A few weeks ago when a sizable order of parts arrived from Dan’s Small Parts And Kits, the plan was to use them (along with the parts from a few smaller orders from kitsandparts.com and Mouser) as the basis for some fun home-brew projects. I got my feet wet by building some small circuits Manhattan style – an audio filter and RF preamp for my VRX-1 DC receiver, and a crystal oscillator in an Altoids tin to check out the MeSQUARES I had just received from QRPMe. These are detailed in recent posts.
Like anyone who builds circuits, I have a mental list of things that I’d like to build which is updated constantly. Some ideas get pushed to the back of the queue to make way for newer ones, and some stay pretty close to the front for long enough that eventually opportunity and desire collide and magically, it gets made. This is what happened with the WBR regen receiver.
As a teenager growing up in England, I had a one-tube shortwave regen that I built from a kit. It was an HAC Model DX which used a mere handful of components and used a big rectangular 90V battery for the high tension supply. It used plug-in Denco coils for band-changing (I think I remember having 3 of them). I spent countless hours in my bedroom with the high-impedance headphones clamped around my ears, constantly tweaking the regeneration and tuning capacitors and listening mainly to shortwave broadcast stations. I could figure out which band I was on, but had little idea of the actual frequency. It really didn’t matter though, because the likes of Radio Australia, Radio Tirana Albania, Radio Moscow, Radio Nederland’s “Happy Station” shows, Radio Prague and many, many more kept me glued to that little set. It was just a small bent aluminum chassis with 3 variable capacitors, a battery triode, a set of headphones, a coil, 3 fixed resistors and 2 fixed capacitors, but it was pure magic to me.
I think I spent the majority of my teenage years in my bedroom, listening to my record collection and radios. As an adult who recently semi-retired, it feels as if I’ve come full circle. The chance to spend all the time I want building and listening to radios is an absolute gift, and the WBR Regenerative Receiver (Aug 2001 QST) that I’ve had the pleasure to use these last few days has brought the magic of radio listening back in a big way. It is sensitive, very stable, suffers from no microphonic effects at all, and thanks to the ingenious Wheatstone Bridge tank circuit has very minimal radiation of the local oscillator signal from the antenna port and so no common-mode hum problems. Oh – and no hand capacitance effects either. I haven’t yet measured the drift, but after I’ve switched it on and let it warm up a little, I can set it to a frequency to listen to a net or QSO, and it stays there. Obviously, there must be some drift, which I hope to measure soon, but I’m not hearing any. One more thing, not only is there little long-term drift, but my unit is very frequency-stable when subjected to physical shock too. It’s a great little receiver and has pleasantly surprised me with it’s performance.
I won’t be publishing the schematic for the WBR receiver here, as it’s not mine to publish. It appeared in the Aug 2001 edition of QST, so if you’re an ARRL member, you should be able to download it from their site. The article was also reprinted in the ARRL book “More QRP Power”. There were a few errors in the original QST article and although the parts list has been corrected in the “More QRP Power” reprint, the schematic hasn’t. For the record, when you look at the schematic, R17 should be labelled R7, C22 should be 0.01uF, C19 (the capacitor connected between R15 and pin 5 of U2) should be 0.01uF, and “the other C19″ – the capacitor connected between R16 and ground, should actually be labelled C20. If they’re going to reprint articles (which I’m very glad they do) I wish they’d make sure that all the corrections are included.
This was the first time I had ever attempted to fabricate an enclosure from PCB material and I’m quite pleased with the results. I won’t fully detail my construction methods, as I got them from K7QO and WA4MNT, and they both already have excellent tutorials available online on how to make PCB cases. Ken WA4MNT’s PDF tutorial is here, and Chuck K7QO’s is here.
One or two people have asked me how I cut PCB material. If you don’t have a bench shear you can get nice clean cuts by scoring it with a utility blade and breaking it. After using a sharp pencil to accurately mark the line where I want to cut, I use a metal rule and fresh sharp utility blade to make deep scores on both sides of the board. Expect to go through a few blades, and don’t be shy about replacing them; cutting with a nice new blade is a pleasure, while an old one will just give you grief and raggedy cuts. Make sure that the cuts on each side are exactly opposite each other. Then sandwich the board in a vice between 2 bits of wood at the score mark thus:
Then, bearing down hard on the whole sandwich, firmly flex the board up and down until it breaks cleanly off. You can run the board back and forth a few times over a piece of sandpaper, sandcloth etc. on a flat surface to smooth it a little, and you’re done. Easy! If you have a vice, you can use it for the above step.
Work progressed on cutting the panels for the enclosure (the 2 triangular pieces were used later to strengthen the cover) -
and before long I had a drilled front panel, a back panel, a base and 2 strengthening side-struts, which cleaned up with a scotch-brite pad and a little dish soap (for de-greasing). I made sure to rub the Scotch-Brite pad on the board mainly in straight lines as it does give the copper-clad a slight brushed look:
As suggested by WA4MNT in his tutorial, I used an angle iron to ensure 90 degree corners. The one I’m using is actually aluminum angle stock. One thing that I would have had to learn the hard way were it not for this tutorial is the fact that when you solder the pieces of board together, as the solder cools, it shrinks, and you end up with something closer to 88 or 89 degree corners. That can really mess up your nice square box, but Ken tells you how to square up your angles. Little by little the enclosure took shape and the thought occurred to me “Wow, I can actually do this!”:
I gave the finished enclosure a final cleaning with Tarn-X and sprayed it with a light coating of lacquer to prevent against oxidation of the copper. It didn’t fully work, as you can see from pictures of the completed receiver at the end of this post. I’m thinking that I didn’t apply a thick enough coat of lacquer.
I thought it would be fun to fit the controls and stick-on feet to the enclosure to get a feel for what the finished receiver was going to look like:
I have a fondness for quality multi-turn wirewound pots, in the same way that I used to like using high quality air-spaced variable capacitors in my projects. Similarly, the way that a regular one-turn pot feels when operating is important to me. The pot that I originally was going to use for the AF gain felt a little “scratchy” in the way it rotated, so I bought one from Radio Shack that feels silky smooth. Even the first BNC connector that I used for the antenna socket (which was left over from an earlier project) was a cheap component, and I noticed that plugging and unplugging the antenna took more force and fiddling than it should have, so I used a new higher quality part (Mouser # 161-9323). If this seems like a little too much attention to detail, let me explain. The connectors and controls are the way that you, the operator, interface with the radio. Higher quality components used here will greatly improve your experience of the radio. What would you rather use – a radio that takes force and fumbling to connect the antenna, has hard to adjust tuning due to the use of a cheaper one-turn pot, and a volume control that turns roughly, or a radio with controls and connectors that operate and rotate smoothly? These kind of things make a big difference. It might seem illogical to spend so much more on connectors and controls than on the rest of the circuit, but given how positively they can impact the user experience, it’s an investment worth making. I was hoping that this receiver would become a permanent part of my station, so spending a little more wasn’t a problem.
You’ll notice in the next picture that I used two 10-turn pots. One was for the tuning, and the other for regeneration. In the QST article, N1BYT details his method of using a one-turn pot with a separate preset to set the regeneration range on the main regen control. I’m sure that this works very well, but I happened to have an extra 10-turn pot available and was wanting an excuse to use as many of these wonderful things as possible, so I omitted the preset (R6 in the article) and used a 10-turn for the regen control. You do get a “whizzing” sound when adjusting the regen if you use a wirewound. To eliminate this, you can bypass between the wiper and ground for both audio and RF with a 10uF and 0.1uF connected in parallel. I’ve left mine as is for the time being, as I quite like the whizzing sound! (Edit: The novelty of the whizzing sound wore off swiftly, so I placed a 33uF cap between wiper and ground at the regen pot. It took care of things nicely.)
Daniel N1BYT’s original design for the WBR didn’t use a volume pot in front of the LM386, relying just on the 1K RF attenuation pot in the antenna lead. I decided that I wanted a little more audio gain in order to drive a loudspeaker, so I used the audio chain from N1BYT’s OCR II receiver, but connected a 10uF electrolytic between pins 1 and 8 of U2 to get the maximum gain of 200 out of the LM386 IC. Running the LM386 with maximum gain, and also fed with a preamp, I wanted to be able to control the gain of the AF stage as well as being able to attenuate the RF input, hence the extra pot. The toggle switch will be used to switch audio filtering at a later stage.
The enclosure build had gone so well that I started hoping that the performance of the receiver, when I built it, would be good enough to justify putting so much time and effort into the case. In retrospect, I probably should have built and tested the circuit first before deciding whether it was worthy of an enclosure. Luckily it was.
I built the circuit Manhattan style using pads from W1REX at QRPMe. I talked about the MeSQUARES in an earlier post and liked them so much that I ordered a sheet of MePADS from Rex. MePADs are Manhattan pads for IC’s, as you can see in this picture. I had already broken off the pad I was going to use in this project:
As you’re building with these pads, if you find that you’ve glued one in the wrong place (I use Super Glue in the gel form), just slip a sharp utility blade underneath the pad and it comes right off. No need to fear a bad circuit layout, as you can change it if you make a wrong move.
The first part of the circuit to be built was the detector and regeneration circuit. I didn’t have an MV104 dual tuning diode as specified in the original article, so used two back to back MV209′s, which you can see in the following picture, along with Q1 and Q2, the 2N3904 regeneration transistor and the MPF102 regenerative detector respectively:
Now I’ve added a few more components, including the 78L05 voltage regulator, which is hiding behind the grey 2.2uF electrolytic in the foreground:
The completed circuit. At this point, I hadn’t added the 10uF electrolytic between pins 1 and 8 of U2 (LM386). The 2N3904 audio preamp is just below the LM386 audio IC:
Some more views of the completed circuit. I knew that I was giving myself plenty of space to work with, but didn’t realize that there would be so much of it left over. This is rather handy though, as it allows room for some low-pass audio filtering that I want to add later:
All very well and good, but would it work, and how well? The next picture shows the board installed in the case. I added a 10K trimpot in place of R12 in order to adjust the tuning coverage of R11, the main tuning potentiometer. You can see the orange trim-pot near the front left-hand side of the board. It was only just now looking at this picture, that I realized I had left out C22, the 0.1uF capacitor in the antenna lead:
The same shot from a slightly higher angle:
By adjusting trimmer capacitor C8 and the trimmer resistor in place of R12, I achieved coverage of approximately 6970 – 7311 KHz. The extra 11KHz at the top of the band was important to me so that I can listen to the BBC World Service on 7310. Vatican Radio broadcasts in English daily at 0250 utc on 7305, so the extra coverage above the top end of 40 allows me to receive that also. Adjust C8 to set the top end of your coverage, and the trimmer in place of R12 to set the bottom end of the coverage. The tuning rate is a bit higher at the bottom end of the band than the top, being about 50KHz/turn in the CW sub-section.
I spent a few days enjoying the radio in this state before making a top cover for it. The RF attenuation control doesn’t take the input signal completely down to zero. No doubt this has a lot to do with my decision to use an unshielded piece of copper wire to connect the antenna socket at the back of the rig with the 1K pot. N1BYT does this in his receiver pictured in the QST article. I liked it because it’s a reminder of the days when complete radios were wired using this technique, but I think I’ll probably replace that wire with a length of co-ax (after fitting C22 inline with the antenna lead) from the BNC to the 1K pot. That should allow attenuation of the input down to zero (or very close). (Edit: I just did, and it does.)
A top cover for the receiver:
The completed WBR Receiver (darn those front-panel fingerprints!) -
The space left at the top right-hand side of the front panel was in case I found a way to hook up a frequency counter to the regenerative detector while in oscillation in order to provide a frequency readout. I have heard of people achieving this with the WBR by placing a pick-up coil near the main tank coil. Still not sure whether I’ll pursue this.
Some of the things I’ve heard on this wonderful little radio in the first few days of owning it are the Vatican Radio transmitter in Sackville, New Brunswick on 7305 and Radio Australia (it was great hearing their Waltzing Matilda sign-on melody). I also heard FO8RZ in French Polynesia on 7001 and immediately switched over to the Norcal 2N2/40 to work him with 4 watts! One of the reasons for building this receiver was in order to listen for AM activity on 40 (amateur, as we all know that there is plenty of broadcast activity there!) and I was happy this morning to hear W6LHQ running 200W of AM from his QTH in Modesto, CA on 7293KHz. This is one great thing about a regenerative detector – it will receive SSB and CW as well as AM. I believe it can receive FM too via the slope detection method, though I haven’t had a chance to try this out.
The Wheatstone bridge circuit in the tank is an elegant way to resolve the problem of oscillator radiation from the antenna that is common in simple regens, without resorting to adding a stage of RF preamplification, and it works very well. I made no particular effort to match C5 and C6, so either I was lucky, or the circuit is forgiving, because I have encountered no issues with re-radiation. Without re-iterating the summary of this receiver’s performance that I gave in the second part of the 4th paragraph of this write-up I’ll just say this; I was hoping that this regen’s performance would be good enough for me to have it as a permanent part of my station, and it is.
The next step will be to replace the antenna input wire with a length of co-ax, and then build some audio filtering. At that point, I’ll make some recordings of the audio and post a YouTube video.
I’m also thinking about bread-boarding another version of this receiver to encompass a larger portion of the shortwave spectrum. I have some MV108 varactors, and am thinking that the wider capacitance swing of one of those could give me broad coverage of a large part of the HF spectrum, with a fine tuning control provided by another diode. It would be great to have a regen to give me continuous coverage of, say, 3 – 10MHz, or even higher.
Lots of fun and experimentation to come. Many thanks to N1BYT for this fine little receiver.
Notes added after above post was written -
*Current consumption is low too – I measured around 12-13mA in regular use.
*A number of other builders have experienced problems with low sensitivity with their WBR’s. LA3ZA found that substituting a 0.22uH inductor for Z1, the 1″ length of stiff wire between the center-tap of the coil and the ground plane, did the trick. In the QRP-tech group on Yahoo, Steve AA7U did some experimenting and found 1uH was the optimum value for him. Other builders have not had these issues with theirs. My suggestion would be to build it as in the original design, adding the inductor if it seems necessary.
*As mentioned earlier, I added an audio pre-amp stage to my WBR, as suggested by N1BYT in the original article. The schematic of the simple stage I added can be seen in this post.
*If I were building this again, I would change the configuration of the LM386 amp stage. Using a 10uF cap between pins 1 and 8 to get maximum gain introduces quite a lot of hiss. The design that VK3YE used in the Micro 40 utilized a couple of ideas that had been discussed in SPRAT and is much lower noise. I would try using the 2N3904 pre-amp as detailed in this post with the LM386 circuit as used in the Micro 40.