX and Maurice G3MMK with others. As you back the drive off, significant spurs (mostly LF) appear and can actually be larger than the intended frequency. The linear amplifier compounds the problem as high levels of drive seem to induce oscillation in the output stages of the transmitter. Of course all the usual suspects for this type of crime have been thoroughly interrogated and released cleared of any wrong doing! Sutton at G3PCJ

I took the rig down to see Tim Walford of Walford Electronics and it is now much more stable. The culprit was mostly too much capacitance introduced into the link between mixer and modulator through using coax! The coax was replaced with plain wire and a buffer stage. His points and other learnings are:-

1. Don’t assume coax is always the right answer for RF links inside a rig. It adds significant capacitance and can be the cause of problems! If the link is short (4 or 5 inches) plain wire, close to the ground plane might work better.

2. If using coax, grounding both ends can create ground loops and other capacitive problems. It’s better to ground only on the input end of the coax and leave the output end free. The principle being to screen the small signal from further amplification getting back into it and to limit the capacitive effect.

3. If reactive imbalances in the output cause instability, try a 1:1 transformer on the output with it’s output ground connected to RF earth and isolate RF earth from rig earth.

Notch Filter and Audio Derived S-Meter
The notch filter is a design from Richard Booth G0TTL. It isn’t a match for a Datong FL3 but it is very easy to build and set up. For most situations I find this notch filter is a valuable cost-effective addition and makes all the difference; remember that Direct Conversion receivers are listening to both side-bands at once and QRM can be a problem.

The Audio Derived S-Meter with LED bar graph display is from a Sprat design and was published in PW by Rev George Dobbs G3RJV. Again, easy to build and inexpensive however I did find setting up was a challenge as there are three variables to set – s-meter amp gain, s-meter output and LED bargraph scaling. click to zoom in

If you’re wondering what the rather Hindu-esque red phono socket is doing on the Sutton’s ‘forehead’ (above the main tuning dial), it connects to the modulator input so that I can pick up the precise frequency on my frequency counter – another kit construction, this time from Cumbria Designs. It might have been more aesthetic to put the socket in what is now the black space on the left side of the front panel but this would have meant longer leads attached to a sensitive part of the circuit and the space was occupied by a moving coil s-meter until April 2008.

20w output on 160m (aka. How to be heard above domestic QRM on top-band!)

With mounting frustration that most amateurs I talk to on top-band couldn’t hear me above their 8 s-points of local QRM, I determined to do something about it! A reasonable output on all bands had been a compromise and on 160m I was only able to put out 4.5w so more watts out were needed!

The linear amplifier has two IRF510 MOSFETs in push-pull and IRF510s have a maximum voltage of about 100v, however typically they would fry very quickly at these voltages. +40w amplifiers have been made using a pair of IRF510s with 1w of input so the gain can be excellent. The trick of course is to run them at 24-28v and employ effective cooling – see WA2EBY’s Write Up – part 1 and part 2 .

My objective was rather more limited, to raise the output to at least 15w and for the rig to cope with long waffle overs.  Being a therm-neurotic, I had visions of IRF510s going up in smoke and so I planned to site a fan on the top of the linear heat-sink from the start (with rubber washers) and to site another fan behind the transmit board heat-sink.  There are many quiet fans available now aimed providing PCs with quiet cooling. The two I chose were Fractal Designs 12v 50mm ‘silent’ PC fans and with a 2w 50 ohm resistor in series with 13.8v they were running quietly at around 11v.

For the linear power supply, I bought a 24v 6A switch mode supply which was ex-rack equipment to a high spec and 6 of 50v 6A Silicon Diodes. My thought was that the power supply would tweak down to 21.5v and the diodes in series would potentially bring the voltage down perhaps to 17v. The diodes only dropped 0.3v each however, so a bit of a non-starter!

The rig continues to have a 13.8v supply for everything other than the linear and I wanted to optionally run the linear either on 13.8v or the higher voltage linear supply. The easy solution was to use  10A 3-pin mains extension lead connectors. The idea being that 24v nominal could be passed to the rig through one of these for normal running and a ‘dongle’ could be made out of another connector to feed the 13.8v back in to the linear if lower voltage running was required…

10A 3-pin mains extension lead connector for passing 24v in and 13.8v out of the rig diagram of use of the power connector linear twin heat-sinks with fan mounted above with rubber washers

I started tests at 19.5v but quickly brought the volts up to 24.0v to produce up to 20w output on 160m.  At 24v on long waffle overs it produced around 20w output on 160m, the heat-sink warms up only mildly. It could take a bit more but 24v is enough for me and I’m more than happy with the results!

Thanks again to Tim Walford. I’m very grateful for all his help through the trials and tribulations of the Sutton which I have extended to be a much more complex rig than he envisaged in his design. Tim’s encouraging approach is so helpful to anyone venturing into construction as a novice. Tim’s approach through the Walford Electronics kits is great and by far the best kits I have come across for someone wanting to learn from what they’ve made. He’s a thoroughly nice chap too!

Reference Data
Sutton Montis transmit board – transistor voltage measurements

Last updated: August 2010