Dave Richards AA7EE

September 9, 2016

A Scratch-Build of N6KR and Wilderness Radio’s SST for 20M

Note – all links in this post open in a new browser window. It would be a good idea to clear your cache from time to time, to make sure your browser loads the latest version of this post. As an example, I just found an error on the schematic, and uploaded a newer, correct diagram.

I’ve been wanting to build an SST for a few years now. It’s a plucky little rig, with a lot of character. Designed by Wayne N6KR in the late 90’s, appearing as a full article in QRPp, and a kit sold by Wilderness Radio, it ignited the imaginations of a whole generation of builders for it’s combination of simplicity, performance, and willingness to accept modifications cheerfully. The review from Adventure Radio Society was quite positive. It is a VXO-based QRP CW transceiver, with a simple superhet receiver (SST = Simple Superhet Transceiver), and a TX that puts out up to 3W, depending on your choice of transistor in the final (it can be dialed down for battery-powered outings). It has very fast, clean QSK – so fast, in fact, that it feels as if I can hear the band all the time I’m sending (W6JL would approve). You actually listen to your own signal as you’re sending – there is no separately generated sidetone. The sidetone level does vary with the volume control, as opposed to being a fixed volume, but I only find this to be an issue when I have the AF gain all the way up, in which case I either quickly adjust the volume knob, or partially pull the earbuds away from my ears while sending. By the way, the sidetone on this little rig sounds really nice. It’s a feature which helps to make operating the SST an enjoyable experience.

QRP’ers loved their SST’s. There was a lively discussion about the minimalist rig on QRP-L, with builders reporting back on the frequency coverage and performance of their builds, with details of mods they were trying. The kit came with a light gauge unfinished aluminum enclosure. The raw-finish aluminum was a blank slate which invited many different creative solutions to the age-old question of how to show off your project. Some folk endowed theirs with professional-looking paint jobs, while others used dymo labels, or simply scrawled right onto the panel with a Sharpie® for that authentic home-brew look. All approaches worked admirably well. I saw one SST that had been painted with a US flag, and looked great. Some of them were taken out on the trail many times, and showed many knocks and scratches on the case which, of course, just made ’em look better still.

original

SST Kit Version – Image from Wilderness Radio

original1

SST Kit Version – Image from Wilderness Radio

JN3-bong;y modified his SST with 2 varacters for extra frequency coverage. He also added a speaker, and a spot button. Click on this photo to go to his site, and read more about his SST.

Koichi JN3DMJ modified his kit version of the SST40 with 2 varacters for extra frequency coverage. He also added a speaker, a spot button, and expanded the LPF for better filtering of the transmitted signal. Click on this photo to go to his site, and read more about his SST. He says that this SST is his favorite rig. He has made over 1300 QSO’s on it so far. Thank you for the permission to reproduce this photo and link to your site Koichi.

Recently, I decided it was time to build my own SST, only to find that I had missed the boat on a kit, as Wilderness Radio had discontinued it at some point in the recent past. I called QRP Bob on the phone, hoping that there would perhaps still be a board and/or enclosure available, but I was out of luck. In fact, it meant that I was in luck, as I would have to scratch-build one, and that’s a good thing.

There is plenty of documentation for this rig online. The initial write-up was in the Spring 1997 edition of QRPp, available from Chuck K7QO here. Note that the preceding link is a PDF of all 4 volumes from that year. The entire QRPp archive is also on Chuck’s site and accessible from the main page. Ken WA4MNT has a copy of the manual for the Wilderness SST kit on his site. Other essential documentation is the mods and information collected from QRP-L messages from 1997-2004, from Ken Larsen AL7FS, here as an HTML file, here as a text file, or here as a .doc file.

Mine differed just slightly from the original. Here’s my hand-drawn version of the schematic, reproduced here with kind permission of N6KR. Don’t rely just on the following schematic, as my drawing is a bit goofy. Best to refer to the original circuit diagram in the manual, and use mine to see how it differs –

 

Note – I do not recommend using the LPF comprised by C34, C35, C36, L2 and L3. Although it just met FCC specs at the time of release, it is very unlikely to provide enough filtering for this rig to meet current FCC rules regarding suppression of spurious emissions. For more satisfactory filtering, see my post here.

Component designations (e.g. C27, R11 etc) are the same as in the schematic in the Wilderness Radio manual. There are 3 parts in the above schematic that do not have such designations – they were added by me (the series capacitor and resistor from pin 5 of U3 to ground, and the 1N5817 diode in series with the 10-16V DC supply line).

Differences between the stock SST and mine are –

1)  Inclusion of a series  diode in the supply line for polarity protection. I did consider using a P-channel MOSFET so as to avoid the voltage drop, but decided to go with a Schottky barrier diode. Some diodes of this type have a big enough reverse leakage current such that they are not effective in this role, but not so the 1N5817. It’s forward voltage drop is about 0.34V in transmit. I still don’t like losing that much, but it’s an improvement on the higher drop of a regular silicon diode.

2) The use of trimcaps in the RX and TX oscillators in order to place the received signal in the center of the passband, and to put the TX signal on the same frequency as the RX signal. I’m still about 20Hz off due, I think, to touchy trimcaps, but it’s close enough – until I get the urge to tweak and adjust again 🙂

3) The use of 68pF capacitors for C19 and C20 instead of the 100pF values specified. With 100pF feedback caps, my VXO wouldn’t oscillate, so I swapped them for 68pF ones, and it sprang into life (thanks to LA3PNA for the help on that). It may well also have oscillated with 82pF caps, and that is an option if you want your frequency coverage to be a little lower.

4) An MRF237 was used for the final instead of a 2N3553. This substitution was suggested in the manual for higher output power, as the MRF237 has higher gain. If you want a cheaper and more modern alternative to the 2N3553, I’m thinking that a BD139 should work as a direct replacement. Joel KB6QVI just told me that W8DIZ has the 2SC5706 at 10 for $4, and I’m wondering how it would work in this application.

5) A Zobel network was added to the output of the LM386. Mine was unstable at high volume settings. A series capacitor and resistor from pin 5 to ground is very commonly used in these circuits, and the inclusion of these 2 parts tamed my instability immediately.

6) Alternate values of the capacitors in the crystal filter were used to widen out the response. The original values were reported to be giving a particularly narrow bandwidth of around 200 – 300Hz at the -6dB points. I wanted something a bit wider. There were several suggestions in the QRP-L archived discussions. K7SZ tried widening his SST20 out with the help of these suggestions, but it still wasn’t wide enough for him. He suggested the use of 47pF for C6 and C9, and 120pF for C7 and C8, which is what I used. Thanks Rich. A few builders went further, and implemented the ABX (adjustable bandwidth crystal filter) mod that was used in the Wilderness Radio version of the NorCal Sierra. EDIT – K7SZ notes in his ARRL book “Low Power Communication” that his mod brought the center frequency of the filter down too low for him on his SST30, so he ended up going back to the stock values. I have found that using the values of C7/C8 = 120pF and C6/C9 =  47pF that Rich suggested on QRP-L for his SST20, I set the sidetone at 400Hz ( a new thing of mine – I’m experimenting with lower than normal sidetone pitch), and the center of the filter passband was still about 20Hz lower, which I guess is pretty close. If you like higher sidetones though, you may be better off with one of the 2 sets of stock values in the SST manual. SECOND EDIT – I plan to tighten the response of the filter, by using the alternate values quoted in the manual. After using the SST for a while, I think my version is a little wide.

Although the SST didn’t come with a keyer, many users added their own, the Wilderness KC1 keyer and frequency readout being popular. At the time of writing, this is still available – the only kit that Wilderness still supplies. I decided that I wanted to build a keyer onto the same board as an integral part of my SST. I didn’t need much in the way of special features, my only 2 requirements of a keyer for this rig being that it will operate in iambic mode B, and that it has a speed pot – a feature I think of as essential. Changing speed on the fly during a QSO is tricky if the option is accessible only via menus, but a piece of cake if all you have to do is reach out and twist a knob. Perhaps I didn’t look hard enough, but the only freeware I found didn’t support a speed pot. I remembered how well the N0XAS Super PicoKeyer that Dar W9HZC had given me had worked, and a light went on in my head. Dale sells spare chips for his PicoKeyer Plus at $6 each. I purchased 4, used one in this rig, and saved the others for future projects. The manual on Dale’s site will give you the info on all the features of this keyer, and how to access them. The surrounding circuitry is simple (the genius is in the coding), so it was easy to incorporate into the SST –

The on-board keyer used a replacement PicoKeyer-Plus chip from Dale N0XAS

The on-board keyer used a replacement PicoKeyer-Plus chip from Dale N0XAS

The keyer uses a piezo transducer to announce the responses to command inputs made via the CMD pushbutton and the paddle. It would be possible to feed this audio into the AF amp of the SST so that it can be heard in the earphones, but I elected to fit a small piezo transducer on the edge of the board. I had intended to punch a small hole in the side of the case to make it easy to hear, but this appeared not to be necessary. Note that in the schematic, I have called it a piezo “buzzer”. It is actually a transducer, but allow me to explain. There are piezo buzzers available to which you apply a voltage, and the unit rewards you with a loud piercing tone, generated by an internal oscillator. Some of the units available are called piezo buzzers, but they don’t contain the audio oscillator – just a transducer, which is often sharply resonant at a specific audio frequency, to enhance the volume of the emitted tone. I bought a 5-pack of so-called “piezo buzzers” on eBay. They looked too small to contain an internal oscillator and I was correct – they consisted of just the transducer, which was exactly what was needed.

This build looked great when it started (as they all do 🙂 ). A nice, clean board, with nothing but potential. As projects progress, I tend to become more anxious that in a single unconsidered moment during a late night soldering session, the iron will slip, the odor of burning plastic will waft into my nostrils, and all the hard work will be undone in a careless fraction of a second. In truth, there aren’t that many errors that can’t be corrected, but this early shot of the board was the best it ever looked! As with all my projects, all the Manhattan pads were MePADS (for IC’s) and MeSQUARES (for everything else) from Rex at QRPMe

If you compare the above photo to later pictures, you’ll notice that I ended up changing the layout of the front panel controls.

In the next picture, the AF amp and the VXO have been constructed, as well as the 8V regulated supply line. Temporary DC and headphone jacks were also connected, so the circuit can be plugged in to see if it works. The shielded cable that connects to the tuning pot was installed, but left longer than needed to allow for the final install in the enclosure. Not too much of the circuit had been built at this point, but it was already possible to test the voltages at the input and output of the regulator, as well as ensuring that the LM386 made a nice, loud noise when the input terminals were touched with a screwdriver (a highly controlled and accurate test 🙂 ) The VXO was tuned in on a nearby receiver and tested for frequency coverage. With the 20M version, the VXO is in the 18MHz range, and by subtracting the IF of 3.932MHz from the highest and lowest frequencies it oscillates at, you can estimate the final coverage of the SST, and make adjustments at this stage if you wish. The discussions on QRP-L (which are linked earlier) contain a lot of info on tailoring the coverage, so I won’t repeat it all here, but your options involve using different varactors, connecting a second crystal in parallel with the VXO crystal, using different values of rubbering inductor, and adjusting the value of R5. I won’t explain how these all affect the frequency coverage and stability, as this is discussed at length in the QRP-L archive and also, to a certain extent, in the manual. There is plenty of homework reading to be done if you are thinking of building this rig!

Here’s another view, with the VXO in the foreground –

Suddenly, the product detector and BFO burst onto the scene. In the next shot, the 3.932MHz BFO crystal is the one closest to the camera, with the trimcap for centering the passband just behind it. I used 60pF trimmers, as that is what I had the greatest quantity of. Something a little smaller might have made the adjustment less touchy though. I’ll leave you to experiment, if you want to. Things are getting pretty exciting at this point, because when you a touch a wire or metal screwdriver screwdriver to pin 1 of the product detector IC U2, you hear honest-to-goodness atmospheric noise – a distinctly different sound from what you hear when touching the input of the AF amp IC. It’s instructive, not to mention really cool, to hear this progression in the sounds you hear, as you touch the inputs of stages closer and closer to the antenna, as the build progresses. If you have a signal generator, you can inject that into the circuit, and look at the output on a ‘scope. Don’t despair if you don’t have a full stable of test gear though – it’s important not to underestimate the power of touching and listening. Once you’ve done it a few times, you get used to knowing what sorts of things you should be hearing. See the curved red power wire that supplies 8V regulated to the BFO/product detector? You’ll notice in later photos that it was replaced with a different-shaped wire. It’s rarely possible to get everything right the first time you construct something, so one-off builds like this tend to morph somewhat as they progress. It’s OK to change things as you go along –

The next stage to construct was the crystal filter. Do you notice how, on the board for the kit version of the SST, the crystals for the filter were lined up with the short edges parallel to each other, so that the filter takes up a significant length of one side of the board? You usually see filters with the long edges of the crystals lined up parallel to each other. I don’t know why Wayne did it this way, but it did occur to me that with this physical configuration, the input and output of the filter are further apart than they would be with the more conventional placement pattern. Perhaps this was an attempt to decrease the possibility of filter blow-by? It seemed like a good idea, so I replicated it in my Manhattan copy. The crystals in the filter are not yet grounded in this next shot –

As far as matching the 3 crystals for the filter, I placed them into an oscillator circuit, and measured the frequency of oscillation. My cheap Chinese stand-alone frequency counter only had a resolution of 100Hz, but then I remembered that my K2 had a built-in counter with a 10Hz resolution, so I used that. I needed 5 x 3.932MHz crystals total – 3 for the filter, and the other 2 for the oscillators in the TX mixer and the BFO/product detector, so I picked the 5 that were closest in frequency. Out of that group of 5, I took the 3 closest and used them for the filter, while the other 2 were used for the local oscillators (but not the VXO, which required an 18MHz crystal).

To verify that the receiver is working, you’ll need to also build the antenna LPF, consisting of L1, L2, and associated parts. Without it, you won’t be able to peak the antenna input trimcap C1. Notice that if you touch the input of the crystal filter, the noise from the phones sounds much more restricted than when you touch the output of the filter. In fact, you can work your way back through the filter, with the rushing atmospheric noise becoming more and more restricted-sounding as you touch each stage of the filter with your metal screwdriver. These quick checks help to confirm that your project is pretty much on track. Adding the receive mixer means that the receiver is complete. After peaking C1 for maximum band noise, you should be able to receive off-air signals. Congratulations! If you substituted a trimcap for C10, you can also adjust it to place the received signal in the center of the passband, an adjustment that will depend on what pitch of sidetone you like to listen to.

My receiver didn’t work particularly well at first – I was getting very low audio out of it. One or two posts in the QRP-L archive made the same observation. I was beginning to talk myself into believing that the design was deficient in the audio department, and resolving to substitute a different audio chain, when I discovered that the coax which delivered the output of the VXO to the input of the RX mixer wasn’t properly soldered at the output of the VXO, resulting in low drive to the RX mixer. Re-soldering the joint solved the problem, and I can happily report that the audio output is more than adequate to drive a quality set of earbuds or a pair of reasonably sensitive headphones. If you attempt to drive a speaker, you will find that the level is only adequate for monitoring whether a frequency has activity or not, in a quiet room. That’s fine, as this was designed as a trail-friendly rig, with low current consumption in mind, and it certainly achieves that. VK3HN mentioned to me the idea of adding a lower noise AF chain designed to drive a speaker, and retaining the original AF output stage, feeding the inputs of both in parallel. The advantage of this would be that you’d retain the AGC action provided by the LED.

See the VXO in this next shot, with it’s 18MHz crystal? It has a total of just 10 parts, including the tuning potentiometer. I know that it represents old, well-established technology, but I feel that it still has it’s place in ham home-brew. Only 10 parts, and yet it has great stability and signal purity too. As long as you can deal with the fairly limited frequency range, a VXO is still a great choice as the frequency control in a simple rig –

This is always the point, when building a transceiver, where I slow down and spend some time playing with the receiver. I was dead chuffed, as we Brits say, that I had successfully built a little superhet receiver with a narrow crystal filter, that was sensitive, and sounded good.

But at some point, the momentum needs to be capitalized on before it is all gone, and so the build proceeded, with the addition of the transmit mixer. I also added the keying line (the green wires around the edge of the board) so that I could key the TX to see if it worked. If it did, then all that would be required would be to amplify the output of the TX mixer with the driver/buffer, and the PA. We were really getting close at this point! You’ll notice that there is a “channel” of space separating the crystal filter from the rest of the circuit. I did this for two reasons – firstly, as I thought it couldn’t hurt to physically separate the filter a little, to help prevent filter blow-by. Secondly, if there was excessive blow-by, it would give me enough space to erect a screen made from PCB material. C39, the 470uF AGC capacitor, is not present in this shot, nor in the later overhead notated view. I was planning on mounting it off the board, on the inside front panel, but eventually decided to mount it on the board. It ended up occupying the space between the AF amp and the edge of the board –

If you look carefully at these pictures, you may notice one or two components changing position slightly. As the build progresses, I will occasionally move a part or two in order to refine and improve the layout. I’m not going to point out which parts this applies to, as I don’t show these photos with the intention that you follow the layout closely. I started out by following the layout of the stages on the board from the SST manual but as the build progressed, realized I’d be able to move the position of the driver/buffer, thereby freeing up space for the on-board keyer, in one of the back corners of the board. Here’s another view of the board in the same state as in the above photo, with the receiver fully built, as well as the TX mixer. If you have a scope, you can measure the output of the VXO, which should be between 200 and 500mV RMS (that’s 0.565 – 1.414V peak – peak). You can also adjust C28 to peak the signal that will drive the buffer –

This was really the point at which I felt that I was home free. The TX/RX switching was working well, and the rig was putting out a small signal on the operating frequency in the 20M band. All that was left was to amplify it – and even if that didn’t work, I still had a cool little receiver and let’s face it – receivers are cooler than transmitters 🙂

The next shot shows the rig fully built, with the exception of the keyer, with the board temporarily mounted in an enclosure. I ended up changing the layout of the front panel controls, which necessitated the use of another enclosure. In both cases, I used the LMB Heeger 143 enclosure in plain aluminum finish (they also have it in black and grey). I used an MRF237 instead of the 2N3553 in the PA, in order to provide a bit more output, and you can see that transistor, wearing it’s heatsink. To the right of the PA transistor is the orange top of R12, the drive control, and to the right of that is the LT1252 driver/buffer stage. The RF input to the LT1252 is carried by a single wire underneath the board. There are only 2 wires under the board – the one just mentioned, and a length of RG174 coax connecting the output of the VXO to the input of the RX mixer –

A view of the completed board, with the N0XAS keyer in the rear left-hand corner (which is actually the rear right-hand corner, if you are looking at the board from the front panel end). I mounted the keyer chip in a machined socket. I run as many of the control cables as possible underneath, and drill holes in the board for them to enter. I think it looks neater that way –

This next view shows the layout. I got carried away and labeled a few too many parts. The side of the board that faces the front panel is the left edge. As with the previous overhead anotated view, C39, the AGC capacitor, is not shown here, though it did end up being mounted on the board. Remember those SMD SA602’s I was giving away for the price of postage a while back (courtesy of KV7L)? I hadn’t used any myself, until now. 3 of them are in this little rig –

Time to get this thing in a case. The LMB Heeger 143 is an ongoing favorite of mine. It measures 4″ x 4″ x 2″ high, and has 2 small lugs at the front and back of the cover that engage with the front and back panels to prevent them from being pushed in. This feature adds rigidity and stoutness. One of the things I don’t like about most clamshell cases is that the front and back panels can be flexed; not so with this model. It comes in grey and a sort of black wrinkle finish, if you don’t want the raw aluminum. All the pots are Alpha brand. The 3 small ones were $1.29 each from Tayda. The larger tuning pot is also an Alpha part, but is very slightly smoother in action, and I wanted to optimize the experience of tuning this rig. It is Alpha part # RV16AF-10-20R1-B10K-LA (I got it from Mouser).  There is a small dummy load plugged into the back in this next shot, because I was having fun using the rig as a code practice oscillator 🙂

This SST is quite a triumph for me, as it is the most complex project I have built so far with Rex’s MeSQUARES and MePADS.

The piezo transducer for the keyer was fixed to the edge of the board with a small spot of hot glue (on the high temperature setting of the hot glue gun, as it flows better) –

The keyer CMD pushbutton was a 22 cent cheapie from Tayda. These types are available with plastic and metal shafts. The ones with the metal shafts have a slightly smoother, more positive action. Get those ones. 4 vinyl bumpers from the local hardware store keep the SST from slip-sliding on my desk –

This enclosure is higher than the kit version, at 2″ high. I wasn’t initially planning on such a high case but the advantages are that it supports a larger tuning knob and, as you can see from the next shot, there is room for an internal battery pack, speaker, ATU or other add-on –

Here’s a view of the SST-20 upside-down and from the rear. From left to right – RF gain (rarely used), Antenna, DC power, and paddle –

I have a couple of spare covers for this enclosure, from projects that didn’t go as planned, and am thinking that it would be possible to have different covers with different accessories built in. For instance, one cover could have a speaker and extra AF amplifier, for operation at home. Another cover could contain a battery pack, for portable ops –

The red and yellow knobs look a bit garish, and I’m still getting used to them, but the thinking is that yellow = audio (AF gain and headphones), while red = keyer (speed pot and CMD button). It was also a way of using the cheap knobs I got from Tayda for 49¢ each 🙂

So how does it perform? Well, in 2 words – very well. I don’t operate a lot, but I do spend a lot of time listening. I’ve had 6 QSO’s so far with a horizontal loaded dipole (a Buddipole) at 25 feet above ground at my home QTH. 3 of them were with stations in Colorado, about 900 miles distant, one with KE5AKL who was doing a SOTA activation in NM, also 900 miles from me, and one with a mobile station in Hooks, Texas, who was running 25W. He was 1600 miles away as the crow flies. The other was with a local station. The receiver is as sensitive as you’d need a receiver to be, and there’s a good amount of opposite sideband suppression. I haven’t measured it, but you only hear the opposite side of the signal weakly when tuning through a very strong station. The RF gain only needs to be backed down when in the presence of very strong stations, as the use of an SA602 in the front end can cause it to crumble under such circumstances. I haven’t needed to use it yet, and from what I’ve read, it doesn’t need to be used very often – hence the reason it is on the back panel. My frequency coverage, with an MV209 varacter, is approximately 14055 – 14064KHz, a swing of 9KHz, which is about as much as you’d want when tuning with a 1-turn pot. Many users mounted a switch on the front panel to switch in another varacter (usually an MVAM108, which was also supplied with the kit) to extend the coverage downwards. With the MRF237 in the final, my WM-2 wattmeter indicates an output power of about 2.25W with 11.61V at the input to the rig (11.27V after the polarity protection diode). When supplied with 13.8V from a PSU, the power output was about 2.8W. I didn’t measure the current consumption on transmit, but on receive it is between 26 and 27mA. This is low, but somewhat higher than the 15-16mA quoted in the manual. The keyer consumes <1mA, so that isn’t the reason for the difference.

The AGC LED is a rather unique feature. I’ll let you read up about it in the manual but for the addition of a few extra parts, it will save your ears from the worst ravages of sudden loud signals – and the LED is fun to watch too 🙂 Most red LED’s have a forward voltage drop of about 1.7 – 1.8V. If you want to raise the AGC threshold, look for a red LED with a higher Vf – some of them go as high as 2.2V.

Although all the parts for this little rig are still available, a few of them are a bit harder to find than others. I purchased the LT1252 from Digi-Key – they have them in both through-hole and SMD versions. Chuck K7QO tipped me off to a supplier on eBay who was selling them in 10 packs. I couldn’t resist purchasing a pack. Thanks Chuck 🙂 W8DIZ has the MPN3700 PIN diodes, though see the next paragraph for a worthy substitute.  There are several different choices for the PA transistor. 2N3553’s and MRF237’s were available on eBay when I was looking. Try to buy legitimate parts from a reputable supplier (my gut feelings seem to serve me well in this regard). I’m thinking that a BD139 would work in this position too. All the crystals are still available from Digi-Key. The part numbers are the same as in the SST manual, with the exception of X1-X5 for the 30M version. The manual quotes the Digi-Key part # as X007-ND. It is, in fact, CTX007-ND. Perhaps it changed. It has, after all, been 19 years since the kit was introduced!

Even though this design is now quite old, I think it is still very relevant. An experienced home-brewer can build this into a fairly small case, and take it on the trail with a simple tuner and, say, an EFHW, for a compact and effective portable set-up. All of the parts are still available, though it would be great to see a partial redesign, utilizing more modern and widely available parts. I’m thinking of a redesign of the buffer/driver and PA stages. BS170’s are cheap, and 3 of them in parallel, in class E, could provide close to the full QRP gallon. The original SST had room in the case for a 9V lithium battery, and could be dialed down to lower output powers to help battery life. Nowadays, newer battery technologies make more power available in a light and small package, so running 4 or 5W while portable with a small rig like this is practical. Kenjia JH1PJL used an NPN transistor in his driver, instead of the LT1252 IC. He also used a 1N4004 instead of the MPN3700 PIN diode (you can see pictures of his SST scratch-build here). In fact, all the diodes in the 1N4001 – 1N4007 series have the relatively slow recovery time of 30µS, giving them PIN characteristics. Any of them should work fine in place of the MPN3700. If a 1N4000-series diode is good enough for RF switching in the Elecraft K2 (the 1N4007), then it’s good enough for us!

Here’s a brief video of it in action, with a surprise appearance by Jingles the blind kitty. I fed her just before starting the video, and forgot that her routine after eating is to jump up on the desk to relax and digest her meal for a few minutes. I’ll work on producing a slightly better video, though videos are not my strong point. Apologies for the slightly crackly audio. It’s a combination of operator error and a camera that was designed primarily for stills, and not video –

The yellow knob was making me uncomfortable. It has since been replaced with a black one, and I am feeling much calmer now 🙂

10 minutes later, and the red knob has now been changed for a solid and dependable black knob also. I finally feel that I know where I am in the world again 🙂

For the near future, the next tasks are to –
a) add an extra stage to the LPF between the antenna and the rig for greater harmonic suppression  and
b) tighten up the crystal filter a bit. I have decided that it’s just a little too wide 🙂

Both the above were done here.

 

Note – as of early Oct 2016, I received this very informative message from Walt K3ASW –

I have an actual SST20, with several mods.

If you’d like more VXO range, try adding a small value NP0 or C0G from the crystal-RFC junction to ground. Mine has a 3 pF and I get about 14042-14064. However, the VXO voltage to the two mixers (RX, TX) drops by about 1/4 to 1/3 over the lower 2-3 kHz. Also, I have a 1u8 RFC in series with the 5u6 below the crystal.

Currently, it has the K8IQY filter mod, but I’m likely to change that per my crystal measurements and modelling (via W7ZOI’s GPLA).

I added a JFET amplifier (MPF-102) between the crystal filter and product detector ‘602. This reduced the RX noise level quite a bit and I don’t have to run the AF gain as high. This simple circuit is from NA5N’s little handbook he published some years ago. If one does this mod, a dual JFET keying switch needs to be added between the 602
‘product detect and LM386; otherwise, the sidetone is way too loud. (My first version of the JFET switch circuit didn’t work, so I’ll have to try again. It is a tight fit! – on the   Wilderness Radio PWB  version.)

I’d also noticed the output of the TX mixer needs another filter section. I’m trying to figure out how to shoehorn it in on the existing PWB. (You won’t have that problem with your HB layout.)

I’ve worked quite a bit of DX on mine over the years. When I lived in a condo (before 2000), the antenna was a `98 foot horizontal loop in the attic above the fourth floor and I regularly worked into EU from here in MD.

Hope you like yours; it’s a neat little rig.

73 Walt K3ASW

– and a few days later. Walt sent these extra SST tips –

A couple other simple mods: Add a 0.033 or 0.039uF between the product detector pins 4 and 5; this reduces some of the high frequency noise. Also, a 10 or 15 uF from the phone jack + to ground will reduce the noise a bit more (22uF is too much – its starts attenuating the desired signal). The latter is a RC low pass filter for audio; the R is the AF gain control.
73 Walt K3ASW
Thanks for the helpful tips Walt!

May 11, 2013

The NA5N Desert Ratt 2 Regen

EDIT – If you’re thinking of building the Desert Ratt 2, although the pictures in this post are numerous and quite large, I do recommend reading all the text too, as I have included what I thought were relevant details on the construction as part of my narrative. Also make sure to read the comments and replies.  Previous blog-posts have taught me that readers often ask pertinent questions, so you may be able to glean a little more information from them too.  In fact, just before I wrote this, Paul NA5N made a comment which includes a usefiul piece of information about the 2 x 1,000pF (0.001uF) capacitors in the regen stage.

EXTRA EDIT – Please read the update at the end of this post, after the videos.

I’ve been wanting to build NA5N’s Desert Ratt regen ever since I first found his very attractively drawn schematic for it online. I then found the updated version, called the Desert Ratt 2, and a very good description of how the circuit works – all of these documents available on Paul’s website. What more could an avid regen builder want? Not much, it turned out. Late last year, when N2CX and N2APB dedicated an episode of Chat With The Designers to the Desert Ratt (and to the subject of regens in general), I just had to listen and of course, it fueled my interest in building the DR2 even more. The whiteboard for this particular episode of CWTD is here, and the podcast audio is here.

The WBR was a successful regen for me and while it worked well on SSB/CW, it didn’t seem to quite have the gain with AM stations. This makes sense, as a regenerative detector has to be set below the point of oscillation for AM reception, at which point it has less gain than when it is oscillating (which is where you set it for SSB/CW reception.)  Even so, I had read that bipolar transistors tend to work better as regen stages for AM, as they have higher gain when not oscillating. The search was on for such a receiver, and this was one of the key deciding factors in building the DR2 for me. In fact, Paul has mentioned (I forget where I saw it, as I have done so much reading on this receiver) that the Desert Ratt doesn’t do so well with SSB/CW as it does with AM. My experience with it backs up this assertion, thought it’s a pretty neat receiver for AM.

In particular, I wanted a receiver for covering the 49M SW BC band as although my Elecraft K2 covers a few of the BC bands, 49M is not one of them. There were a few things I found interesting about the design. The use of a phase splitter transistor to convert the single-ended output of the detector to a balanced output in order to drive the LM386 in differential mode was novel. Paul talks about how much RF is flying around inside regen receivers, and how the common-mode rejection of the 386 when used in differential mode can be advantageous in such an environment. I was also intrigued by the detector consisting of 2 germanium diodes – I think I was just looking for an excuse to build something with Germanium diodes again to remind me of my crystal-set building days as a kid 🙂

If you look at the schematic of the DR2,  you’ll see that one of the changes in the design from the original DR is that instead of a variable capacitor, it uses 1N4004 diodes as varicaps. I have a bit of a “thing” for nice air-spaced variable capacitors, and I had in mind a nice Millen 50pF capacitor that I picked up on eBay for a very fair price last year. Combined with a 6:1 reduction drive, it made a good combination with a very useable tuning rate for tuning in AM stations.

Anyway, I’m getting ahead of myself here. I did make a few changes to the original schematic for my version, so allow me to introduce my rather wobbly circuit diagram –

The differences between my schematic and Paul’s are as follows –

– I added an RF attenuation pot at the antenna input. After building the DR2, I found that using a relatively short piece of wire indoors as an antenna was causing a lot of common-mode hum.  On top of that, I wanted to be able to increase the signal level into the receiver with the use of my regular outside antenna (A 40M dipole fed with 300 ohm balanced feeder.)  Using the attenuation pot allowed me to use the large outdoor antenna without overloading the receiver.  Use of my outdoor antenna created enough separation between the receiver and antenna that the hum problem almost entirely disappeared.

– Earlier versions of the Desert Ratt included instructions for winding the coil on a plastic 35mm film canister and on an IC shipping tube. The DR2 schematic doesn’t include such instructions, but I wanted to use a toroid, so I experimented a bit and came up with a scheme that seems to work OK.  I used a T68-6 former and the turns info is on my schematic above – a T50-7 would take up a little less space. More about this later.

– I had a few 2-position center-off switches that I wanted to use, so I used one of these for a bandswitch instead of the SPST switch in NA5N’s DR2 schematic. I had originally thought that using the 50pF tuning capacitor with no padding would make the upper limit of frequency coverage too high, resulting in too large a frequency swing in one band, but there must have been more stray circuit capacitance than I had anticipated, as the coverage with no extra padding was about 7.3 – 13MHz. This band became the center position.

– I was attempting to power the DR2 from my shack power supply, which is about 45AH of sealed lead acid batteries with a float charger constantly connected.  This also powers my K2, and the DR2 was picking up processor noise from the K2, as well as a low-frequency “burbly” kind of noise of undetermined origin. The problem went away when I powered the receiver from a separate SLA. but I decided to add extra filtering to the power line anyway.  I found that a 1mH choke as well as a 1,000uF electrolytic almost (but not quite) got rid of the unwanted interference on the power line.  For good measure, I added a 0.01uF RF decoupling capacitor across the power line at the input connection.

– I added an AF preamp stage directly after the diode detector to ensure enough power to easily drive a speaker – even with weak signals.

– The inputs to the LM386 are the opposite way around from the way indicated in NA5N’s DR2 schematic.  With the inputs connected as shown in Paul’s diagram, the LM386 emitted a loud screeching sound.  Swapping the inputs cured this. I was not the only person who had this problem, as I discovered from this post in the GQRP Yahoo Group (you need to be a member of the group to read the post).

–  I left pin 7 unconnected. I don’t understand the way that NA5N has it connected to the junction of the series resistor and capacitor connected between pin 5 and ground in his diagram.  Most circuits that use pin 7 call for a decoupling capacitor direct from pin 7 to ground (usually about 10uF).  This helps reduce large signal distortion, though Paul does say that in this application, it may not do a great deal to help and is therefore optional.  I elected to leave it unconnected.

Now for some pictures.  I didn’t want to spend a lot of time constructing an enclosure, so decided to make a simple PCB L-shaped chassis and build the circuit directly onto that.  With the variable capacitor mounting bracket, it still ended up taking quite a while to construct though. All my projects begin like this, with the main components and control being laid out on the front panel, while deciding on the basic layout –

I’ll spare you the words at this point and apologize for all the pictures that are about to come. If you’re living in a remote area and are still relying on dial-up, then I feel a bit sheepish about the sheer number of images to follow!  I’ve talked before about constructing enclosures from PCB material, so won’t repeat that information here. As well as constructing the chassis from PCB material, I also made a mounting bracket for the variable capacitor and a tuning pointer to attach to the reduction drive with 2 small screws – all from double-sided copper-clad laminate.

I applied several thin coats of lacquer from an aerosol spray.  It was sprayed from a distance, resulting in a light, and stippled coating, which you can see in these pictures. I’d rather apply too light a coat than risk overdoing it. The downside of this is that oxidation will being to affect the appearance of the copper fairly soon. Oh well. The capacitor mounting bracket received a thicker coat. You can see the smoother, shinier finish.

I got the 6:1 reduction drive from Midnight Science. A number of others sell them, and one place that springs to mind is Mainline Electronics in the UK. They are the suppliers for Jackson Bros components (I think they have the rights to manufacture and sell the parts).  They sell on eBay using the name anonalouise.

The enclosure looked a little bit different by the time the DR2 was finished, as the hole for the nylon toroid mounting hardware hadn’t been drilled in the base at this point.

Look at that gorgeous variable capacitor!

A close-up view of the Millen 21050 50pF air-spaced variable capacitor and mounting bracket. This component is silver-plated (the vanes are probably brass), and has double bearings and a ceramic base. It is a very nice variable capacitor, and had never been soldered to before being used in this project. It is at least 35 years old – most likely older!

Boy, was I glad to finish the chassis so that I could start work on wiring it all up.  I decided to build the AF amp first and work backwards, my thinking being that the AF amp would be relatively straightforward. The act of touching the input with a metal screwdriver and hearing a hearty buzz in the loudspeaker would give a welcome psychological boost! If I started by building from the antenna end, I’d have to wait until the entire receiver was built before getting any clue as to whether it was working.

Here’s the chassis with the LM386 amp, the 2N3904 phase splitter, and the 2N3904 preamp built. As has been the case with all my projects since I started using then, I used W1REX’s wonderful MePADs and MeSQUAREs to build the circuit –

Here’s a close-up. The 2N3904 preamp is just below the 6:1 reduction drive, and the 2N3904 phase splitter is to the left of the LM386.  The 100uF capacitor that decouples the supply line to the LM386 straddles it. I read that it is best to ground it to pin 4 instead of to some other point on the chassis to avoid instability, hence the reason for this placement. The other electrolytic that is straddling the chip is the 10uF capacitor between pins 1 and 8 that sets it to the maximum gain of 46dB. The black shielded cable connecting the AF gain pot to the circuit on the PCB is lavalier mic cable.  It has 2 conductors, each of them in it’s own shield, which is ideal for wiring up potentiometers. It is fairly thin and very flexible. I use it in all my home-brew projects. I bought it from a local pro-audio store which recently closed down, so will now need to find another supplier.

In this view, you can clearly see the extra DC supply line filtering that I added, consisting of a 1mH choke in series with, and a 1,000uF electrolytic across, the DC supply. After seeing these pictures, I noticed that there wasn’t very much solder on the joint connecting the choke to the power jack, so I re-flowed the joint and melted a bit more solder onto it.

The power indicator LED’s main function is as a voltage regulator. NA5N marked the various voltages on his schematic for the DR2, and I chose an LED with a forward voltage drop to match those voltages as close as I could.  A green LED in a variety pack I got from Radio Shack had a forward voltage drop of 2.1V, which seemed about right.  The 1N4148 had a forward drop of about 0.65V.

The next stages to be built were the detector and impedance converter/buffer stages.  The description of the DR2 on NA5N’s site gives more info on these stages (as it does for the whole receiver). I couldn’t be sure these stages were working, but bringing my finger close to the diodes resulted in a pleasing cacophony of stations in the headphones – and at a louder level than in doing the same to subsequent stages, so I figured there was some detection/amplification going on 🙂

I didn’t know how many turns I was going to use on the toroid, but using the calculator on W8DIZ’ site and an online resonant frequency calculator, I figured that 36 turns on a T68-6 should be a good starting point for the whole winding from pin 3 to pin 6. In Paul’s version, with the coils wound “traditional style”, the tickler winding was about 1/3 of the whole winding.  Coupling between windings is tighter with a toroid than a “regular” coil, so I reduced the number of turns on the tickler. I found that regeneration was occuring at only about 25% rotation of the regen pot, so further reduced the number of turns. Using the turns shown on my schematic at the beginning of this post,  the regen stage moved into oscillation at anywhere between 40 and 50% rotation on the pot, so I left it at that. For the same reason of tight coupling, I used fewer turns on the antenna winding too and because I am using an outdoor antenna, could probably have used even fewer turns.

The toroid was fixed to the PCB with nylon nuts, bolts and washers that I got from my local Ace hardware store.

Here are some pictures of my Desert Ratt 2 with the circuit finished –

The red wires running along the back of the front panel are the regulated 2.1V and 2.75V lines.  I would have run them on the main board but ran out of room due to lack of planning, so went vertical.  Incidentally, although I refer to the 2 regulated lines as 2.1V and 2.75V,  the exact voltages aren’t important.  That’s just what they turned out to be in my case.

The RF amp and regen stages can benefit from transistors with high hfe. I got a cheap Harbor Freight DMM that measures hfe from an eBay vendor for under $6 including shipping.  hfe varies depending on the collector current, but I was doing this mainly for comparative purposes rather than absolute values, so the fact that I didn’t know what value of collector current was used to measure hfe in this cheap meter didn’t matter. It just so happened that my 2N2222A’s tended to have higher hfe than my 2N3904’s, so I ended up using a 2N2222A that measured in at hfe = 203 for the RF amp, and a 2N2222A with hfe = 223 for the regen stage.  The other stages don’t require high-gain transistors. NA5N talks about it in this post on QRP-L from 1999. Bear in mind that he was talking about the original version of the Desert Ratt in this post (just so you don’t get confused when he identifies the various transistors).

I did promise that I’d give a bit more detail on the toroid. Mine was wound on a T68-6 former. The main winding was 30 turns tapped at 27 turns from the top (3 turns from the bottom). The antenna coupling winding was 5 turns.  All turns are wound in the same direction. I used 26 gauge wire, but the precise gauge isn’t important. 26 gauge was narrow enough to easily fit all the turns on the former, yet stout enough to lend some stability to the oscillator, as the toroid isn’t sitting close to the board, and the leads are relatively long. When putting taps on coils, I used to not cut the wire i.e. I would simply make a loop in the wire, twist it, tin the twisted part and keep on winding.  Now I find it is easier to treat them as 2 separate windings connected together. If you can get heat-strippable wire, please do – it makes winding toroids so much easier and more pleasurable.  I wound the first winding of 27 turns, stripped and tinned the end, then stripped and tinned the end of another piece of wire, twisted and soldered them together, and carried on winding the last 3 turns in the same direction (this is important).  The separate antenna winding of 5 turns is also wound in the same direction.  I’m afraid I didn’t write down (or if I did, I have since lost it) the lengths of wire used. I did notice that the turns calculator on W8DIZ’ site (linked earlier in this post) was quoting lengths that are too short for the T68-6 former.  All you have to do is wind one turn around your former, measure that length, multiply it by the number of turns you’re going to wind, add an extra inch or two for the leads and, as we say in England, Bob’s yer Uncle and Fanny’s yer Aunt (meaning – you’re home free!)  When winding toroids, I often find that the first 1 or 2 turns aren’t quite as tight as the rest so when I’ve finished winding, I will unwind one turn from the beginning of the coil, then wind an extra one at the end, to keep the total number of turns the same.  Sometimes I will repeat that exercise a few more times until all the turns are nice and tight.  For this reason, I use enough wire to leave several extra inches at each end.

The next picture shows an anti-hiss filter that wasn’t in the earlier pictures, which I tried and ended up removing due to a low-frequency oscillation it was causing at the higher volume settings.   It was a series 0.01uF capacitor and 4.7K resistor connected from pin 1 of the LM386 to pin 5.   From what I have read, too low a value of resistor or too high a value of capacitor can cause the oscillation. I have seen other anti-hiss filters that used a 0.01uF cap and a 10K resistor, so it is very possible those values would have cured my problem. However, I was near the end of the project and itching to move on, so I just removed it. You can also see the 0.1uF capacitors on the inputs of the IC that have been swapped over to stop the uncontrolled oscillation, and are now crossing each other.  You may not have to cross these caps if you plan your layout accordingly –

Other than the problem with the loud screeching that was solved by swapping over the inputs to the LM386 (my schematic reflects the way the inputs were finally connected), the only other problem I had was with what appeared to be a defect in the 0.001uF (1,000pF) capacitor that leads from the tap on the coil to the emitter of the regen transistor.  I wasn’t getting any regeneration at all but on replacing this capacitor, the circuit broke into a nice loud hiss when advancing the regen pot.

I do have one ongoing issue that I hope someone can shine a light on for me, and that is a loud crackling sound when adjusting the tuning capacitor. At first, I thought a dirty rotor connection was the problem, but it only happens when extra padding capacitance is switched in by the band-switch   With no extra capacitance switched in, the tuning is smooth, but on the lower frequency bands, the receiver crackles when being tuned.  I need to try bypassing the band-switch and soldering the padding capacitors into circuit in case the switch is the problem. I’ll report back when I’ve done further work on this.

Incidentally, the main tuning range on mine covers approximately 7250 – 13000KHz.  Switching in a 47pF capacitor changes the range to 5825 – 8050KHz. I’m a bit limited with my receiver and test equipment here, so haven’t yet been able to determine the coverage of the lowest frequency band.

When first listening to the DR2, I had no idea what frequency I was listening to – only that I was probably somewhere between 5 and 12 MHz. I had no antenna connected (and at this point, hadn’t even built the RF amp stage) but started hearing CW. Lo and behold, it was Hank W6SX 180 miles away from me in Mammoth Lakes, CA. His CW signal was coming through well and in fact, this was the only time I have received CW in a satisfactory fashion on the Desert Ratt. There was no antenna – he was being picked up directly by the toroid.  Any concerns I might have had about the sensitivity of this receiver would have been immediately allayed.

I know the main question that is probably on your mind is – how does it sound, and what is it like to use? How does it “handle”? There are some videos of my Desert Ratt 2 in action at the end of this post. Apologies for the poor video quality, but my only video camera is 10 years old (and has a faulty CCD sensor). You’ve probably read articles about regens that describe the many and subtle adjustments that need to be made when tuning a regen in order to coax maximum performance from it. If you haven’t operated a regen before or if it’s been a while, it does take some time to get the hang of getting the best out of it. As you get further away from the setting of the regen pot where it breaks out into oscillation you lose selectivity and gain, so you need to try and keep the control set just under the point of oscillation. Loud stations can overload the detector, resulting in audio distortion, so it’s worth keeping an eye on the RF attenuation pot too. Also, if the attenuation pot is set too high (too little attenuation), you may get breakthrough from stations on other frequencies. There’s quite a bit going on to keep under control, but if you manage to keep all controls adjusted well, you can coax some pretty decent performance out of the set. I think this is why regens appeal to some people – we are incurable knob-twiddlers!

Stability is easily good enough for AM reception and with a logging scale fitted to the front panel, I don’t think it would be hard to find specific frequencies, as the majority of SW BC stations stick to 5KHz channels. In my casual listening so far, I have heard The Voice Of (North) Korea on 9435 and 11710KHz, Radio Habana, Cuba on the 49M band, Radio Australia on the 31M band, coastal station KLB (South Korea) on 8636KHz, the BBC World Service (forget which band or frequency), China Radio International on 9790KHz, WTWW on 5830KHz, and a number of other evangelical Christian stations (sorry, I tune them out and don’t pay them much attention.)

To sum up, you can definitely have a lot of fun and engagement with the bands on this set.  Being a regen, it is not the easiest receiver to operate, but you shouldn’t let that put you off. The best analogy I can think of is to reference the way that although an older British sports car may not have the finesse and performance of a newer sports model, it’s a lot of fun, and it’s lack of suspension gives you an exhilarating feel for the road that the more expensive cars cannot.

The Desert Ratt 2. A logging scale fixed to the front panel would make frequencies in the SWBC bands easy to find. I must do this sometime 🙂

Please note that in the following videos, an MFJ-281 ClearTone speaker was used. My understanding is that this speaker has a slight resonant peak at around 700Hz (helpful for CW) and a relatively restricted overall bandwidth that is good for communications applications. This probably means that it’s not optimum for getting the maximum fidelity from an AM SW broadcaster (not that those stations have a lot of fidelity, but they tend to have a bit more than your average SSB transmission). On top of that, the audio was captured with the built-in mic in my old Canon A80 compact. Please don’t judge the quality of the Desert Ratt 2 audio from these clips. It’s better than this! I’m working on a few audio only recordings that will better demonstrate what the DR2 sounds like, and will put them up in the next blog-post (hopefully within a week or so).

Update – It has been about a year since I built my version of the Desert Ratt 2 and I feel compelled to provide an update. Whenever I first build a project I am often so thrilled that it works at all, that I tend to gloss over any shortcomings, particularly in my blog write-ups. Some of this is due to the possibility that any deficiencies are due to my layout and construction, as opposed to a problem with the circuit design. In the case of my DR2, I am still not sure whether the issues arise from the circuit itself or from my construction, as I have only built one of these. I did, however, want to document what I have observed, as my DR2 has laid on my shelf for the past year, largely unused, while I drag my WBR out and take it for a spin on a regular basis. Here are the issues I have observed –

* There is a lot of scratchiness in the speaker when tuning the DR2. This happens on some frequency ranges more than others, but it happens a lot.  At first, I wondered if it was due to inadequate grounding of the rotor plates but I don’t think this is the case. There is a solder tab for both the rotor and the stator, and the rotor is grounded to the chassis by a direct wire. Also, it is a quality Millen variable capacitor, and it is clean (the oxidation has been cleaned off).  I’m still considering the possibility that it as something to do with my variable capacitor, or the way that I have connected it.

* The set does seem to overload very easily on my outside antenna. Breakthrough from other frequencies is a common occurrence. This got me to thinking about the RF amp stage. The instructions call for picking a high hfe transistor to use in this position but thinking about this, I’m not sure why. Surely the purpose of an RF stage in a regenerative set is to provide isolation between the detector and the antenna, with gain actually being undesirable, due to the tendency of the detector to overload? The more I think about it, the more I think that different configuration for this RF stage would be more appropriate.

* Hum, though not always apparent, does still occur from time to time.

A commenter who goes by the name of Mast does mention that the tank circuit is very tightly coupled to the collector of the regen transistor. I’ll cut and paste his comments here, as I now wonder why I didn’t pay more attention to his input at the time,

“A nice schematic for general use. But the tank circuit is tightly coupled to the collector of the regenerative stage. You will suffer a lot from changing internal stray capacitances of the transistor when setting the regen level. And strong SSB signals will change these capacitances too, causing an unintelligible reproduction of SSB signals.”

At this point, the DR2 has gone back on the shelf while I move on to planning other projects, but I’d be very interested to hear what the experiences of others have been with this circuit.  I know there are folk who found N1TEV’s beginner’s regen to be a little hard to tame – and the DR is based in part on that circuit.  In contrast, both my WBR’s are well-behaved, and have been used regularly since I built them.

June 3, 2012

The DSB80 – A Direct Conversion DSB Transceiver for 80M By G4JST and G3WPO – First Stage Of Building

I was 19 years old in March 1983 when the UK magazine Ham Radio Today published the article “A Low Cost DSB/CW Transceiver for 80M”. Being short of cash and wanting to get on the air, I sent away for the kit and soon after, was surfing the phone portion of the UK 80M band on my new DSB rig.  I didn’t get too many QSO’s due probably, to my poor 80M dipole, although G3UMV who lived just a mile down the road heard my home-brew signals and came over to see where they were coming from.  The receiver seemed to work very well, and I spent many hours listening to the chatting between 3.6 and 3.8MHz (80M only goes as high as 3.8MHz in the UK). The whole thing was enclosed in an aluminum case and tuned with a Philmore vernier attached to a polyvaricon.  I don’t remember any drift, so it must have been stable enough for sideband, and it didn’t have any noticeable microphonics either.  As it was my first DC receiver, I didn’t even know that this type of circuit often suffered from microphonics, as this one didn’t have any to speak of.

That little rig made it with me across the Atlantic and met it’s end one day in my apartment just a block from Hollywood Blvd. In a passing wave of nostalgia for my earlier radio days, I hooked it up to 12V DC to see if it still worked. It would have, had I not connected the 12V the wrong way round, and if I’d had the foresight as a kid to provide it with reverse polarity protection. I still don’t know why I didn’t just put it aside so that at a later date I could have replaced the damaged active devices. Unfortunately, I tossed it into the dumpster of my apartment building. What a shame – and it had an SBL1-8 mixer too!

From time to time either when moving or thoroughly tidying my apartment, I come across the copy of the original article that came with the kit. Trouble is, whenever I looked specifically for it, I could never find it, and the only copy of the article I was able to find on the internet is of too low a resolution to be of much use. I’ve been wanting to recreate this rig for a while and recently, when the desire became too strong to ignore, decided that I was going to find that article even if it took a day or two of searching. It did, but I did.

Pure nostalgia wasn’t the main reason for my wanting to build this rig again. A big reason is that I have always been drawn to simple receiver topologies such as regens and direct conversion receivers, yet not all designs are created equal. I remembered this one as working well and on top of that, it used something in the circuit that you don’t see too often in DC receiver designs these days – a passive diode ring mixer package (NE602 anyone?)  I wanted to build a DSB rig that used a diode ring mixer package, so this is why I am here.

The schematics for this rig aren’t that easy to come across.  I eventually a found a low-res version of the article online after some searching but it’s not really good enough to work from.,Ham Radio Today is no longer being published, and the company that sold the kit back in the 80’s, G3WPO Communications,  went out of business a long time ago. On top of that, Tony Bailey G3WPO is no longer an active radio amateur. On this page on his website he gives a link to a reprint of an article about another of his projects, the Minisynth VFO. Judging from this, and what he says in the whole of that 3rd paragraph, I don’t think he’d mind my publishing the schematics for the DSB80 here on my blog.  That is what I am hoping as I’m pretty sure that some readers will want to see the schematics, and there doesn’t seem to be any other way to get them. I’m having a bit of trouble with the VFO (more on that later), so by showing you photos of my construction and the schematics, I’m hoping someone may be able to help me.

The plan is to get this working well as a receiver first, after which I’ll add the transmitter stages.  So to kick things off, here’s a block diagram of the receiver, a pretty standard diagram of a direct conversion receiver:

I built the VFO first of all. It seemed to work OK and be reasonably stable, with drift of less than 80Hz/hour after warm-up. I know that’s not stellar, but a bit of temperature compensation could help that.  Somewhere in between adding the buffer and adding the rest of the receiver, I noticed that the VFO was drifting a bit more and FM’ing, which makes SSB sound pretty bad. However, if I can lower the drift on the VFO and get it to stop FM’ing, I think I’ll have a nice-sounding direct conversion receiver on my hands. There are virtually no microphonics – you have to turn the volume way up and really be listening in order to hear them. For all practical purposes, microphony is just not a problem; something I like very much in a DC receiver.

I’m getting ahead of myself. Here’s the original circuit for the 80M VFO in the DSB80. I did leave out a trimcap, 1N4148 diode and associated components that were used to switch in a CW offset, as I won’t be using this rig for CW:

Important – please note that I experienced instability with the buffer transistor Q2 (it wasn’t doing a lot of buffering). I don’t understand why Q1 was coupled to Q2 with a 100 ohm resistor instead of a capacitor, but at the suggestion of K4AHO, I replaced it with a coupling capacitor (I used a 39pF NPO) and put a 100K resistor from the gate of Q2 to ground. My problems with the buffer cleared up and the receiver sounded really great. I’ll publish the schematic of the entire receiver section of this rig in a future post.

The oscillator is a Colpitt’s configuration and the 260pF variable was, in the original design, a polyvaricon.  I wanted to modify the VFO for varactor tuning (at what point did we stop calling them by the more descriptive term varicaps and start using the name that makes them sound like a prehistoric bird?) and also figured that a 78L08 regulator in place of the 8.2V zener diode could only help. This is what I came up with:

All the caps marked “poly” are polystyrene. I changed the values of the 2 x 1000pF caps to 1200pF simply because that’s what was available.

Projects always look pretty when I first start them, before I’ve had a chance to mess them up –

Here’s the VFO, although I have yet to add the varactor at this point – it will be located at the far right end of the board –

One more view just for the heck of it –

Somehow, by the time I got around to adding the mixer, AF preamp, AF amp and bandpass filter to complete the receiver, the whole thing started to look just a little bit messy. You’ll notice that I ditched the nylon mounting hardware for the VFO toroid in favor of a single nylon strap. I figured it would be one less material in contact with a frequency-determining part of the circuit that might cause drift. I didn’t clean up the board for it’s photo-op, as this is a work-in-progress that may not make it out of the emergency room –

VFO drift was steadily downwards after the initial warm-up and probably something that could be brought to within useable limits with some temperature compensation work.  There are almost no microphonics to speak of (always a good thing in a direct conversion receiver), and only a small amount of broadcast band break-through which is only coming through at the kind of high volumes that will rarely be used. This breakthrough is not coming from the DBM, but from the preamp, which is set for a gain of 1,000 (60dB). If this rig makes it to a later stage of building, I may reduce the gain of that preamp just a bit – we’ll see. The receiver sounds pretty good on 80M SSB with one big problem – the VFO FM’s when receiving signals, and that IS a problem.

The documentation that came with my kit for this rig in 1983 had something to say about  FM’ing of the VFO:

Hmmm….but I was using J310’s in this re-creation and was still getting FM’ing of the VFO.  As far as I can remember, it was not happening in the original version I had.

I decided to build the original VFO circuit with the zener diode regulation and with an air-spaced variable capacitor instead of a varactor to tune the circuit (the first schematic in this post and the second image down). I wanted to do this on a separate board, before connecting it to the rest of the receiver.  This is where it started. It sure was exciting looking at a bare board (blank canvas) with just an air-spaced variable capacitor. The variable capacitor was given to me by a friend and boy, what a great gift. Thank you – you know who you are. I was looking at this thing and thinking of all the possibilities – a signal generator, crystal set, or a regen perhaps?  Air-spaced variable capacitors are very inspiring to me –

Here’s the VFO circuit built – no buffer yet, and you’ll notice that I have not yet installed C2 and C3.  I wanted to see what the frequency coverage was without those capacitors first.  It was pretty wide, so I ended up installing C2 and C3 in the values suggested, and removing a few turns from L1 to achieve coverage of 3600 – 4000 KHz with one gang of the capacitor, which was about 330pF at full capacitance –

And with the buffer added, and temporarily terminated with a 51 ohm resistor for drift testing:

I noticed that on touching the output of the buffer with a small metal screwdriver, the frequency of the VFO changed by about 600Hz when terminated with the 51 ohm resistor. I wonder if this is the reason the VFO I built on the main board FM’s when receiving signals? The only difference between this one and the one that is incorporated into the rest of the circuit as pictured 5 images above, is that the one directly above is tuned by a variable capacitor, whereas the other one is tuned by a varactor. Either way, it suggests to me that I need another stage of buffering. Before I even look at the drift and figure out how to compensate for it, I need to tackle this issue.

To be continued……..(unless another project derails this and it ends up on the shelf, with the variable capacitor used for something else…..)

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