Courtesy Of F5VJD – A Working G4JST/G3WPO DSB80 80M DSB Transceiver!

As I’ve explained in earlier posts, my interest in the G4JST/G3WPO DSB80 rig goes back a long way – back to the mid 1980′s in England when I read the description of this little rig in Ham Radio Today and scratched together the money to buy the basic kit. An enclosure was also available but I didn’t have enough money for that as well, so built it into an aluminum enclosure I already owned. That little rig gave me much fun until one day in my apartment in Hollywood in the mid-1990′s when I accidentally connected the 12V power to it the wrong way round and it instantly stopped working. I didn’t have the patience, time or experience to get it working again, so (and it pains me to say this) I tossed it into the trash. There have been several times since then that I have regretted doing that, and since then, the inability to see my DSB80 only served to further memorialize that rig in my mind.

Recently I dug up the reprint of the article that came with the DSB80 kit (I did keep that) and tried to rebuild it Manhattan style – here and here.  The receiver worked as well as I remembered the original working, but I experienced a problem with the transmission of a residual carrier on transmit. After a few weak attempts to cure that issue, I shelved it in favor of other construction projects. It’s a weakness of mine – if something doesn’t work well the first time I switch it on, unless the fix is relatively simple, I don’t always pursue it. I didn’t consider it a failure, as the receiver worked well – and it was wonderful to once again listen to a direct conversion receiver that utilized a diode ring DBM . The TX seemed to work up to and including the final,  so there was really not that much to troubleshoot if I ever wanted to give it another go.

Then a very exciting comment was left by Richard F5VJD on the blog post about my DSB80 build. It read as follows -

“Hello Dave

I also built one of these in 1983 and I still have it… but like you I fried it during a fit of nostalgia.

I do still have the original instruction sheets and the original Ham Radio Today articles which I could copy and send you if you like… and as I can’t really see myself replacing all the fused components in the RX, I would also be happy to send you the radio if you promise to repair and use it. It has some nostalgia value for me because it was the first kit I ever built and I had my first ever QRP QSO using it, so it would be nice if it was back on the air again after all these years.

Let me know what you want to do and I’ll get things started my end.

73

Richard”

Well, how do you think I replied? This was a chance to see that little rig again – and to see how another ham had built it too.  Richard told me that he had used the original case suggested by Frank and Tony, so I was curious to see a more “original” version of the DSB80 than mine was.  Richard packed up the mighty little transceiver and even took a picture of him with it outside his home in Northern France to mark the beginning of the journey to it’s new home in California:

About a week later, it turned up on my doorstep in California:

Wow – talk about well-packed! The DSB80 was very well protected with much packing material, and Richard had written his address inside each layer while packing, just in case it became partially unwrapped in transit.  We needn’t have worried, as it made it’s way to me in one piece. It’s just as well that I’m patient, as it must have taken the best part of 10 minutes before I finally saw the object of my desire. What a great looking transceiver! As well as the several address labels in the package (including one taped to the rig itself), there was an envelope inside the enclosure, with a note from Richard and some extra J310′s and BC182′s. The BC182′s are near-equivalents to the BC238′s and BC239′s used in mic amp, AF pre-amp and RF driver.

I could barely wait to open it up and see that circuit board once again:

I could hardly contain myself.  It was so great to see that circuit board that I remembered so well from my youth.  Notice Richard’s ingenious use of Meccano to help support the mounting bracket for the tuning capacitor. I think that Meccano sets were called Erector sets in the US:

Here’s a view from the other end of the rig, showing the input/output bandpass filter, with the 2 inductors wound on T68-2 cores. Fixed to the back of the case, that board with the relay gives full break-in operation:

In case you’re not familiar with the architecture of this rig, or don’t remember my previous posts on the subject, it is a CW and DSB transceiver based around a Mini Circuits SBL-1 diode ring double balanced mixer package.  The SBL-1 was rated down to 100KHz so gives excellent out of band rejection at the design frequency of 3.5MHz. My earlier Manhattan version used an ADE-1 which is only rated down to 1MHZ, but there was not a trace of any out-of-band breakthrough.  Here’s a block diagram and device breakdown:

Block diagram and device breakdown of the G4JST/G3WPO DSB80 80M CW/DSB transceiver, details of which were first published in the March 1983 edition of the UK magazine “Ham Radio Today”.

This DSB80 of Richard’s had been inadvertently connected to 12V with reverse polarity, just as I had done with mine. The first order of the day was to remove the board from the case and, well, make it work again! Richard mentioned that one or two of the electrolytics had exploded, so I knew that at the very least, I’d need to change some of the electrolytics and one or more active devices.  On getting the board out of the case and onto the workbench, one of the big advantages of boards without plated-through holes became apparent – with the help of desoldering braid, it was very easy to remove old components and replace them with new ones. After replacing the big electrolytics that performed service as smoothing capacitors for the 12V supply (one of which had very obviously exploded) I realized that it would be very little trouble to go ahead and replace every single electrolytic. After all, the rig was about 25 years old, and those caps might be getting a bit dry by now.

After replacing all the electrolytic capacitors (even the small signal ones – including replacing the 1uF electrolytic in the diplexer with a polyester cap just for good measure), I connected wires for the volume pot, antenna and speaker, connected a 12V supply (the right way round this time!) turned the volume up and – as predicted – heard absolutely nothing. This was not a surprise as I had a feeling that amongst the devices well and truly cooked would have been the LM380.  This rig used the 14-pin version of the LM380 and I had planned in advance and purchased one. In it went (thank heavens for de-soldering braid!) On re-applying power, I heard hiss in the speaker, and on touching the input to the DBM, I heard atmospheric noise.  It couldn’t be this easy could it? Well yes, it could!  Further investigation revealed that the VFO and buffer were working and the receiver was fully operational.  Not only that, but the transmitter was working too – and without the issue of residual carrier emission that I had experienced with my Manhattan version. Woohoo!

This was the point at which I became a little uncertain what to do next. I still felt that it was not really my rig – it was Richard’s that he had sent to me.  I knew that he had given it to me, but still felt a reluctance to change it too much from the way in which he had built it, for fear I was being disrespectful to the history of this fabulous piece of home-brew. After all  it was the first transceiver he had ever built, with which he had conducted several hundred QRP QSO’s.  Richard put my mind at rest by saying that it had ceased being his rig once it left his house in that brown paper package. He was happy to know that it would continue having a life in California, and wanted me to do whatever I wanted in order to “make it mine”. What a helpful and wise sentiment. I wanted to retain something of the original rig other than (obviously) the circuit board. Richard suggested that perhaps I could fabricate an enclosure from copper-clad laminate and use the original top cover. What an excellent idea!

First of all, I wanted to modify it to cover the entire US band of 3500 – 4000KHz. The original version, being a UK design, covered the UK 80M band of 3500-3800KHz. I used a different tuning capacitor – a 365pF air-spaced variable with a built-in 8:1 reduction drive that I bought new from Midnight Science. I also re-wound the VFO toroid and changed a few of the other caps in the VFO circuit to achieve a coverage of 3485 – 4019KHz.  Even with the reduction ratio of 8:1, this still gives a tuning ratio of over 125KHz per turn of the tuning knob.  In order to help, I added a fine-tuning control consisting of a 1N4001 diode across the VFO tank circuit, acting as a varactor, and tuned with a 1K linear pot. The schematic of the modified VFO, with values, is as follows:

The other modification I needed to perform was to redesign the input/output bandpass filter in order to accomodate the wider US band.  Thank goodness for the software that comes with EMRFD – the program DTC.exe makes the business of designing a double-tuned filter quite straightforward.  The filter used in the original design of the DSB80 uses taps on the inductors for impedance matching into and out of the filter, while the circuit that the EMRFD software is based on uses a tap point at the connection of 2 series capacitors.  The filter I used keeps the output of the transmitter relatively constant over the 500KHz bandwidth of the US band:

Here’s what the board looked like with the mods finished.  The redesigned bandpass filter is at the left-hand side. Just to the right of it, the ferrite toroid wound with the green wire is the transformer that matches the low output impedance of the MOSFET final to the 50 ohms the bandpass filter needs to see. The VFO coil is on the far right of the board, just above the LM380.  The double-balanced mixer package (the heart of the rig) is the silver rectangular package just to the right of center:

Here you can see a closer view of the VFO toroid.  It’s not very easy to see, but very close to, and to the right of it is the 1N4001 diode that is used as a varactor to provide bandspread. The lead sticking out to the right is for the volume control and the lead leaving the board in the foreground is for the bandspread potentiometer:

Even though this particular board is 25 years old and has been worked over a few times by my soldering iron, I still think it’s pretty nice-looking:

I have only reproduced the schematics of the parts of the circuit that I altered. It didn’t seem appropriate to show the full schematic of the DSB80 here without permission. However, although the original article doesn’t seem to be available on the internet in good resolution, it is available from the right people if you know who to ask

Now that the board was fully operational in both DSB and CW, the next task was to fashion a new enclosure.  I did want to retain some of the flavor of Richard’s version of this fine little rig and had wanted to keep the 2 small pieces of Meccano that he used in his tuning capacitor mounting bracket. After the decision was made to use an air-spaced variable instead of the polyvaricon that the original used, this didn’t seem possible.  I went with Richard’s suggestion to make a new enclosure from copper-clad laminate and use the original top-cover.

I cut 3 pieces of laminate for the bottom, back and front panels, and started marking the positions of the front-panel controls on a piece of paper.  When doing this, I mark the outline of the front panel life-size on a piece of paper and keep re-arranging the controls to find a suitable layout. This picture gives you the idea, though when this picture was taken, neither the paper layout nor the partially-drilled front panel were complete. To the right of the horizontal rectangular cutout for the frequency display are 4 holes that I drilled at what were to be the 4 corners of the vertical rectangular cutout for the modulation meter.  When making square or rectangular cut-outs, I drill a series of overlapping holes around the perimeter of the rectangle, remove the center piece, and use a small file to file the edges of the cutout smooth.  It’s slow work, but with care you can make some very acceptable cutouts:

Wherever possible, I mark the cutouts and hole-centers on the copper side of the board:

The finished front panel (before I realized I was going to have to enlarge the hole for the main tuning capacitor) -

The finished 3 panels with all cutouts and holes.  I think I might have cleaned and lacquered the boards at this point.  I scrub the boards clean with a Scotch-Brite pad, a little dish soap (washing up liquid for the Brits!) and plenty of elbow grease. This was the first time I decided to drill extra holes for those little “spigots” that help to keep switches and pots from rotating off-center -

 I use a hand punch to make holes wherever possible. It’s quicker and more convenient than drilling.  For larger holes, I start with a hole made with the punch, then enlarge it with a T-handled reaming tool. I got lucky and bought my punch from Harbor Freight for around $20.  They don’t carry this anymore, but similar punches are available online.  Mine came with a series of different-sized dies and looks like this:

I think it turned out pretty well:

Aw heck – I’m just dying to show you how the rig looks in it’s new case with it’s fancy digital display from N3ZI. The 2 knobs on the upper left hand side are the volume and bandspread respectively. Under the bandspread knob is a switch that will be used to switch between 2 different bandwidths when I fit a single-stage audio filter in between the AF pre-amp and the LM380 output stage. Under that switch is the headphone/speaker socket.  I did buy a nice speaker and grill cloth from Elecraft (a K2 replacement speaker) to fit in the top cover, but realized that drilling holes in the top cover might possibly damage the crinkle finish.  I didn’t want to risk that, so will probably live without an internal speaker.  I could always make a top cover from copper-clad laminate and drill holes in that for a speaker, but I like using the original top cover. Underneath the right-hand side of the frequency display is a toggle to switch the LCD backlighting on or off.  The meter is for monitoring modulation level in DSB transmit. The bank of switches on the far right are (top to bottom) TX/RX, DSB/CW and the on-off switch for the whole rig -

You’ll notice that I’ve used a hot glue gun to secure the VFO toroid to the board. This is also the first view of the 365pF air-spaced variable cap from Midnight Science. -

A few more views -

There are a few small issues that I’m working on. The TX does chirp a bit on CW transmit.  It didn’t do that on the bench, and I know that Richard’s version didn’t either. I’m pretty sure that a few more well-placed  RF bypass caps and/or feritte beads will cure the problem.  I think that I need to do a bit more work on how the board is grounded also.  Future projects will be to add a switchable bandwidth audio filter.  The later version of the DSB80, called the DSB2, had one, and I plan to use the same circuit.  It uses a single op-amp.  Richard’s version had a sidetone oscillator for CW and I’d definitely like to add one.  My antenna is rather inefficient on 80, so if I want much success on this band, CW will be the way to go.

Richard and I have kept up a regular e-mail correspondence in the last couple of months and in one e-mail, he told me how his DSB80 started it’s life -

A bit of history for you:  the rig first squawked it’s mighty 2 watts in anger at 00h08 27 January 1986; signals received by G4BMR Derek (599 about 40 miles away from my then QTH Swindon) but it wasn’t really the first proper QSO as I was already in QSO with Derek on 2M FM and had been for the previous 4 hours.  He was helping me to try and solve the hum, drift and chirp problems and we were also in QSO on 80M CW so all I did was just change rigs when I thought that I had the problems licked – it was quite a thrill to at last hear a nice clean CW signal coming back to me via 2M.  As soon as that had been achieved and we had swapped signal reports, Derek went QRT – not surprising after 4 hours!

I then finished boxing the little chap up but at 01h03 with the adrenalin still running high, there was only one thing for me to do and that was to call CQ.  I received an immediate response from OK3CSA Juro in Myto who gave me a 569 report;  for me, this has to be the first real QSO with the rig and also my first QRP QSO and my first QSO with something I had put together myself.  (Paper logs do still have their uses – the one in question is in front of me as I write – which is why I still to this day log this way!)

Here is that log entry of which Richard speaks – the QSO with Derek G4BMR and the first “proper” QSO, with OK3CSA -

Richard’s note continued -

Enjoy your time with your visitors and I look forward to reading the next instalment of the saga of ‘The little rig who found a new home in America…’

Well, the next installment begins here! The big success of this rig for me is that it has connected me to my ham radio past and filled in the gap on a long-lost piece of radio nostalgia.  It all happened because Richard G0BCT/F5VJD shared with me his first ever home-brew rig and allowed me to let loose on it with my soldering iron.

For that, I thank you OM!

Note – although the wiring on my version may look a bit neater, my version chirps and Richard’s didn’t.  He found that it was important to keep the various leads to the board well-separated, and I think I will end up bringing my connections out to the top of the board like he did in order to experiment with them to find a layout that works best.  He told me that his choice was between pretty yet defective, or scruffy and functional.  I am going to have to make the same decision the next time I take the top cover off this rig!

The intention when taking this photo was to set the DSB80 to 3985 – the North American QRP SSB calling frequency. It wasn’t until later I noticed that I had set it to 3895. These senior moments worry me!

PS – I liked Richard’s original version of this rig so much that I have kept the case, tuning mechanism and associated components intact in case I wish to recreate it one day.  I do like the ability to see where I am on the band with the LCD frequency display but in my opinion, his version of this DSB80 looked more like a “real home-brew rig”. I have not had a QSO with my version of the rig yet, but have spent many hours listening to it already. I love listening to this DC receiver! There is still a slight chirp for me to get rid of before I use it on CW and truth be told, I am more of a CW person than a phone person.  I was 100% phone in the earlier days of my ham career and maybe the pendulum will swing back in that direction one day but for some reason, after a working life in which I earned my living by talking (DJ’ing and voice-overs for radio and TV commercials), I now can’t think of very much to say on a microphone!  Even on CW, I tend to be more of a listener than “talker”, but I do love to listen. Besides – listening is the perfect thing to do while I’m building , and this little rig sounds great

Stop Press – I fixed the chirp.  It was a wiring error on my part and a simple fix. Will explain in a future post. I also had my first QSO with the rig this evening (Oct 4th) on 75M DSB! More info to follow.

Rest In Peace My Dear, Sweet Chloe-Rug

Our sweet little kitty passed away on Sunday morning and the place feels terribly quiet without her.

I took her in as a stray a year and a half ago. We didn’t know anything about her – where she came from, who her previous owners were, or whether she even had previous owners, though she wasn’t feral so I think that she did at some point.  I still remember how fast Ruggie would run towards us when she saw us. She came at us like a bullet, lay down at our feet and rolled around, showing us her tummy, just crying out to be picked up and petted, which of course, we did. She was so desperate to get inside the house. The first night we let her stay inside, she curled up on a cushion and purred all night long. I think she might have slept, but I do know that every time anyone entered the room, she was purring loudly. She was just so happy to have a safe place indoors to sleep. How could we not take her in?

My sister-in-law is a cat lover, but she already had 2 cats that weren’t exactly friends with each other. Adding this particular kitty to the mix would have created a problem. I volunteered to let her stay with me but firmly stated that I would only keep her until we found her a new owner. She knew some people that might be looking for a new kitty and I was hopeful that one of them would want this little furry bundle of purring and friendliness. They didn’t.  I took her to the local SPCA thinking of giving her up for adoption, but it was obvious that she’d be one of a sea of animals all waiting to be adopted. I wanted to know who her new owners would be, and I actually wanted some say in the matter but was told that once I surrendered her, I would not know anything more about her.  Even though I kept telling myself that I didn’t want to be a cat owner, I certainly wasn’t going to leave her there, and walk away never knowing her fate. I guess at that point I was already falling in love, but didn’t yet know it.

There were other local adoption agencies that would allow me to at least know who she was going to, but I didn’t pursue that option. Deep inside, I wanted to keep her but hadn’t yet admitted it to myself. Slowly, the little annoyances became part of her appeal. I had to cover my bed with towels and my nice leather sofa with an old comforter to protect them from scratching and the occasional throwing-up incident. My room didn’t look quite as nice as it had before, but what I began to realize was that it was beginning to feel more like a home with this adorable little creature as resident. My partner Antoinette, who lives just a mile away, wanted to call her Chloe, and I wanted to call her Rug, so we called her Chloe-Rug.  On an everyday basis we called her Rug, Ruggie, Rugster, Rugsto and other variants on her name. Occasionally we’d call her Chloe, but most of the time it was simply Rug – and boy, did that little Rugster work her way into our hearts.

Every morning she would do her best to wake me up for her breakfast. She was a mainly wet-food kitty, so I didn’t have the luxury of leaving dry food out for her at all times and leaving her to take care of herself. Nope – just before first light every morning, she would jump onto the bed, meow, then sit next to me purring and waiting patiently for me to get up and feed her. If I didn’t do that, her next move was to jump on the desk and start punching away at the computer keyboard keys. I guess she must have seen me do that all day, and figured that the computer was something that was important to me, so if she messed about with something that obviously was an object of my attention, she knew that she could get my attention too.  Most mornings it worked but on the days that I really didn’t want to get out of bed at around 6 to feed her, she moved to part 3 of her plan, which was to start knocking parts from my latest home-brew project off the shelves above my desk. She was smart and just KNEW that once she started knocking radio parts off the shelf, I would be up and out of bed within seconds. Smart little kitty! I could never get mad at her. If you’ve ever watched a little cat clatter away at keys on a keyboard, or knock things off a shelf just to get your attention, even if it happens before you’re fully awake, it’s just one of many things about pet ownership that will open your heart. Many a morning I was shuffling around, barely conscious and feeling slightly grumpy, but with my heart open and full of love for that little creature.

After her breakfast, she would sit patiently next to me while I ate my yogurt, knowing that I would leave a small amount in the dish and then place it by her side. Some mornings, we’d have our yogurt together on the balcony. If I sat at the desk with a bowl of cold cereal, she’d smell the milk, jump onto the desk and sit down patiently right next to the bowl while I ate. I knew what she was waiting for, of course, and would leave just a couple of teaspoonfuls of milk in the bowl. She was so good – she’d wait until I backed away from the bowl before taking that as a cue to move in and have her milk. She never wanted much – barely more than a taste, before she jumped back on the bed and curled up for another nap.

I spoiled her in a way that some pet owners wouldn’t agree with, but it was just her and I living together, so no harm was done to anyone else (unlike the outcome if you spoil your children when raising them). After eating her food on the kitchen floor, she would invariably leave her dish after a few minutes and jump onto the bed or sofa, waiting for me to move the dish up there so she could finish her meal in the ambience of those more exalted surroundings. She had a lifelong problem with her digestion and quite frankly, I was happy to do anything that would encourage her to eat. Even first thing in the morning after dragging my weary carcass out of bed to put a dish of food on the floor, after 5 minutes, she’d jump on the bed and start meowing again, just as I was going back to sleep. This was my cue to get out of bed again and move her dish onto the bed so she could finish her breakfast at my feet while I attempted, usually without success, to get a few more winks.

She had many little quirks of behavior that made it all the easier to love her and since she’s been gone, I’m realizing more and more how very much I did love her. It will seem a bit silly to many people who have never owned a pet, but little Ruggie was very important to Ant and I. When Ant woke up in the morning and called me, one of her first questions would be “How’s little miss Ruggie?” to which I’d reply with an update of her activities so far on that particular day. Sometimes there was a funny story to tell, like the morning that I walked to the bathroom in bare feet and stepped in a little Ruggie dingle-berry while still half-conscious.  It took me a few seconds to realize what the squishy feeling between my toes was. More often, I’d simply tell Ant whether Rug had yet had her breakfast, how well she had eaten, whether she was now lying on the bed giving me her famous “I’m the queen of this territory so please keep a respectful distance” look,  or perhaps sitting at my feet meowing for food. She had many little behaviors, all of which were very, very endearing.

I hope you don’t think this is too much information, but The Rugster’s bathroom habits were quite comic at times.  When performing a number 2, at the very moment of finishing, she’d leap out of her litter box and fly down the corridor, turning the corner into the main room at high speed, whereupon she would race around the room with her tail high, executing a kind of “victory lap”.  Unfortunately, being a long-haired cat, she didn’t always successfully complete her mission in the litter box, and while turning that corner at much velocity, a dingle-berry would fly from her rear end, only to be discovered by my feet at some later time.  We loved her all the more for these little mishaps.

She was a good radio ham too. She was my assistant op in the 2011 Zombie Shuffle:

We never got very high scores in contests though, despite her valiant attempts to dig the weak ones out of the noise:

We don’t want to say goodbye to you Chloe-Rug, but we have to.  We’re both finding this hard to deal with, but I’m hoping that time (and it has only been 2 days so far) will make things better. Our lives were so much the richer for knowing you, and I wish so much that we’d had a lot more than 18 months together.  From my initial conviction that I didn’t want a cat,  I changed to thinking of my future life with her and knowing that wherever I went and whatever I did, Ruggie was going to go on that journey with me. It hurts to know that she won’t be able to do that now. She left us far too soon.

Rest in peace my sweetie, and thank you for opening my heart.

Adjusting The Crystal Filter Settings On The K2 And Achieving Maximum Intelligibility with Narrowband SSB

When I first built my K2 about 8 months ago in November and December of last year, I initially set up the variable bandwidth crystal filter as per the suggesting settings in the K2 manual.  Shortly afterwards, I downloaded Spectrogram v5.17 and followed the filter set-up procedure described by N0SS on this page.  Incidentally, as of the time this post is being written, the page that is linked to for downloading Spectrogram no longer has the download. Version 5.17, the last free fully free version, is more than adequate for setting up the K2 filters and it’s still available if you look around the internet a bit.

I did reasonably well aligning most of the filters but noticed while on the narrowest CW setting – 200Hz, that it attenuated the signal quite a bit. On looking at the response curve in Spectrogram the other day, it became obvious why. Tuning the receiver so that it received a tone of 500Hz (my selected frequency offset) placed the signal halfway down the lower skirt of the pass-band of the filter. Heaven knows that I was thinking. With every other bandwidth I had set up, the filter pass-band was centered nicely on the 500Hz mark, but the 200Hz filter pass-band was way off.

This incorrect filter setting had arisen as a result of  a misunderstanding of the way that the K2 filter adjustment process works. I don”t understand exactly what is happening inside the K2 during this but the best way I can currently describe it is as follows: When adjusting the BFO frequency for a particular filter setting in order to move the passband, the CW note (or pitch of the SSB signal) also changes. When you move to the next filter setting, or hit the menu button to finalize that particular setting, the BFO frequency is re-calculated, and you will hear the signal at the correct pitch again. Call me slow, but I wasn’t noticing that recalculation step. As a result, I was reticent to move the filter pass-band very far during adjustment of the settings for fear that once they were finalized, as I stepped through the different filter settings, the CW note (or SSB signal pitch) would change significantly. What a handicap my misunderstanding presented!

The other difference in the way I was adjusting the filter settings this time around was that instead of using band noise, I decided to build a simple wideband noise generator. The circuit is described by N0SS on this same page on the Elecraft site.

Boy, I like simple circuits. The ubuiquitous Altoids tin was the way to go for this one. I could have built it ugly-style, but the MeSQUARES are so easy to use.  No need for a circuit board – I just glued the MePADS directly to the tin and used the tin as the ground plane:

Here are the Spectrogram plots for the 4 CW filter settings I chose.  For the widest one, 1000Hz, I didn’t center the filter pass-band on the 500Hz mark, as that would have placed part of the filter response curve on the other side of the BFO signal,  meaning that the receiver would no longer be a single-signal receiver. For this bandwidth, I placed the 500Hz mark (indicated by the vertical red line) a little to the left of the center of the pass-band. Although the nominal bandwidth of this setting as reported by the K2 was 1000Hz, I estimated the 3dB b/w as about 500Hz and the -6dB b/w about 690Hz:

The nominal 700Hz b/w was closer to 335Hz at the -3dB points and 440Hz at the -6dB points:

The nominal 500Hz bandwidth looked to be about 250Hz at the -3dB points and 380 at the -6dB marks:

While the 200Hz nominal bandwidth checked in at the only slightly lower figure of 240Hz at the -3dB points and 340Hz at the -db points, it sounded a lot narrower than the 500Hz nominal setting. This was only an estimate.  The shape of the curve in Spectrogram is not static – it does move around a little,  so there is room for variation in the measurements:

I’m really happy with the way the 200Hz setting sounds. It’s a shame that it was incorrectly adjusted for so long.  I had read that the signal is attenuated if you wind the filter down to 100Hz, so I assumed that the attenuation I was experiencing was normal. Turns out it was normal – for a filter in which the signal is centered halfway down one of the skirts! This got me thinking about the small number of complaints I’ve read from K2 owners who say that their K2 sounds terrible, and can’t help wondering if they have not yet learned how to set up their filters properly.

Which brings me to the next part – adjusting the filters for SSB. I never used to think of anything other than bandwidth when thinking about filters. For some reason (and this is evidence of my particularly inflexible way of thinking) all I thought about was the width of the filter. “How narrow is it?” was my only question.  I never gave much thought to the importance of exactly where in the pass-band the BFO is placed. Once again, I don’t know why. I seem to never pay attention to things until I’ve been beaten over the head with them many, many times.

Things aren’t so critical if your SSB filter is 2.5KHz or wider, but as you dial the pass-band of your SSB filter down it can get pretty hard to retain good intelligibility.  I Googled the general subject and found this really interesting article by G8JNJ titled “Improving the Intelligibility of SSB Transmissions”.  Originally published in Radcom in Feb 2009, it gives some good tips for achieving the maximum clarity of SSB transmission and reception in the limited bandwidths we amateurs use.  To simplify, the article says that the range  up to 8KHz is all that is needed for intelligible speech.  Trouble is, we amateurs use bandwidths of much less than that.  In western languages, most of the energy in the vowels takes place under 500Hz. The vowels are what help another person determine that it is you who is speaking – they do a lot to give your voice it’s unique identity that makes it sound like your voice.

That’s all very nice (and it is) but the vowels don’t contribute anywhere near as much to intelligibility as the consonants do. Consonants occur at higher frequencies than the vowels, and the range 800 – 5000 Hz is particularly important. That still represents a bandwidth of 4200Hz though.  Going further, Martin says that the area around 1600-2000 contributes the most in terms of consonants.

Now we’re getting somewhere.  So the range from 800 – 2000 Hz is particularly critical. That’s a bandwidth of just 1600Hz!

I didn’t follow these figures faithfully when adjusting my settings for the SSB filters in the K2, but I did follow some general rules based on what I had just learned. Incidentally, I don’t yet have the KSB2 SSB option for the K2, so am using the CW filter to receive SSB:

The wider bandwidths were easier to adjust. I set them to allow more bottom end and as a result, they are more pleasant to listen to. By the time I got down to the 1600Hz (nominal) setting, I had to set the lower cut-off at about 800Hz to achieve maximum intelligibility.  Without having first read the Radcom article by G8JNJ, it wouldn’t have occurred to me to set the lower cut-off so high, but I was surprised at how clear the audio is at that setting, even if it’s not really that pleasing to listen to.

In the following screen captures, the markers are set at 300Hz and 2500Hz. That doesn’t mean anything – they’re just markers to show you where the 300Hz and 2500Hz points are, in case that helps you to interpret the filter response curves. The 2490Hz (nominal) setting was easy. The -3dB point at the lower end looks to be around 300-400Hz. It sounds fine:

As we go down in b/w to the (nominal) 2200Hz setting, the lower -3dB cut-off looks like it’s around 450-500Hz (my rough estimate only). It definitely sounds more restricted at the lower end but still has plenty of clarity:

The 2100Hz response curve looks very similar. I’m not sure why I picked 2 bandwidth settings that are so close to each other. I may well change this:

Look at the 1600Hz (nominal) setting. I’ve set the lower -3dB cut-off point to what looks like around 800-1000Hz.  There’s a lot less fidelity than the wider filter settings, but not much less in the way of intelligibility:

I’m pretty sure that at some point in the next few months, I’ll be building the KSB2 SSB option; not so much because I want to become active on phone, but more because I’m interested to see how the filter compares to the CW filter in the basic K2. The CW filter does have quite a bit of ripple in it at the wider settings, and it’ll be good to have an SSB filter with a flatter top to the response. I’m also keen to spend a little time setting up the transmit audio to see how it sounds. It wouldn’t hurt to have the SSB option fitted.

The Other Half Of The ZL2BMI DSB Transceiver – The Transmitter!

In the last blog post, I described the receiver part of my build of the ZL2BMI DSB transceiver and noted that I’d been having problems with the TX transmitting a constant carrier, even with no modulation.  Everything seemed to be OK up to and including when I built the driver stage. On adding the BD139-16 final though, pesky RF feedback started having it’s way.

During my early years as a radio DJ, one of the pieces of advice I received from my program director was that if I didn’t know something, it was better to not talk about it at all than admit it on the air. As a general rule, it’s probably better to spend time imparting what you do know to your listeners, than to waste time telling them all the things you don’t know.  However, occasionally admitting to holes in your knowledge, I think, increases your relatability and well, is just more honest. In the same way, after my partial success with the G3WPO DSB80 rig, I decided that I wasn’t going to keep blogging about the circuits I built that didn’t fully work. However, a very nice comment from Rogier on my last blog post about the receiver in the ZL2BMI DSB transceiver made me change my mind.  Rogier said that even if my circuits don’t work, they still look good (or words to that effect). Well, I’m not posting the pictures here because I think they look good (after all, what good is a nice-looking circuit that doesn’t work?) but for two reasons. Firstly, to share what I’ve been up to, and secondly, someone may look at my layout and know why I’m having the problem with RF from the final getting back into the earlier stages of the rig and causing it to emit a constant residual carrier (of about 3/4 watt if I remember correctly – not an insignificant amount in such a rig).

So without further ado, here’s what my ZL2BMI transceiver looks like from above.  I hadn’t yet added a TX/RX switch, so the power supply line is connected in the TX position, and the 12V supply to the receiver is disconnected:

My version includes a mic pre-amp as used by G3XIZ and detailed on the GQRP site:

I added an extra 01.uF cap from pin 8 of the NE602 to ground in order to try and stop RF getting where it wasn’t supposed to go.  I also temporarily disconnected the output of the mic amp from pin 1, which didn’t help. I’d read that some people have experienced problems with RF getting into the antenna input coil on TX, so temporarily removed that coil, but that didn’t make a difference either.

Another view of my build:

It is now sitting on a shelf until inspiration (or help from someone else) moves me to take another look. I’m taking a brief break from building but am feeling the desire to build a kit

Rogier – was it you that I heard Bill Meara mention in the Soldersmoke mailbag in the just-released episode 145?

Half Of The ZL2BMI DSB Transceiver – A Simple 80M Direct Conversion Receiver :-)

A few weeks ago, Jason NT7S mentioned the ZL2BMI DSB transceiver as a rig I might be interested in building. He was right – I had seen it in SPRAT but for some reason hadn’t seriously considered making it into a project. The mention from him somehow got me to take another look at it and, well, it was such a simple design, it didn’t seem as if I had anything to lose by giving it the old college try.

The great thing about building a transceiver is that if the transmitter part doesn’t work, you’ve still got a receiver. That’s what happened to me (or at least, until I figure out my problems with the transmitter.) Everything was working fine up to and including the building of the driver stage. Once I added the BD139-16 final, I started experiencing problems with a constant residual carrier being transmitted when no modulation was present.

However…….once I had finished the first part of the build, which consisted of building the receiver, I took a little time to enjoy listening to the receiver and generally being surprised that such a small collection of parts would allow me to listen to the ragchewing on 75M. Little things like touching a wire to the antenna terminal and hearing atmospheric noise coming out of the speaker always give me a kick.

Here’s the schematic for my (only ever so slightly different) version of the receiver part of the ZL2BMI DSB transceiver:

There are no RF or AF gain controls in this schematic.  The circuit is still on a board, not yet in an enclosure, and in the experimental stage. If I ever get it into a box, I’ll add an RF attenuator pot in the antenna input circuit. This is even easier to build than a receiver with a VFO, as there are fewer toroids to wind. In fact, there is only only one – the single antenna input inductor.  Coverage was about 3911 – 4009KHz, so I didn’t bother winding a rubbering inductor, figuring that 100KHz of coverage was already pretty good for such a circuit.

The frequency drift wasn’t very encouraging. I was expecting a little better from a ceramic resonator VXO, being around 200 – 300Hz/hour upward drift after an initial 15 minute warm-up period.  The free-running VFO in my 40M NE602 CW DC RX had a better stability – on 7MHz! This board wasn’t in an enclosure though, whereas the 40M receiver was.  I wonder if that could have made the difference?

However, the receiver sounded pretty good, and there wasn’t much to it:

Here’s another view:

Looking at the schematic, you’ll see that as well as the 0.1uF coupling capacitor from pin 5 of the NE602 to pin 3 of the LM386, there are 2 x 0.1uF decoupling capacitors – one from pin 5 of the NE602 to ground, and the other from pin 3 of the LM386 to ground. I saw a wonderful looking version of the ZL2BMI transceiver built by a ham in the Czech Republic, but looking at his schematic, saw that he left out these 2 decoupling capacitors. There was a 0.1uF coupling cap from the output of the NE602 to the input of the LM386 and that was it. I thought that perhaps he was onto something, so I also left out these 2 caps. Well, they are quite important. The 3 capacitors between them form a kind of simple diplexer – as far as I can tell from my limited knowledge. With just the coupling cap, I was hearing stations, but was also hearing breakthrough from nearby strong in-band signals. Adding the 0.1uF from the output of the NE602 to ground cut out the breakthrough as well as cutting down on some of the higher-frequencies in the audio. Adding the second 0.1uF cap from the input of the LM386 to ground helped shape the audio a bit more and cut down on some more of the higher frequencies, making the receiver more pleasant to listen to for long periods.

I shouldn’t admit this in public, but my first thought on seeing the 3-capacitor network that connects the two chips was “How can a 0.1uF cap bypass RF to ground, when the same value is also coupling audio to the input of the next circuit? If the 0.1uF coupling cap passes audio to the next stage, why doesn’t the 0.1uF bypass cap short all the audio to ground?”

After a bit of thinking, I realized that the capacitors form RC filters with the circuit impedances which determine which frequencies they pass and which they don’t. Imagine you’re an audio signal coming out of the NE602 and heading towards the coupling cap for the input of the LM386. You are going to see that 0.1uF capacitance as well as the input impedance of the LM386, which is about 10K. The 3dB cut-off frequency of this high-pass filter is given by:

But what about those 0.1uF caps to ground? Well, they also form high-pass filters, and the impedance in this case is the impedance of the connection to ground which, if the capacitor is connected properly to the ground plane, should be very low. Therefore, the cut-off frequency of this simple filter is much higher. Audio frequencies are blocked and subsequently passed on to the LM386 AF amp, while RF is bypassed to ground.

While pondering the really nice-looking ZL2BMI rig that had been built by the the Czech ham, I decided to do some testing of my own.  Maybe he had a reason for leaving out those 2 coupling caps?  I decided to replicate this in my circuit and found very quickly that without at least one of those 2 bypass caps, the circuit experiences breakthrough from strong stations on nearby frequencies.  I made a recording with and without one of the bypass caps, and here are the results:

If I were going to build this as a simple little receiver to listen to the ragchewing and general chat on 75 and 80M, instead of using a ceramic resonator, I would use a varactor-tuned free-running VFO as with my Hi-Per-Mite DC RX so that I could cover a wider portion of the band. I’d also use a double-tuned bandpass filter for the antenna input and include an RF attenuation pot as well as possible an AF gain pot.

Naturally, these simple receivers have their limitations, and it doesn’t stop me from dreaming about owning a Ten-Tec RX-340, but I get a real kick out of receiving good-sounding signals from a handful of parts.

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 listening to double that bandwidth, 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.

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:

An Air-Spaced Variable Capacitor With Brass Vanes

I think I might be closing in on the ideal variable capacitor for a VFO.  This arrived in the mail yesterday from an eBay seller:

The capacitance swing is 9-50pF with wide gaps between the vanes.  Rotation is fairly smooth, though the bearings could use a re-lube. Brass is a good metal for the vanes, as it has a lower temperature coefficient than aluminum.  However, this particular capacitor interested me because the fixed vanes are brass, while the moving vanes are aluminum. Why not all brass?  This would mean that as the vanes are engaged and the capacitance increases, more of the aluminum vanes will be meshed with the brass vanes, meaning that the temperature coefficient of the capacitor will increase.

Why would you need a variable capacitor that has a (positive) temperature coefficient that increases with decreasing frequency?

According to the seller this was a National Radio part, so I’m sure there was a good reason.  I need to put my thinking cap on. I should have cleaned it up a little more for it’s photo op, but it’s still quite a looker:

The DSB80 Part 2 And A New Addition To The Shack

I did make some progress on the DSB80 a couple of weeks ago and then put the project aside.  I became a bit sidetracked by other things, some of them non-radio, so want to provide an update in case the DSB80 project gets shelved for a while.  I received a wonderful e-mail from Frank Ogden G4JST, co-designer of the DSB80. He sent me a picture of his prototype DSB80 with my letter to him alongside. What a thrill! He said that he powered up this fine little rig, and it was still putting out 4W of DSB on 80 with fine audio and a stable VFO:

Frank told me that when he designed the DSB80, he scratched the circuit out on a piece of paper and sent it straight to Tony G3WPO, who came up with the PCB design. It was never bread-boarded.  Amazing!  Frank has many happy memories of making many QSO’s with southern England and northern France from his brother’s boat on this rig.  As I’ve mentioned before, the DSB80 I built from the kit supplied by WPO Communications worked fine.  How I wish I still had it

If you recall from the last post, I had been having some problems with the buffer amp in the VFO not buffering too well.  Jim K4AHO suggested that I capacitively couple the output of the VFO to the input of the buffer instead of coupling it resisitively. He also suggested a high-value resistor from the gate of the buffer JFET to ground to act as a self-biasing resistor. This cured the problem nicely – on terminating the output of the buffer with a 51 ohm resistor and touching it with a screwdriver, there was no noticeable shift in VFO frequency (unlike before). The output from the VFO transistor was a nice sine wave:

The output from the buffer transistor when terminated with a 50 ohm resistor was a little less than ideal though:

The above waveform is about 0.9V pk-pk which, by my calculations, across a 51 ohm resistor, translates into around 2mW – just +3dBM, which is a little low for a level 7 diode ring mixer. Also, that waveform – I don’t know how important it is that the VFO buffer outputs a clean sine wave into a 50 ohm load, but I’d sure like to see it.

I decided to press on and see if I could finish the transmitter. This is where I got:

If you saw the earlier iteration of this board from a previous post, you’ll see that I have removed the on-board VFO.  In it’s place is the mic amp for the TX section. I also built the TX driver and TX final. The TX seemed to be generating a pretty nice-sounding DSB signal from the driver using a 2N3904 in place of the originally-specified BC238 device. G4JST very kindly offered me a VN66AF (the device originally specified for the final) for my experimenting, but I wanted to adapt the circuit to use the commonly available IRF510. Heck – you can even get ‘em at Radio Shack.

I’ve had a couple of problems with the IRF510 final. The first was solved with the help of NT7S. Jason did a bit of brainstorming with me and we found that there is a choke in the collector circuit of the driver transistor that is designed to resonate with the input capacitance of the MOSFET final.  It turns out that the input capacitance (Ciss) of the IRF510 is in the range of 135 – 180pF, while that of the VN66AF (the final in the original DSB80 design) is around 50pF. Changing the choke from 33uH to 12uH increased the output from the final from 3/4W to something like 2 – 2  1/2W (I didn’t write it down). However, I also noticed that the rig was putting out significant carrier in the absence of modulation. I don’t think this was due to poor carrier suppression – rather due to a spurious response somewhere in my circuit layout.

This is where I’m at with my version of the DSB80. The receiver sounds good and has given me fresh inspiration to build some more direct conversion receivers. I almost wish that I had not built the TX section, as I don’t do a lot of transmitting on phone anyway, and this is the only part of the circuit I’ve had a few problems with! I’m sure that it’s my layout, or my use of different active devices, as my original kit back in 1983 worked well.

So my DSB80 board is sitting up on the shelf for the time being while I give it a rest.

By the way, there’s a new addition to the shack, which you may have noticed from the pictures above. It’s a Tektronix 465 oscilloscope. A very generous local ham gave me his old Tek 465.  I’m not sure whether he wants to remain anonymous or not but he knows who he is, and to say that this was a generous gift is a huge understatement. Thank you very much – you know who you are!

Photography and Air Spaced Variable Capacitors

Several people have commented to me that they enjoy the pictures I take for this blog, and Mike VK1oo recently asked if I could write a post detailing how I take them. I don’t want to turn this into a photography tutorial and acknowledge that there are other (and perhaps better) ways to take pictures for a blog, but this is how I take mine.

Firstly, I use a DSLR. There is a noticeable difference in the image quality that comes from compact digital cameras with their smaller image sensors, and that of the images that emerge from a larger-sensor camera like a DSLR (Digital Single-Lens Reflex).  The number of megapixels is not that important, especially if you’re taking pictures mainly for the internet.  The common standard for images in print publications is 300 dpi (dots per inch), while you need considerably less than that to produce a great looking image on a computer monitor. Perhaps more important than the number of pixels on your camera’s sensor is the quality of those pixels. With the larger sensors of DSLR’s you can make the pixels larger, which makes for lower noise in the final images.  However, all these discussions aside, there is one factor that I think makes more difference than anything else in the perceived higher image quality of DSLR’s and that is the control over DOF (depth of field) that they give you. Put in layman’s terms, a DSLR makes it much easier to exercise some control over what parts of the picture are in, and what parts are out, of focus.  You can even control how out of focus they are. With a compact camera, almost everything is in focus.  This is good when you want everything to be in focus, but terribly frustrating when you don’t. With a DSLR, if you want very selective focus, you just open your lens aperture up to something nice and big, like f2.8, or even bigger if your lens is capable of it and you’re brave.

Why would depth of field make a difference, especially to pictures of radio and electronic parts? It doesn’t if your only interest is purely seeing how things look and how they go together, as in kit assembly instructions, for example. However, when we look at things in real life, we don’t look at everything – we concentrate on the things in our field of vision that interest us. The things that don’t interest us are not necessarily out of focus, they’re just not there (to our brains anyway).  The selective focus of a camera with a larger sensor can approximate this by throwing the things that are not central to the main purpose of the picture, out of focus. Occasionally, I have employed this technique in a rather extreme fashion. One or 2 of the pictures of my K2 build were like that, but I think the technique is more effective when it’s subtle. As an example, I’d like to offer up this photo of an air-spaced variable capacitor that turned up in the mail this week:

The background is moderately out of focus. If you look closely, you’ll see that the vanes of the capacitor are also very slightly out of focus. The front part of the capacitor is in focus, and so is the rule. Because the capacitor vanes are only slightly less sharp, there is no loss of detail, but this slightly limited depth of field helps to make the picture look natural and appealing (in my opinion). The fact that that the subject is a Millen 50pF air-spaced variable capacitor with ceramic end-plates and what looks like silver-plating is an important factor in the appeal too!

Another issue that is very, very important when you’re attempting to take good pictures is lighting.  When I say a “good” picture, I mean one that achieves what you initially set out to do. I don’t know much about how to control lighting for photography indoors.  If you don’t have flash, studio strobe lighting, reflectors, snoots, diffusion panels and all the other lighting gear that many serious photographers have, there is a quick and easy fix that will help a lot – go outside and take your pictures either out in the open if it’s overcast, or in the shade if it’s sunny. What you need is bright diffuse light. Direct light creates a lot of contrast and you don’t want that, because cameras can’t handle it. Photographers spend a lot of time controlling the contrast in their photographs, so our best defense is to look for lighting conditions that don’t create that contrast in the first place. I love bright cloudy days because it means that I can take pictures for my blog pretty much anywhere outside.  If the sun is out, I gravitate to a place in the shade that is still fairly bright.

Even when you’re photographing on an overcast day, the light does still cast shadows – they’re just not as distinct.  There may be certain parts of your project than need a bit more light so the viewer can see the details (especially the innards) so try and orient it so that a little more light gets into and illuminates the parts where you want viewers to be able to see details.

One more thing about light and this is really getting a bit esoteric, but late morning and early morning light has a special quality. The above photo of the variable capacitor was taken early in the morning. The sun had only just risen and there were some very weak diffuse rays of early morning sun lighting the capacitor. See that very slight warm coloration on the metal parts? I think it’s gorgeous but I wouldn’t go out of my way for that. I just happened to be taking photos for the blog at that time.

It’s important to really pay attention when you’re taking photographs. Make sure you get everything in the frame that you want your viewers to see, and don’t include anything that you don’t want them to see. That leaf or bit of dirt lurking behind your project – get it out of the way. It’s much easier to move these things at the time than erase them in Photoshop afterwards. Also pay close attention to the angle from which you take your photograph. Do you want the final image to show a certain detail to the exclusion of most others, or are you trying to give the blog-reader an good overall view of what your creation looks like?  If you make a habit of always asking yourself what the purpose of your picture is, then it sets the stage for the next step, which is doing what is necessary to have your picture achieve that purpose.

Many of my pictures are taken with the project at an angle and with the subject of the photograph not exactly in the center of the frame.  The reason I often orient the object at an angle is because I’m trying to give you an overall feel for what it looks like. If I just show you the front panel, you don’t get a sense of how deep it is.  The reason for not always putting the subject slap-bang in the center of the frame is more of an aesthetic one; I just think it looks nicer. If the foreground of the picture is a blurred piece of ground, and the project is set off-center, a bit further back, and is sharply in focus for example, I think that this helps to draw the viewer into the frame.  I could be wrong; this is all just opinion.

When you’re taking pictures, take your time and take lots from different angles. Don’t be afraid to lie on your tummy and get really close. This thing you’ve just built is your pride and joy and you want to show it off in the very best light to your online readers. If you’re using a DSLR, experiment with different apertures, so you can see the effects of different depths of field on the final image. I spend quite a bit of time crouched low down and lying on my stomach on the ground looking at my projects through the lens of my camera from different angles.  It’s fun.

Here are my bullet point suggestions for good home-brew blog photography:

• If you have a DSLR (or other camera with a larger image sensor) and know how to use it, this should (IMO) be your camera-of-choice
• If you don’t have a DSLR, use whatever camera you have
• Take your pictures in bright diffuse lighting (unless you’re experienced with studio lighting). Outdoors on an overcast day works well. If the sun is out, look for the shade. The idea that only sunny days are good for taking pictures is a hold-over from the time of cheap film cameras with fixed (and small) apertures that, combined with the medium to slow speed films of the day, needed the bright light of a sunny day to get an image at all. Nowadays, sunny days are not your friend (unless you have the lighting tools to control the high contrast that direct sun causes);  bright overcast days are, as well as bright shade on sunny days.
• Explore your subject – your home-brew project in this case, from different angles, both close-up and from a slightly further distance.  Shooting down from directly above might be the best way to show the layout of components on a circuit board if your main aim is show people what stages and components go where.  Pay attention to what aspects of the project you wish to display in each shot.  Take your time and enjoy looking at your creation through a lens.

Enough about photography, and back to my new preoccupation – air-spaced variable capacitors. Todd VE7BPO has a great page on his site about building VFO’s.  He has details of a very stable Vackar VFO for 40M with which he achieved a drift of the order of 5Hz/hour. That is about as good as you can get with a free-running VFO, and it’s a pretty astounding figure.  If I could get 10Hz/hour, I’d be pretty happy, and I’m thinking that the variable capacitor pictured above (which I got from an eBay seller for $12.50) might do well in this VFO. Here it is again:

I’m a novice on the finer points of variable capacitors for VFO’s but the fact that this Millen 21050 has 2 ceramic end-plates makes for a nice stable construction – more so than the ones whose frame is a single piece of metal bent in a u-shape. The vanes appear to be silver-plated (I don’t know if and why this is a good thing). I don’t know what metal the vanes are made of.  Brass is the best as it has a lower temperature coefficient than aluminum, which is a more common material for variable capacitor vanes. There are no bearings.

Another contender for a VFO could be this one, which was part of a care package of parts from a very generous ham:

The ham who sent me that variable also sent me this one  (this picture featured in an earlier post on the DSB80) :

I also received this variable capacitor, reduction drive and mounting bracket from The Xtal Set Society this last week (the candy and copy of The Xtal Set society Newsletter was also in the package):

It’s great to find a supply of new reduction drives (I don’t know of too many). The air-spaced variable capacitors are also new and while they are not the very highest quality in terms of mechanical stability and Q, there are many uses I can think of for them.  Even for non-critical VFO applications I think they’d do OK – they have to be significantly better than the polyvaricons which are used in some VFO’s nowadays.  It’s a single gang 365pF with a built-in 8:1 reduction drive, which has a nice smooth action.  The actual maximum capacitance is closer to 390pF.  I’d quite like to make a broadcast band regen with this little fellow – or perhaps a direct conversion receiver for 160 or 80M. The neat thing is that the mounting bracket in the above picture is specifically designed to hold the reduction drive at the right height for their variable capacitors, making the mechanics of connecting it all together a lot easier.

Here’s one more photo of the little beauty:

There has been some progress on the DSB80 this week – more on that in a future post.

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…..)