Dave Richards AA7EE

October 19, 2013

The VK3YE Micro 40 DSB Transceiver

I have attempted to build 2 DSB transceivers now with limited success – a Manhattan version of G3WPO and G4JST’s DSB80 – here and here  (the original kit version which Richard F5VJDD sent me for reclamation, worked fine) and the ZL2BMI rig, here and here. Both of them worked FB up to and including the TX driver stages but as soon as I added the PA, I had constant feedback/oscillation, even when not modulating the TX.  In retrospect, I think a simple partition to separate the driver and final from the earlier stages of the TX would have done the trick in both cases (or even building the driver and PA on the other side of a double-sided board.)

The kit version of the DSB80 that Richard F5VJD very generously sent me was a fantastic piece of nostalgia (I owned one as a young man) and a very satisfying project, but I still wanted to be able to build at least one DSB transceiver from scratch and have it be fully operational.

Enter Joel KB6QVI from stage left. Joel is an avid homebrewer of QRP rigs – both from kits (he’s currently working on a BitX using the original board from India, which he is putting on 40M) and from scratch, Manhattan style. Joel is a fan and big user of the MePADS and MeSQUARES from QRPMe (as am I) and has constructed several QRP rigs using them. Joel and I communicate on Twitter, on which he was singing the praises of the VK3YE Micro 40 that he built. I think he was trying to get me interested in building something again, and his enthusiasm couldn’t help but pique my interest. I’ve made a number of jokes in the past aimed squarely at that trusty favorite of many a QRP homebrewer – the LM386. I usually end up using it with a 10uF cap between pins 1 and 8, which gives lots of gain but also quite a lot of noise.  Joel told me several times about the configuration of the LM386 AF amp that Peter uses in this little rig which still gives enough gain to easily drive a speaker, but has much lower noise than the typical high gain configurations of this chip. Then one of my other non-ham projects came to a temporary pause and I got to looking at this enclosure which I originally made for the second beta run of NT7S’ CC-series transceivers. That beta run ended up using a much smaller board and a smaller custom case, so this blue enclosure has been sitting on the shelf for the last 2 years, just waiting for something to be built in it –

The blue enclosure that was originally made for the second beta run of the Etherkit CC-Series transceivers. The 2 pushbuttons on the front were intended for the CC-Series beta. Only one of them would remain for the Micro 40 DSB rig.

Joel got me to thinking that a little DSB rig in this case sure would be neat, so I rummaged in the parts drawers and fitted the controls and connectors I’d be using if I were to build the Micro 40. I kept telling myself that, as I was trying hard not to commit myself at this point 🙂 Note the little electret condenser mic insert in the middle. I thought an internal mic would make it easier to use, especially if out in the field. Also note that even though the enclosure is 2 years old, the coat of lacquer I applied has kept the copper looking pretty good –

The trouble is, on seeing a neat little case like this with a few controls and connectors installed, it’s hard not get enthusiastic about actually building it. Notice the small hole drilled above the right-hand pot for the locator lug. I used to break these little spigots off until an incident with my Fort Tuthill 80, in which the volume pot came loose and twisted round. I don’t know exactly what happened, but one of the potentiometer terminals contacted something else, causing a blue LED that was being used as a voltage regulator to blow. From that point on, I started using the locator lugs to help keep the pots in the same position –

At this point of course, I was committed, and set about building what I hoped would be the first DSB rig I’d build from scratch that would actually work.   I have made a few changes to VK3YE’s schematic, and will describe them here.  I hope you don’t mind that instead of using the conventional symbols for the 2 chips, I have represented them as rectangular blocks. It makes it a bit harder to figure out what’s going on with the circuit, but easier to visualize the physical layout when building –

If you do use the BC band filter, a better one is described at http://swling.com/blog/2016/02/a-simple-homebrew-high-pass-filter/

I do have one problem with this rig. In fact, it is the only issue I have with my version, and that is a loud feedback howl from the speaker on going from TX to RX. I am thinking that Peter’s method of directly keying the mic amp with the PTT button would switch the mic amp off a fraction of a second before the relay kicked in and switched the LM386 RX AF amp on, thereby avoiding the feedback perhaps? This loud howl, which you can hear in one of the recordings linked to at the end of this post, was the only thing I wanted to cure. Everything else about the rig is great.  Note – see point 10) at the end of this list.

2) I changed the value of the cap that couples the output of the mic amp to pin 1 of the NE602 from 1uF to 0.1uF (100n). On-air reports indicated that my audio was a bit bassy. Admittedly, I was using a microphone that was designed for recording and broadcasting applications, and was way overkill for this use, but I figured it wouldn’t hurt to gently roll off some of the lower frequencies in the TX, regardless of what mic was used. It did help, but I’m now thinking that the value of that 1uF cap in the base lead of the mic amp could stand to be reduced also. Feel free to experiment 🙂

3) In Peter’s version, the cap that couples the collector of the BD139 final to the output network is a 47nF.  I didn’t have any of those. I could have put two 100nF caps in series but figured that a single 100nF would work just as well.

4) Peter bypasses the wiper of his tuning pot to ground with a 47nF cap.  I used 100nF.  No biggie. Perhaps I should have used a 10nF instead……..

5) Peter bypasses pin 5 of the NE602 to ground with a 47nF cap. I used 100nF.  He couples pin 5 of his NE602 to the top of the AF gain pot track with a 220nF cap, while I used 100nF. My substitutions are based on what I have in my parts box, rather than any meaningful analysis of the circuit 🙂

6) For tuning, Peter uses 2 banks of diodes, each consisting of four 1N4002’s with a switch to achieve the frequency coverage in his rig. With the switch in circuit, both banks of diodes are used, and with the switch out of circuit, just the one bank of 4 diodes are connected between the resonator and ground. He also has a 10uH inductor in series with the ceramic resonator. My 7.2MHz resonator was obtained from hamshop.cz and seems to have very desirable properties. With no series inductor, and just one 1N4004 diode (I didn’t have any 1N4002’s so I used what I had), I achieved coverage of 7207 – 7335KHz.  Placing a 3.3pF cap across the diode (shown as Cx in the schematic) changed the coverage to 7183 – 7295 – almost all of the phone portion of the US 40M band. What luck! Both Jason NT7S and Joel KB6QVI did tests with 7.2MHz resonators from hamshop.cz and achieved very similar coverage. They don’t always have these resonators so my advice would be buy a small stash of them when you see them in stock. These things are like gold! Not all ceramic resonators are created equal – others have different amounts of coverage.

The key advice with ceramic resonators in rigs like this is to experiment in order to get the coverage you want. However, if you are in the US, with the band going up to 7.3MHz, and you have one of those resonators from hamshop.cz, this circuit should give you excellent coverage. Other resonators will most likely give very different results, and you may need to experiment with different diodes, different numbers of diodes in parallel, and perhaps a series inductor (which I believe has the effect of extending the bottom end of the frequency swing.)

7) Pin 1 of Peter’s LM386 is connected to ground via a 47uF cap and a 33 ohm resistor. I didn’t have a 47uF, but I did have a 33uF.  Given the wide tolerances of electrolytics, it probably doesn’t matter much but I substituted a 33uF cap and a 47 ohm resistor. There is an interesting article in SPRAT 116 on page 4 that talks about the use of the feedback resistor and capacitor between pins 1 and 5, as well as the use of an RLC network between pin 1 and ground to create a high gain amp that has a peak at 500Hz for CW reception. With a resistor as low as 3.3 ohms, gains of 74dB and even higher were achieved. This configuration doesn’t use an inductor, or such a low value resistor, but still has plenty of gain without resort to the the more common method of connecting a 10uF cap between pins 1 and 8 – a method that has (in my opinion) done a great deal to give the 386 it’s reputation for high hiss. It does have a lot of hiss when used this way, so don’t do that – use this circuit instead. It is far more pleasing to listen to!

8) I added a 1N4148 diode from pin 8 of the LM386 to ground as detailed in SPRAT 155 page 26. This is designed to help with squeal on going from RX to TX. It did seem to help a bit, but my bigger issue was the squeal in going from TX to RX. Feel free to leave it out, or put it in. Whatever you’d like to do!

9) I really liked the receiver and was surprised at how good it sounded, considering the simplicity. However, at certain times of day, I did experience a small amount of low level breakthrough from AM broadcast stations in the 550-1700KHz band. Joel KB6QVI didn’t have this with his Micro 40 but then, he lives in a less built-up area, about 12 miles outside Medford, Oregon.  I am in the city of Oakland, in the San Francisco Bay Area, and close to many AM broadcasters. This breakthrough didn’t actually stop me from copying any ham stations but it was there and as such, was mighty annoying. Then I noticed that while the problem occurred when I connected the Micro 40 directly to my outside antenna, it disappeared when my ATU was inline. A quick look at the schematic of the ATU revealed that it was a high pass filter (as many ATU’s are). Aha – problem solved!  I installed a simple high pass filter permanently in the receive antenna lead and the breakthrough completely disappeared. The receiver now sounds great.

If you don’t live close to many powerful AM broadcasters, or you are planning to use this rig only out in the field, in the boonies, then you could most likely leave the AM BC band filter out. However, if there is any uncertainty about the circumstances under which you’ll be using it, why not install it? It’s just a couple of toroids and 5 caps (unless you have 2,000pF caps in which case it’s only 3 caps, as you won’t have to double up on the 1,000pF caps).

10) A word about the bypass cap on the TX +ve supply line – the one marked Cy. In Peter’s version, this cap is 220uF. His mic amp is permanently connected to the +ve supply and switched off by a 100nF cap in the emitter lead, which is shorted out by the PTT button on TX. To achieve this, his PTT button keys the -ve side of the TX/RX relay. I understand now why he did this but in my “wisdom” I decided to permanently connect the mic amp +ve supply line to the TX driver final supply line and key them together. A side effect of doing it this way is that when the PTT is released, the remaining charge in the 220uF bypass cap on the TX supply line keeps the mic amp energized for about a second, causing a loud squeal in the speaker. I found that decreasing the value of Cy to 10uF gave a much shorter squeal that I could live with. I am hoping that this lower value of capacitance will still bypass any audio on the TX DC supply line.

As is usually the case with such projects, I built the AF amp first. Touching the input of the amp chip (in this case an LM386) to hear a loud buzzing sound always provides good positive feedback (pun intended 🙂 ). In the following 2 pictures, the AF gain pot hasn’t been hooked up yet. The curved red lead is a temporary power connection –

The MePADS and MeSQUARES from Rex at QRPMe have become a firm favorite of mine. Every Manhattan project I build uses them. I just realized that I can buy SMT chips from now on if I like, as the sheets of MePADS contain pads for mounting SMT devices too.

The next stage was the point at which things started to get interesting. This is the VXO using a 2N3904 and a 7.2MHz ceramic resonator. Thru-hole resonators for frequencies such as 3.58 and 3.68 are easily available, but ones for 7.2Mhz are a little harder to come by.  When I discovered that http://www.hamshop.cz stocked them, I ordered 3 and gave away one, leaving me with just 2. Now I’m realizing that I should have ordered more, because on firing up the VXO, I found that the coverage with just one 1N4004 diode used as a tuning diode and no series inductor, was 7220 – 7335KHz. Of course, 115KHz of swing is quite a lot but what surprised me more was the fact that this 7.2MHz resonator was happily being pulled so high above it’s nominal frequency. A 3.3pF capacitor placed in parallel with the tuning diode brought the tuning down to 7169 – 7297KHz, which I consider very satisfactory, encompassing as it does the majority of the phone portion of the US 40M band. I like that the upper limit is 7297 as this means I won’t inadvertently transmit out of band. What a cracking little resonator! The resonator is the blue thing just below and to the left of the tuning pot (the top pot) in the photo below –


Then, things started to get really good, because I built the VXO buffer (an MPF102) and installed the NE602. At this point, I could connect an antenna to determine whether I would be able to hear signals. The first thing I usually do at this point is to turn the power on my K2 right down to 01.W and give a few short bursts of carrier. Even without an antenna attached, the little DC receiver picked it up with no problem and I knew we were in business. The antenna input coil is on the lower left of this next picture. You’ll notice that I have also built the 2N3904 mic amp. The blue wire was a temporary connector to the BNC at the rear of the case, so I could plug in the antenna for listening. If you look closely at the AF gain pot, you’ll see that I soldered a short grounding wire from the body of the pot to the chassis.  Without this lead, you may get hum whenever your hand comes close to the pot –

This is always the point at which building transceivers gets tricky for me, as I spend so much time listening to the receiver, I lose momentum. I was even beginning to wish that I had set out to build just a receiver.

Here is a synopsis of what had been built up to this point. I had removed the PTT pushbutton to make soldering in that area easier –

You know how when you move house or apartment, you reach a point where you feel as if you’re very nearly done? That’s usually the point at which you are only halfway through (or even less.) All I had to do to turn this rig into a full transceiver, was add driver and PA stages, and I was in business. It wasn’t quite that simple, as I also had to cut and fit a partition, and wire up the transmit/receive switching. Here’s the first view of what I thought at the time was the completed rig. If you’re sharp-eyed, you’ll notice that the electret mic has been replaced by a phono socket.  This was because I kept getting a motorboating sound on TX which was coming from the mic amp. Peter VK3YE said that I should either try a dynamic mic, or try lowering the gain of the mic amp if I wanted to use it with an electret mic.  I decided to take the easier route, and replaced the internal electret mic with a mic socket. That way, I could experiment with different dynamic mics to find the best one. Also, the 2N3053 driver is fitted with a heatsink, wheras the BD139 final is not. KB6QVO said that his driver got warm, while his final ran cool.  For this reason, he used a heatsink on his drvier, but allowed the final to go au natural.  I simply copied him ( it was easier than doing my own research!) –

Well, this little rig works well. See the video at the end of this post to see and hear it in action. As mentioned before, the only issue I was having with the receiver was low level breakthrough from local AM broadcast stations. It was the only downside to what was otherwise a neat little receiver. I won’t retell the story related in point 9) near the beginning of this post, but the simple high pass filter I installed to attenuate signals in the AM BC band did the trick. Here’s a view of the completed transceiver with the high pass filter installed in the receive antenna line. The 2 toroids wound with green wire and the 5 blue caps in the upper left-hand side of the picture are the receive-only high pass filter –

I cut two small triangular pieces out of the bottom of the partition on both sides to allow wires to pass through. One cutout was a little bigger, as it had to allow more wires through. In the following picture, you can just see one of the triangular cutouts (I cut the pieces out with a flush wire cutter – perhaps not the best idea, but the cutter seemed to be undamaged) –

I suppose that at this point extra images just seem gratuitous, but perhaps one of them will contain an extra detail revealed by a slightly different camera angle that will help a hopeful builder somewhere –

This one might be useful in determining what goes where –

Here’s a view of my VK3YE Micro 40 from the back. The hole on the right is unused.  I will cover it up with a piece of electrical tape on the inside –

And here is what this little DSB beauty looks like with it’s cover on, and viewed from the front –

You might wonder about the practicality of covering a tuning range of over 100KHz with a 1-turn pot.  In fact, you are covering this range with just 300 degrees of rotation, which is not much.  The tuning is a bit touchy but I was surprised to find that I got used to it.  If you want, you could use a 10-turn pot for tuning, or a 1-turn pot fitted with a turns counter. The value is not critical – I often use 10K pots for tuning. One advantage to a 1-turn pot for tuning in simple rigs is that you can see roughly where you are in the band with a quick glance. Also, it is great for quickly scanning the band for activity.  A second 1-turn pot for fine tuning would make it easy to exactly tune stations, while still keeping the cost lower than a 10-turn pot or a turns counter. I’m thinking a 100K pot for rough tuning and a 5K for the fine adjustments.

I plug a little MFJ-281 ClearTone speaker into the phone jack and it sounds great, with easily enough volume for comfortable listening. Current consumption is about 30mA on receive with no signals, peaking up to 100 – 125mA on very loud signals. I didn’t measure the current consumption on TX.

Here’s a recording of me in QSO with KE7NCO 180 miles away. At this point, the 220uF capacitor was still in use bypassing the +ve supply line to the TX, causing the very noticeable feedback when switching from TX to RX –

On changing that bypass cap from 220uF to 10uF, the feedback reduced considerably. Here I am in QSO with N7UVH. He is my greatest DX to date, being 736 miles away – not bad for 800mW of DSB (equivalent to 400mW of SSB) –

Here’s a video demonstration of the receiver (boy, I really need a new video camera. This one is 10 years old and limited in resolution!) –

This was me checking into the daily Noontime Net on 7268.5KHz with Jim W6FHZ, who is 180 miles from me –

On the scale of dollars spent for fun and satisfaction had, this little rig is high up there on the list. I built mine with components I had on hand but even if you had to purchase all the parts, I calculated it would cost you around $23 (not including shipping from the various different suppliers.) This is one fun little rig – and it wouldn’t be hard to whip up a simple matching network for an end-fed halfwave antenna, and take a small battery with you for some portable fun.

As an aside, I went to Pacificon last weekend and had the pleasure of meeting Steve the Goathiker, WG0AT. Here he is at the Buddipole booth holding the packet that contains his entire portable station – a KD1JV MTR with a small key and end-fed half-wave antenna. Fantastic!

Note on Ceramic Resonators – sourcing suitable resonators for projects like this can be tricky. The supply of hamshop.cz 7.2MHz parts seems to have dried up. Mouser have some 3 terminal 7.2MHz ones that, even with the internal capacitors out of circuit, wouldn’t resonate much higher than 7.15MHZ (myself, NT7S and KB6QVI all got the same results). More recently, Patrick W9PDS found some 7.3728 MHZ resonators from Mouser that seem to fit the bill. Joel KB6QVI just reported that with a 27pF cap across this part, and using a polyvaricon for tuning, he is getting a freq coverage of 7.175 – 7.303MHZ. It sounds like this would work in the Micro 40. You’ll need to experiment a bit with parts values to get the coverage you want, but suitable ceramic resonators for 40M are getting hard to come by, so you might want pick up a few while you can. You can find these ones (while supplies last) here.



  1. Great write up Dave, thanks for all the mentions, my head is swelling up as I type this.


    Comment by Joel KB6QVI — October 19, 2013 @ 5:36 pm | Reply

    • You were the inspiration for me building this Joel. I wouldn’t have done it had it not been for your successful build. I owe you a big vote of thanks.

      Comment by AA7EE — October 19, 2013 @ 5:54 pm | Reply

  2. I am alway’s in awe about your very clean construction!

    Comment by pa1zz — October 19, 2013 @ 6:25 pm | Reply

  3. push button PTT and built in mic…you made an HT 🙂

    Comment by imabug — October 19, 2013 @ 8:38 pm | Reply

    • oh, and very nice build, as usual

      Comment by imabug — October 19, 2013 @ 8:38 pm | Reply

  4. The radio is a work of art, amazingly neat construction, Glenn VK3YY,

    Comment by vk3yy — October 20, 2013 @ 10:16 am | Reply

  5. What can I say, other than another masterwork by AA7EE. 🙂

    You and Joel motivated me to get this SSB phasing rig done, so I owe you gratitude for that as well. Hope we can QSO soon!

    Comment by Jason Milldrum (@NT7S) — October 20, 2013 @ 11:55 pm | Reply

    • I’d love to QSO with your phasing rig (and you of course too Jason!)

      Comment by AA7EE — October 21, 2013 @ 12:18 am | Reply

  6. Kudos Dave, beautiful work and works beautifully. Thanks for the writeup and recordings. 73.

    Comment by Elwood Downey, WB0OEW — October 22, 2013 @ 3:55 am | Reply

  7. Hi David, your manhatan style soldering is very nice! Did you make your very neat glued pads yourself ?
    73, Laurent F8GQH

    Comment by F8GQH — October 25, 2013 @ 8:12 am | Reply

  8. Dave, great write-up and videos. You did a great job describing this neat little rig. I may just have to put one together. Your construction techniques are fantastic! Thanks!


    Comment by George Sereikas — November 5, 2013 @ 6:23 pm | Reply

    • Thanks George – I try to put as much information in as I can in order to help anyone wanting to build their own. Glad you enjoyed reading it!

      Comment by AA7EE — December 24, 2013 @ 3:47 am | Reply

  9. Dave, your construction is amazing, and very inspirational! I am a convert and ordered a bag of mePads after reading this. Cannot wait for them to arrive!

    Incredible photos as well. They are beautiful.

    Pat S.

    Comment by Patrick Skerrett — December 24, 2013 @ 12:10 am | Reply

    • If you have any questions about how to use them, please drop me a line. I’m good on QRZ, or you can leave a comment here. Best of luck Pat – and let me know how things go!


      Comment by AA7EE — December 24, 2013 @ 3:43 am | Reply

      • Will do Dave. Thanks. Waiting for the shipping backlog to clear & get a few more components in the mail.

        PS, I got my copy of Sprat in the mail yesterday & your rig is mentioned in it. Congrats!


        Comment by Patrick Skerrett — December 30, 2013 @ 1:16 am

  10. Not the point of your article, of course, but ah, I do miss Pacificon… haven’t found a con out in the midwest to fill the gap. Nice job on the construction!

    Comment by David Adams — February 6, 2014 @ 4:27 am | Reply

  11. Would you recommend the Micro 40 or the Beach 40 ? If you were to build another what might you do different?

    Comment by David Goodrow N1OMO — February 7, 2014 @ 1:12 am | Reply

    • David – I haven’t built the Beach 40 and am not that familiar with it, so cannot give you an informed opinion I’m afraid.


      Comment by AA7EE — February 7, 2014 @ 1:52 am | Reply

    • David,
      I’ve built the beach40 but not the micro yet. From my experience and looking at the schematics I do not think there is much difference in terms of build complexity between the two rigs. Probably just a little more time investment to build the beach vs the micro.
      I’d say your chance of success is greater with the micro, but a better learning experience with the beach. Putting your own diode mixer together is magic & fun!

      I think I’ve just talked myself into building both 🙂

      Comment by Patrick Skerrett — February 7, 2014 @ 1:31 pm | Reply

      • Thanks for this feedback Patrick – I hope David N1OMO sees this. I have some ADE-1’s lying around here and would be torn between using one of those, and using discrete diodes. Rolling your own does sound fun though!


        Comment by AA7EE — February 7, 2014 @ 5:26 pm

  12. Nice job.

    Superb close-up photography.


    Paul N4LCD

    Comment by Paul Quillen N4LCD — February 10, 2014 @ 5:34 pm | Reply

    • “I have decided on the micro 40. I have the above schematic draw in CAD. I just need to label all the components and eliminate a few more crossover. It is a personal thing, but I really don’t like crossovers. Then I am going to CAD out the ME-PAD layout so the lay out can be laser printed and transfered to the copper clad board”.

      Hi Paul

      If you complete the above, would you be so kind as to share your CAD layout with me. I am new to Amateur Radio and Homebrewing and would very much like to build the Micro 40. Your help would be greatly appreciated!


      Garth Williams

      Comment by Garth ZS6GW — February 25, 2014 @ 2:48 pm | Reply

  13. I have decided on the micro 40. I have the above schematic draw in CAD. I just need to label all the components and eliminate a few more crossover. It is a personal thing, but I really don’t like crossovers. Then I am going to CAD out the ME-PAD layout so the lay out can be laser printed and transfered to the copper clad board.

    Comment by David, N1OMO — February 10, 2014 @ 11:05 pm | Reply

    • Keep us posted David, and best of luck!


      Comment by AA7EE — February 10, 2014 @ 11:11 pm | Reply

  14. Hi Dave,

    I saw an article in Sprat 155 “muting the LM386” and all that’s needed is a 1N4148 connected from TX+ to pin 8 of the LM386 (cathode to pin 8). I’ve not tried it but apparently this shuts the ‘386 up as soon as the ptt is pressed, eliminating the squeal.

    I’ve started building a ‘Micro 40’ myself; because I can’t get hold of a resonator, I’m using three 7.2MHz xtals in parallel with a series coil and v/c to deck. The board must be twice the size of yours but I’m experimenting!


    Chris. G0VOE. Somerset. (I’m on QRZ and EEV) or christopher_iles@btinternet.com

    Comment by Christopher Iles — May 11, 2014 @ 7:08 am | Reply

  15. Hi, please let me know how I should use a three pin resonator in the vxo. Should I leave the center pin w/o connection or should I connect it to ground? 73 Guido LU8EQ

    Comment by lu8eqGuido — October 23, 2014 @ 8:33 pm | Reply

    • Check the datasheet for the resonator Guido but I’m pretty sure that if you connect the center-pin to ground, then any internal capacitors are automatically connected in-circuit. Leaving the center pin unconnected leaves the internal caps out of circuit. It’s your choice, depending on what external caps you have access to, and what frequency coverage you want. A certain amount of experimentation will probably be needed here.


      Comment by AA7EE — October 23, 2014 @ 9:02 pm | Reply

  16. Hello.

    Firstly I’d like to thank you for showing your schematic and how you built it step by step. I’m new to this and the pictures have been very helpful!

    I have two problems though. On the schematic there are 3 switches. I know the PTT is to switch between transmit/receive, but I couldn’t quite understand what the two others are for.

    Then near the PPT switch, parallel to a diode there is an inductor, but you haven’t written what kind of an inductor it is.

    Comment by The Crowbar — December 30, 2014 @ 8:47 pm | Reply

    • Also, i’m sorry if my questions seem stupid, but I’m VERY new to circuit designing.

      Comment by The Crowbar — December 30, 2014 @ 9:03 pm | Reply

      • No, they are not stupid at all. Believe me, if you knew half of the questions I had but were too shy to ask……..!!


        Comment by AA7EE — December 30, 2014 @ 11:24 pm

    • Hi there Crowbar –

      The inductor parallel to the diode is the coil in the relay that controls the TX/RX switching. The diode across it is simply there to prevent high transient voltages that can develop across the coil. The push-button switch marked “PTT” is the button you push to transmit. When you push it, it sends current flowing through the coil of the relay, which causes the 2 switches to change from the receive position to the transmit position. One of the switches changes the + 12V supply over to the transmit circuits, and the other one switches the antenna circuit from the input of the receiver to the output of the transmitter.

      It”s a good idea to select a relay that has low current consumption, because all the time you are transmitting, the coil will be drawing current. I used the NEC EA2-12NJ, thanks to a tip from KB6QVI, from whom I seem to get many of my good ideas!

      I hope this helps. Please ask if you have any more questions,

      PS – if you’re fairly new to home-brewing, it might be a good idea to consider building a receive-only version of this rig. That would cut out all of the TX/RX swtiching, as well as the 2N3904 mic amp, the 2N3053 driver, and the BD139 final.

      Comment by AA7EE — December 30, 2014 @ 11:22 pm | Reply

      • Thanks! This helped immensely.

        Comment by The Crowbar — December 31, 2014 @ 7:35 am

      • I’m sorry, but I have one final question.

        At the Pin 8 of NE602, there is something marked 78605 and with i/p and o/p. I can’t figure out what it is.

        Comment by The Crowbar — January 1, 2015 @ 2:23 pm

  17. Crowbar – not sure if you’ll get this reply, as the blog won’t let me reply to your message. I had to start a new comment thread. That’s a 78L05 – it’s a 3-terminal voltage regulator in a small plastic package that makes it looks like a transistor. The 78L05 is a 5V regulator. i/p and o/p are the input and output pins. It takes the 12V across the input and turns it into a 5V regulated supply for the NE602. Hope that helps. You are always welcome to ask questions. If I know something, I will tell you, and will also admit it if I don’t know the answer!


    Comment by AA7EE — January 1, 2015 @ 6:22 pm | Reply

    • Oh, that makes sense.


      And I’ve spent quite a bit of time on WordPress comment sections. In these comment threads, the reply button is usually at the first comment in the thread.

      Comment by The Crowbar — January 1, 2015 @ 6:49 pm | Reply

  18. Very nice and great write up,Dave. It is extensive in explaining every technical detail.
    However there is another good design is “SPOTTO” at ham radio India. You can Google it and I recommed you to try it………..
    73s de Toff.

    Comment by Toff Penfold — February 20, 2015 @ 8:35 am | Reply

    • Thank you Toff. I’ll have a look at that tomorrow. Am just heading off to bed now. Thank you!


      Comment by AA7EE — February 20, 2015 @ 8:48 am | Reply

      • 1.can i change the frequency of this circuits with another ceramics resonator ?
        2.how to increase the power of this circuits to 4W ?

        Comment by Harison — April 7, 2016 @ 3:43 am

      • 1 – Yes. If the resonator is in the same band, no other component changes are necessary.
        2 – You’d have to add another PA stage to increase the power. I haven’t tried it, but a suitable circuit shouldn’t be too hard to find


        Comment by AA7EE — April 7, 2016 @ 6:45 am

  19. Jeeze Dave, I may have to try building one of these. I’ve been off the air for at least four years but looking to build up a a few basic bits of gear to get back on the air with now that I am getting closer to moving to my 5-acres of land in the middle of no where. I was going to start with a few kits to dust off the iron but this will definitely have to go on my short list of projects after watching how much fun you had with it. It’s interesting to note that Mouser still has plenty of those resonators you mentioned towards the end of the post.

    73 – JT, N1ESE

    Comment by N1ESE — March 16, 2015 @ 3:38 am | Reply

  20. Hi Dave
    I have several question for this project

    1. What size AWG for all Coil ?
    2. Can I replace 7.2MHz Ceramic Resonator with 7MHz Crystal Oscillators?
    3. Substitute transistor of MPF102?

    I waiting for your answer 🙂

    Comment by Arif — March 28, 2015 @ 3:43 pm | Reply

    • Arif –

      1) The size of wire is not critical. With toroids, it is the number of turns that affects the inductance. The wire size has little effect. If the wire is too thick, it will be hard to use, and you may not fit all the turns on the coil. If it is too thin, then it is harder for the toroid to be self-supporting. 26 gauge will work fine – I think that’s what I used.

      2) I haven’t tried it, but I see no reason why you couldn’t use a crystal instead. You’d get better frequency stability, though it wouldn’t pull very far.

      3) Any of the standard MPF102 alternates should work – J310 or 2N3819. Try it!


      Comment by AA7EE — March 28, 2015 @ 4:15 pm | Reply

      • And final question.
        How to install Signal Meter and Frequency Counter for this Transceiver?

        Thanks for your answer 😉
        I will build this project next month

        Comment by Arif — March 30, 2015 @ 11:36 am

  21. Arif – I have never installed an S-meter in a direct conversion receiver before. It would need to be some kind of audio derived circuit. There is a circuit at http://www.dl6zb.de/002/smeter.html that you could try, though it does extra complexity. It’s your call as to whether it would be worth it.

    For a frequency counter, you should be able take an output, via a small-value capacitor (less than 100pF) from the output of the VXO or buffer.


    Comment by AA7EE — March 30, 2015 @ 1:13 pm | Reply

  22. Great project Dave. I started off with the Bitx20 and this is a very nice next project. I have just finished building my own case made of double side printboard, it looks just like yours and I may paint mine blue as well. I’ve already started putting the parts in. The MPF102 is hard to get in this part of the woods but I will try using a standard J310 FET, I have some around. Is the an easy way to add two IRF510 in push pull config to get some more output you think?
    I’m also considering building a simple 80 meter QSO rig and make my shack completely homebrew, any suggestions for that?

    Keep up the good work! 73, Floyd PH0KKO

    Comment by Floyd — April 12, 2015 @ 6:17 am | Reply

  23. I finished building the Micro 40 and with a 2.2. pF cap at Cx I get a great coverage of 6.990 to 7.184 which is great because the voice part of the band in my country goes up to 7.200. Instead of the MPF102 which was impossible to get I used a standard J310 and that works as well. Instead of the 1N4004 I used the 4007 version and in the bandfilter 1 nF instead of the double 470 pF caps in the middle.
    Most of the parts were in my junkbox but the ceramic resonator was ordered from minikits in Australia, I ordered 15 pieces in one go.
    The reception is simply great and it is easy to tune into a station with some practise. The only downside is the low output, 400 mW at most, is simply not enough to drive my lineair amp. Any suggestions on how to increase the output, maybe add another 2x stage in the mic amp? Or adjust the bandfilter to a higher frequency?

    Floyd PH0KKO

    Comment by Floyd — April 19, 2015 @ 11:18 am | Reply

    • Floyd – that’s great that you built the Micro 40 and got it to work. Congratulations! I’m really glad the receiver is working well. I don’t have any thoughts on how to increase the output of the TX I’m afraid, but perhaps VK3YE might have some ideas?

      73 for now,


      Comment by AA7EE — April 19, 2015 @ 3:07 pm | Reply

  24. Hi Dave,
    Nice rig.. is there an SSB version of it?
    How hard would it be to make an SSB version with LSB/USB toggle?
    Or NFM?

    Comment by alexunixbrain — April 24, 2015 @ 6:01 am | Reply

  25. this projects is very interesting. I would like to one but i have no fund, i am VU3PPL in india.
    any source get one me sir, thank you.

    Comment by vu3pplvu3ppl — July 25, 2015 @ 5:49 am | Reply

  26. i am very interest this projects. but I have no fund.

    Comment by vu3pplvu3ppl — July 25, 2015 @ 5:51 am | Reply

  27. Can two of these radios or other simple DSB radios talk to each other or do they drift out of phase and go silent?
    I am making two experimental DSB radios using the 602 and 386 chip that seem to be going out of phase and the volume goes up and down at a constant periodic rate during QSOs and research suggests this just happens with direct conversion DSB and that complex methods of recovering the suppressed carrier and it’s phase are necessary but the sound quality of the radio in your video seems much better than my results and leads me to believe I have some other problem than phase synchronization of the radios. On the other hand, all my previous experiments with using the 602 mixer for DSB have had the volume cycle between zero and max when receiving DSB and I wonder why. One clue that I haven’t figured out yet is that the radios seem to detect each other while turned on in receive mode so I just now I think maybe they are either leaking the carrier from the LO or maybe they’re doubling it and maybe that’s causing the problem. I’m not building the same exact radio but similar ones for 6 meters. If you have any other ideas about why my loudness seriously keeps fading in and out please let me know. It doesn’t happen when I use a 6 meter SSB rig to talk to them but only when my DSB home brews talk to each other.

    Comment by _N2QMU_ — February 13, 2016 @ 8:11 am | Reply

    • Bill – I have never tried receiving DSB with a direct conversion receiver, but have read that very precise frequency control is necessary for it to be successful. I don’t know the mathematical reasons for this. What you are experiencing though, seems to be in line with what would be expected. Sorry that I cannot help any more.

      73 for now,


      Comment by AA7EE — February 13, 2016 @ 8:31 pm | Reply

    • In a DSB radio, if the receiver LO is slightly above the transmit frequency, the upper sideband will be slightly higher in pitch than the original audio, while the lower sideband will be lower in pitch. Or I may have that backwards. The important thing is, the two detected sidebands are at different pitches, which causes them to beat against each other, resulting in the periodic variation in volume you report. This happens anytime either the transmitter, receiver, or both allows both sidebands to pass, and the two radios are no perfectly synchronized. Sorry, it’s just the math of the situation.

      Comment by BrightBlueJim — September 18, 2016 @ 12:40 am | Reply

      • Oops – I meant any direct-conversion rig, not just DSB.


        Comment by BrightBlueJim — September 18, 2016 @ 12:44 am

  28. Hi Dave
    I enjoy your work very much. Your construction is a work of art. I am a new Ham, and have the bug for QRP. Looking foward to more.

    Dave VE3DFN

    Comment by Dave — February 17, 2016 @ 1:58 pm | Reply

    • Dave – welcome to this fascinating hobby. I hope you find it as absorbing as many of us do. Just remember to eat the occasional meal and get some sleep every now and again 🙂

      73 for now,


      Comment by AA7EE — February 17, 2016 @ 3:43 pm | Reply

  29. sir , your construction is really great……

    Comment by viswesh — March 13, 2016 @ 9:46 am | Reply

  30. Very good, your style of manhattan construction is very prolix!! Thank you for the circuit, i constructed this radio and works very correctly. You helped me a lot with the broadcast am filter (HPF), and now i use it for all shortwave receivers i make, because a powerful am transmitter is in my city. I dont very experimented with radio because i started with receivers and transmitters when i was have 11 years, and i am 14 years old.

    Greetings from Argentina, 73, Lucas.

    Comment by Lucas Jorgenson — April 5, 2016 @ 11:45 pm | Reply

    • Lucas – congratulations on building your VK3YE Micro 40! I am impressed that you built it successfully at the age of 14. I’m glad that the filter is working on your other receivers too.

      Happy building and 73,


      Comment by AA7EE — April 6, 2016 @ 1:25 pm | Reply

  31. Here is my review of a 40m DSB kit: http://www.examiner.com/review/diy-review-mdt-ham-radio-40m-qrp-transceiver-kit

    Comment by James Hannibal — April 6, 2016 @ 2:09 am | Reply

  32. I am in the process of building this transceiver and have found that the receiver works great.I’m having a bit of difficulty with the transmit portion.I don’t have a BD 139 but have several 2N3866 available.Can I substitute one of these for the final?

    Comment by Paul Schwartz — April 22, 2016 @ 2:35 am | Reply

    • Paul – I don’t know, but they are both NPN bipolars. The 2N3866 is, I believe, good for up to about a watt, so it would be worth a try. Mine, using a BD139 from Diz, only produced about 0.75 watts, which is not a whole lot for DSB. Another amp stage, to bump the power up to something close to 5W would be a worthwhile project. The Norcal QRP Miniboots amp perhaps?

      Glad the receiver is working well so far!



      Comment by AA7EE — April 22, 2016 @ 5:24 pm | Reply

  33. Hi.. dave I want to build two transceiver base on this circuit but could this radio communicates each other like other transceiver can do.. I stay and read this thread for a few minutes ago I see post that claim this transceiver can’t communicates each other.


    Comment by Harison — April 24, 2016 @ 10:47 am | Reply

    • Harison – I haven’t tried it, but have read that for 2 DSB transceivers to be able to effectively communicate, the frequency stability needs to be far greater than can be provided by a ceramic resonator. In practice, at least one of the rigs should be capable of transmitting and receiving SSB. If you want to build a simple SSB rig, how about trying VK3YE’s Knobless Wonder?



      Comment by AA7EE — April 25, 2016 @ 6:26 am | Reply

  34. Beautiful job…both on the build and the presentation!

    Comment by Jack Purdum, W8TEE — October 13, 2016 @ 12:32 pm | Reply

    • Thank you Jack – and thank you for the Arduino articles in QRPp!


      Comment by AA7EE — October 14, 2016 @ 4:14 am | Reply

      • Hello dave,,i was very impress of your microdsb 40m,looking at the schematic may i know what size of magnetic wire in all toroids did you used?


        Comment by Nhoi — March 2, 2017 @ 6:52 am

      • The gauge of wire isn’t critical as with toroids, it is the number of turns that determines the inductance – the wire size has little effect. If you use too small a gauge, the toroid won’t self-support very well, and if the wire is too thick, it will be hard to wrap the wire around the core, and you may not fit all the turns on. For this little rig, I think I used 26 gauge (American Wire Gauge, roughly equivalent to 27 SWG). Any similar thickness will work, as long as you can fit all the turns on.

        Comment by AA7EE — March 2, 2017 @ 5:35 pm

  35. hi dave I am new to construction on your schematics how do I know which ceramic capacitors are uf nf or pf I can not understand your writing thanks john

    Comment by john hurlbutt — August 19, 2017 @ 6:27 pm | Reply

    • John – on my schematic, n = nF = nano farad. A capacitor marked 100n is 100 nF, which is the same as 0.1uF. Also on the schematic, µ = µF = micro farad, and p = pF = pico farad. For instance, there is a capacitor between the output of the 2N3904 VXO and the input of the MPF102 buffer, marked 47p. It is a 47pF capacitor. I hope that helps.

      PS – the capacitors with µF markings are not ceramics – they are electrolytics. Also, you’ll notice that on the input of the LM386 circuit, there are some capacitors marked 0.1 and 0.01 – those are 0.1µF (100nF) and 0.01µF (10nF).

      Comment by AA7EE — August 19, 2017 @ 8:20 pm | Reply

  36. Hi Dave, do you have PCB variant for this project, very interesting hor hamlets it my club.
    73 and tnx

    Comment by Daniel — August 27, 2017 @ 7:57 pm | Reply

    • Daniel – you can try asking Peter VK3YE but, as far as I know, there is no PCB available for this project.



      Comment by AA7EE — August 28, 2017 @ 11:30 pm | Reply

      • Ok tnx Dave
        Best Regards

        Comment by Daniel — August 29, 2017 @ 6:03 am

  37. Great Post Dave…Very nice to read!

    How critical is the use of the T37-2…? Can I use a T43?

    Could be you already know but instead using a 1N4xxx diode as a varicap, maybe you want to give a standard 5 mm Red LED a try. They gave me a very satisfying results.



    Comment by Martin — September 9, 2017 @ 6:18 pm | Reply

    • By T43, do you mean FT37-43 Martin? I definitely wouldn’t recommend using those instead of the T37-2 cores in the input/output filter. The 43 type material is a ferrite core, which allows for higher inductances with the same number of turns, but is lower Q. The powdered-iron cores in the T37-2 toroids are higher Q and better-suited for the low pass filter in the input/output circuit. As for the transformer on pin 2 of the 602 when in receive mode, I don’t know if you could use 43 type material instead of the T37-2 core. It’s possible that you could, but you’d have to use far fewer turns, and it might get tricky to get the correct impedance ratio.

      Good to know that you had good results using a red LED as a varicap. I knew such a thing was possible, but haven’t tried it myself – yet!



      Comment by AA7EE — September 9, 2017 @ 8:37 pm | Reply

  38. Yes…I ment the FT-37-43. I’ve compared them a minute ago and see clearly now want you mean. Well, no problem, then I have to order the T37-2.

    Thanks for the quick answer Dave!



    Comment by Martin — September 9, 2017 @ 9:32 pm | Reply

  39. Hi, Mr. Dave

    I would like to know if there some information about values of inductors and capacitors to operate at 3.5MHz.



    Comment by RICARDO ANDRÉ RIBOLDI — October 5, 2017 @ 8:05 pm | Reply

    • Ricardo – I have not built one for 3.5MHz, but I think it would work well there. Perhaps you could experiment with some values and give it a try?



      Comment by AA7EE — October 6, 2017 @ 3:21 pm | Reply

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