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!

July 27, 2012

Using the NM0S Hi-Per-Mite Filter From 4SQRP To Make A Simple 40M DC CW RX

When 4SQRP brought out a version of the David Cripe NM0S-designed Hi-Per-Mite Filter, I noticed that it could be configured to give up to 50dB of gain. Like many others, I’m sure, my next thought was that it would make the process of building a simple direct conversion receiver even simpler. I decided that at some point, I needed to use this little filter board as the audio chain in a direct conversion receiver for CW.  I wanted to see for myself how it would sound.

In the meantime, my preoccupation with direct conversion receivers continued, as I embarked on building a G3WPO/G4JST DSB80. The receiver in that DSB rig uses an SBL1-8 DBM. That particular mixer package is out of production, but I used an ADE-1 and was pleasantly surprised by the receiver. I had some problems with the TX section, so put it aside and started building a ZL2BMI DSB transceiver.  I’d known about this little rig for a while from the pages of SPRAT, but a comment on Twitter from NT7S encouraged me to have a go at building it. The receiver is a simple NE602/LM386 combination with just a single-tuned filter on the antenna input. Of course, the receiver is not THAT simple, as each chip contains more complex circuitry, but the schematic at the chip level looks almost too simple to work.  As with the DSB80, I had problems with my ZL2BMI rig transmitting a sizeable residual carrier, but was surprised that the receiver sounded pretty good. The seed was sown…..

The ZL2BMI DSB rig was actually only the second time I had built an NE602/LM386 direct conversion receiver. The first was about 20 years ago when I bought a kit for the Sudden receiver on 20M. At the time, I wasn’t that keen on it and it soon ended up in pieces. I’m not exactly sure why I wasn’t taken by it, but I think it was a combination of factors. Firstly, I was living in an apartment in Hollywood with no outdoor antenna and was using just a short length of wire indoors as an antenna. It’s also highly possible that band conditions weren’t great, but I just remember not hearing many signals and also not liking the fact that a half-turn of the variable capacitor covered the entire 20M band. Because of this one not so great experience, I have since harbored a bias against using NE602’s in direct conversion receivers, an attitude that I now realize was unfair.

After achieving only partial success with both these rigs but enjoying the fact that the direct conversion receivers in both of them worked quite well, I wanted to build a simple receiver and use the Hi-Per-Mite kit that had been sitting patiently on my shelf for the last few weeks.

Look at the schematic for any NE602/LM386 DC receiver, and they are almost all the same, differing only in whether they use a crystal, ceramic resonator, or free-running VFO for frequency control, whether they have a single-tuned or double-tuned antenna input filter, or whether they use a differential or single-ended audio output.  The circuit is so simple that there is only so much room for variation, but there were a couple of things I had in mind for my mine.  While the receiver portion of the ZL2BMI rig only has a single-tuned filter on the antenna input, I wanted to do what G3RJV did with the Sudden, and place a double-tuned filter there. It’s an easy thing to do, and living in a built-up urban area, I have a lot more RF around me than ZL2BMI does when he’s using his rig in the bush, so a bit of extra bandpass filtering certainly couldn’t hurt. The other decision to be made was the method of frequency control. I was curious to see how the internal oscillator in the NE602 would work as a free-running VFO.  My apologies to you folk who have built umpteen simple NE602-based receivers and have-been-there-and-done-that.  I guess I need to get this out of my system! Another thing that I had seen in other circuits but hadn’t tried was the use of a 1N4001 diode as a varactor. Some call it the poor man’s varactor. Sounds like it was custom-made for me!

So that was my configuration – double-tuned input filter (just like the Sudden) and a free-running VFO tuned with a 1N4001 diode.  From what I’d read, I knew that I should be able to get enough capacitance swing from the 1N4001 to cover at least the bottom 30-50KHz of 40M. Here’s what I ended up with:

There’s nothing original in this schematic  but as you’ll see soon, it did turn out quite well, so I decided to name mine “The Rugster” after my cat, whose full name is Chloe-Rug, but who gets called several different variants of that name, of which “The Rugster” is one.  For the input bandpass filter, I used the vales of L and C that are used in the GQRP 40M Sudden Receiver. The pre-wound inductors in that design have a value of 5.3uH. I decided that I wanted to use T37-2 cores, so I used the calculator on W8DIZ’s site to figure out that I’d need about 36 turns on a T37-2 core to give 5.3uH of inductance. I fiddled around a bit with the values of the parallel fixed capacitors before settling on 47pF for those. I had originally thought that a higher value of around 68pF should work, but that didn’t put the peak of the filter somewhere in the mid-range of the adjustment of the trimcaps.  Some experimentation revealed that 47pF fixed caps achieved this.  Your value may vary depending on what value of trimcap you are using.  For the VFO inductor, I found a design for a 40M DC RX on the internet that was tuned by a 1N4001, noted the value of the inductance, decided that I wanted to use a T50-7 core (for stability) and once again, used Diz’ site to calculate the number of turns needed.

When building receivers, I like to start with the audio chain first. It’s quite affirming to be able to touch the input and hear that reassuring loud hum and mixture of AM broadcast stations in the speaker! So I got out the 4SQRP Hi-Per-Mite kit. Close inspection of the PCB revealed a small problem that was easily remedied. There was a very, very narrow copper trace connected to the ground plane that had been bent so that it was contacting the input pad.  A DVM test confirmed that the input pad was shorted to ground. I didn’t take this picture until I was part of the way through removing the offending trace, but you should be able to see the problem:

A minute or two with a sharp craft knife and that pesky little trace was history. Here’s the finished board:

The only thing that prevented me from putting this kit together even more quickly was my own compulsion to solder the components into the board so that they line up as straight as possible. The holes for some of the capacitors were a bit larger that they needed to be so that if you just plug them into the holes and solder, they’ll be a bit off-square. It’s not a big deal, and most builders won’t be bothered by it, but I just can’t control my OCD tendencies sometimes. Anywhere, I finally got there:

Although with modern low-flux solders, there is no need to remove the flux, it sure does make a board look nicer. I recently started using flux remover on my boards so that I can show off the underside too. At the suggestion of W2EAW, I apply it with a cheap plumber’s flux brush obtained (for 19c) from the local hardware store. Look at that nice clean board!

Trader Joe’s sells Green Tea Mints in a clear-top case that fits the Hi-Per-Mite board well if you’re looking to use it as a stand-alone device. There’s also room for power and in/out connectors (though you might need to cut off the top corners of the board in order to make room for the connectors.)  The tin in this shot had been knocking around my bench for a while, so the clear-top is a bit scratched up, but you can see the potential:

The next step was to build the direct conversion receiver with the NE602 and some of W1REX’s MePADS and MeSQUARES. The most frequency-sensitive part of the VFO circuit was built ugly-style, with the help of a 10M resistor for a stand-off, to help the stability. I don’t have any pictures of the board without the Hi-Per-Mite audio filter/amp added, so here’s the finished board. In the earlier shots, you can see the VFO toroid before I decided to slather it in hot glue in a bid to make it a bit more stable. There is only one component in the final receiver that I missed from the schematic, and that is a 1N4001 diode in series with the 12V supply for reverse-voltage protection.  I’ve blown too many fuses in my shack supply to not use these little fellows in the future🙂

Another shot from basically the same end of the board, showing the input bandpass filter on the left, VFO toroid on the left at the back and the DC input circuit nearest you on the right (reading from right to left, you see the reverse-polarity protection diode,  10uF input smoothing capacitor, 78L08 voltage regulator, and the 0.1uF DC output bypass capacitor. I’ve had those green trimcaps for a long time and it was time they were used. They don’t take solder well (I think I got them from Radio Shack about 10-15 years ago) but it seemed a shame not to use them in something:

Here you can see the varactor tuning circuit built ugly-style just to the left of the VFO toroid. The 3 connecting cables are for the AF gain pot (which is connected in place of R11 and R12 in the Hi-Per-Mite, as per the Hi-Per-Mite instructions), multi-turn tuning pot, and AF output jack. I’ve used lavalier mic cable that I bought from a local pro-audio store. It has 2 inner conductors shielded by a single outer braid and is very flexible, which makes it useful for wiring up small radios.  RF connections (which you’ll see in later photos) will be made with Belden 8216 coax:

At this point, I was very gratified to discover that the receiver actually worked, and seemed to work quite well too. That fact gave me enough inspiration to start building an enclosure to house it in.  The next shot isn’t ideal, as the back of the case is a little out of focus. You’ll notice that the front, and the two bolts sticking up on the right side are in focus, but the back of the case (and especially the bolt at the back left) isn’t. When I realized this, I was too far along installing the board to go back and take another shot and besides, sometimes I just have to control myself and keep forging ahead:

A view of the board installed in the case, with the lid:

A closer view from the top, in which you can see the VFO toroid now doused thoroughly in hot glue.  I’m not sure if it helped the long-term stability at all, though it did help to make the VFO more resistant to physical shock. Another way of securing the VFO toroid in a vertical position would have been to drill 2 small holes in the PCB and use a small nylon tie:

The coax that leads from the 1K log RF attenuator pot to the board is a little long, and that is because the one you see in the photos is a 10K linear pot. It was all I had at the time of building.  I am going to swap it out for a 1K log pot when the next order arrives from Mouser in a few days, and kept that lead a little long in case I need to chop some of the end off when changing the pot out.

Here’s another view of the innards. I am quite pleased with the way this little receiver turned out. You can see one of the 4 brass nuts that are used to secure the lid to the rest of the case. I use brass, as I can solder to it with regular solder. When placing the nut, and before soldering, I screw the 4-40 machine screw through the side of the case into it so I know the nut is in the right place.  If I were to get a little over-zealous when applying solder and accidentally get solder on the screw it wouldn’t stick, as the screw is made of stainless steel:

This is what The Rugster looks like in her case with the lid on:

If you look closely at these pictures, you’ll see that the PCB panels don’t line up perfectly and that the edges are just a little rough.  I am somewhat detail-oriented, but I am not a craftsman by any means, and I know my limits.  I made a conscious decision not to spend a lot of time filing and sanding edges. After making deep scores in the board and breaking it, I ran the edges across a file on the bench a few times and left it at that. Good enough is good enough in this case, and it’s certainly good enough for me.

A view of the underside:

I’ve spent a few evenings listening to it and most of the signals that my K2 could hear, this little receiver could hear almost as well. With the very weak signals the K2 won, of course.  A small part  of this could be due to the fact that the DC receiver, even with it’s 200Hz-wide filter, is still at a 3dB disadvantage to a single-signal receiver (such as a superhet), as it is hearing on both sides of the local oscillator.  Naturally, I expected my K2 to be the better receiver (duh!) but was really surprised at what an enjoyable experience listening to this one is for such a simple circuit.  As expected, it does overload in the evenings, but all I have to do to get rid of the breakthrough is to adjust the RF attenuation pot back a little from “full gain”, and the breakthrough disappears.   Elecraft use an NE602 in the front end of the K1, and it has an attenuator for the same purpose. I’m not sure whether the breakthrough is from strong in-band amateur signals or from AM broadcasters – it’s actually hard to hear the content due to the narrow bandwidth of the Hi-Per-Mite. One small adjustment of the RF gain pot and it’s gone – then I just bump up the AF gain a bit to compensate. Volume is enough to drive my MFJ-281 ClearTone speaker.  I normally like to listen to CW with a 500Hz note, but because the response of the ClearTone drops off below 600Hz, I kept the Hi-Per-Mite filter at it’s design center-frequency of 700Hz, and the speaker sounds good. EDIT – If the AM breakthrough is from signals in the 550 – 1700KHz AM broadcast band, which I strongly suspect that it is, a simple high-pass filter with the cut-off set somewhere around 2 or 3MHz should work wonders to eliminate this.

Another point I wanted to mention is that even though the Hi Per Mite employs an LM386 for the AF amp, it is used in the low-gain configuration. Because of this, you don’t hear any of the hiss that is heard in other simple receivers that employ a 10uF capacitor between pins 1 and 8 for high gain. This is a surprisingly pleasant receiver to listen to.

The fact that the tuning rate is quite low helps a lot in the enjoyment of this receiver too.  The 10-turn pot covers 6999KHz – 7051KHz giving an average tuning rate of about 5KHz/rotation. The VFO shifts in frequency very little indeed when  the receiver is knocked or moved. Drift is not great, but manageable.  Drift in the first 10-15 minutes is only slightly worse than after that initial warm-up period,  so I’ll give you the results from switch-on, which were 90Hz drift in the first hour,  140Hz drift in the second hour, and 110Hz in the 3rd hour. All the drift was downward – there was no upward drift at all. The drift was steady, and with a little bit of compensation, I think those figures could be improved upon quite a bit. However, for casual listening (and this is all I am going to use this for) it is adequate.

This receiver is certainly not perfect, but considering it’s just an NE602 and an LM386 (oh – and a quad op-amp IC for the filtering), it’s not bad at all.

Thanks Dave NM0S and 4SQRP for this neat little audio CW filter kit.

Here are a couple of videos that I just made of The Rugster. Please excuse the poor video quality – it’s an old, cheap camera:

This one is a little shorter:

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