Dave Richards AA7EE

June 3, 2012

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

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

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

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

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

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

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

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

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

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

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

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

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

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

One more view just for the heck of it -

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

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

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

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

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

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

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

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

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

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17 Comments »

  1. Ah, Ham Radio Today. What a great magazine that was! I can remember building two of the WPO Communications kits, the FET Dip Oscillator and the 6m transverter, neither of which I still have (sold, not junked.) Good luck with the 80m transceiver!

    Comment by Julian G4ILO — June 3, 2012 @ 10:38 am | Reply

    • I’m glad you remember them Julian. Because it was quite a while ago, and also because of where I am now living, not too many people do. Thank you for commenting.

      Comment by AA7EE — June 3, 2012 @ 4:13 pm | Reply

  2. Put a 1 megohm resistor on the gate of the buffer to ground, and a cap in series and I bet the problem will go away. I think it is called the miller effect…

    Jim
    k4AHO

    Comment by Jim Diggs K4AHO — June 3, 2012 @ 8:15 pm | Reply

    • Thank you Jim. I’ll be back at the bench tomorrow evening. Will try it and report back then. I very much appreciate you taking the time to leave this comment.

      Comment by AA7EE — June 3, 2012 @ 9:01 pm | Reply

      • Jim – I substituted a 39pF NPO for the 100 ohm resistor, which led to a dramatic improvement in the stability – haven’t put a high-value resistor from gate to ground yet, but will do so. This direct conversion receiver now sounds really good. Thank you!

        Comment by AA7EE — June 5, 2012 @ 5:24 am

  3. I just found my original dsb80 in the loft built from a kit . It used to work but clearly needs some attention! Shall I put it on eBay !!!! Nick g4iwo

    Comment by Nick g4iwo — June 4, 2012 @ 3:41 pm | Reply

    • I’d be seriously tempted to buy it if you did that Nick and the last thing I need is the temptation to spend more money, so please do not put it on eBay!

      Comment by AA7EE — June 5, 2012 @ 5:39 pm | Reply

  4. Oh my — some Classic Ugly Construction — atta go Dave! Keep going with your buffer. Need that R to ground for proper self-bias. Strong reverse isolation is your friend — consider a BF998 or hycas buffer?

    Comment by 7BPO — June 8, 2012 @ 6:21 am | Reply

    • Todd – great to hear from you. I think this might be the first time I have built anything ugly style (or at least the first time since I was a kid!) I did it because, as you know, it is likely to result in greater stability than doing it Manhattan-style. I’m very happy to join the “ugly-club” – feel like I’m now part of a time-honored amateur radio tradition!

      I just read about hybrid cascode buffers on your site – very interesting stuff. At this point, I’m very happy that this VFO no longer FM’s on receive, so will slowly proceed with the project, but it’s good knowledge to have. There are many gems on your site that I’ll be referring to – will very possibly be building your 40M Vackar VFO at some point in the future too.

      Comment by AA7EE — June 9, 2012 @ 4:19 am | Reply

  5. I read your blog every weekend and always find it interesting. Thanks fortaking the time to share your experiences. By the way, you are taking great photos of your projects. Could you please write a blog article describinbg your method.

    Mike
    VK1OO

    Comment by Mike — June 8, 2012 @ 11:38 pm | Reply

    • Mike – thank you so much for the kind comment. My method is fairly straightforward. I have a way that I take these pictures, and set things up the same way every time. I will either write a blog-post about it soon, or e-mail you the info.

      Comment by AA7EE — June 9, 2012 @ 4:22 am | Reply

  6. Congratulations for “no commercial equipment in shack” !
    Can you to give me the totality of the schéma for that DSB80 ?
    73′s de Patrick F6AWY.

    Comment by Patrick Blanc — June 12, 2012 @ 4:26 pm | Reply

    • Patrick – I e-mailed you the original article about the DSB80 as it appeared in Ham Radio Today. I didn’t hear back from you. Did you receive it?

      Comment by AA7EE — August 16, 2012 @ 6:11 am | Reply

  7. 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. The problem is that I used a standard 2.3mm co-axial power connector but unlike most (all?) other QRP designs, the DSB80 uses a positive earth and all the shack leads had (and still have) negative earth!

    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

    Comment by Richard Sayer — July 24, 2012 @ 3:46 pm | Reply

    • Richard – that is a very generous offer. I am very interested. Will be sending you an e-mail shortly.

      73,

      Dave
      AA7EE

      Comment by AA7EE — July 24, 2012 @ 5:32 pm | Reply

  8. Hi from france ,
    I am very interested to built this rig . I have documentation of HAM RADIO march 1981, but shematic isn’t
    visible for value of components.
    I’ll be glad to find a good shematic (scan or file.jpg) of this rig. Thanks
    joseph (France)

    Comment by joseph Martin — April 4, 2014 @ 11:08 am | Reply

    • Joseph – I can e-mail you a copy of the article if you like. Just e-mail me. My address is good in QRZ. Just a word about it though. I like the receiver, but you do have to take some care in wiring up the transmitter side of things. It can be unstalbe unless you’re careful in the way you route the wiring. Ferrite beads and decoupling capacitors come in handy too.

      Comment by AA7EE — April 4, 2014 @ 3:30 pm | Reply


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