Dave Richards AA7EE

August 21, 2014

The Sproutie – A General Coverage Regen Receiver with Plug-In Coils

I’ve mentioned before in posts how one of my first shortwave receivers as a teenager growing up in England (in fact, possibly the first) was a one tube battery-operated regen built from a kit. Many of the popular electronics magazines at the time, including my favorite, Practical Wireless, carried advertisements from a company called H.A.C. (“Hear All Continents”) who sold kits for simple HF regenerative receivers. This was the ad I remember best. To the teenage me, this receiver was the holy grail. With this receiver, there would be no stopping me. I would be the king of the hill, if only I could have this magnificent shortwave receiver -

Ads like this for H.A.C. shortwave receiver kits were common in the UK up until the early 1980’s. Image taken, with permission, from Louis Meulstee at http://www.wftw.nl/

I saved my pennies and eventually sent off for the H.A.C. Model DX Mk. 2. It wasn’t as fancy-looking as the one picitured in the ad, as it didn’t have a front panel or a calibrated dial but hey – those kinds of regens were only for the truly well-heeled, and I was just a kid with a modest allowance. The kit that arrived used an HL23DD valve (or equivalent). It was a battery operated double diode triode, with a coated filament, to maximize emission from the filament that operated from the low voltage of just 1.5V (2V maximum, with a current consumption of just 50mA). This set didn’t have a front panel or a calibrated dial, sporting just a modestly-sized aluminum chassis with 3 chicken head knobs on the front, but with my 2000 ohm headset and 90V high tension battery, I truly was the king of the shortwave hill. I don’t have any pictures of my H.A.C. Model DX Mk. 2, but featured here are pictures of someone else’s taken from Louis Muelstee’s great website, which is where the ad shown above came from too.

The H.A.C. Model DX Mk. 2 cost me all of £14.50 in the late 70’s. (Photo taken by Philip McNamara and taken, with permission from Louis Meulstee at http://www.wftw.nl/)

This one tube regen used an HL23D double diode triode tube with low current consumption 1.5V filament. The blue lead on the left leads to a connector that plugged into a 90V high tension battery. (Photo taken by Philip McNamara and taken, with permission from Louis Meulstee at http://www.wftw.nl/)

Truth be told, this wasn’t exactly the most sensitive receiver ever created, but it mattered little to a teenager in England in the 1970’s, with plenty of loud shortwave broadcast signals. There was much to keep those high impedance headphones firmly glued to my head. They were fairly cheap quality, and the way the metal headband tensioned the earpieces against my ears made them a little red after 30 minutes of use. Did I care? Not at all – I wore them for hours on end, as I was captured by the sounds of Radio Nederland, Radio Prague, Radio Tirana Albania, Radio Moscow, The BBC World Service and many other broadcasters, as well as all the weird-sounding utility stations and the very mysterious numbers station from East Germany. I had no idea back then what it was, but the female voice announcing strings of numbers in German was strangely compelling. Every day, I would rush home from school, eager to get into my bedroom, plug the low-tension battery in, wait for a short while for the tube filament to heat up, then connect the high tension battery and clamp the headphones to my head, cup of tea by my side, as another listening session began. Weekends were heaven. As soon as all my homework was out of the way, there was nothing but blissful hours and hours of potential shortwave listening time stretching ahead. From time to time, the 90V battery would run down, and I would walk the 2 miles into the village of Astwood Bank to buy a new one from the local gas station.  It didn’t take me long to figure out how to power the filament from a transformer in order to save money on low tension batteries. I eventually figured out how to do the same for the high tension supply too. On good days, I could even pick up some local amateurs on 80M SSB. Man, I was indeed the king of the shortwave hill! At the back of my mind, though, was the idea that somewhere out there was still a truly dreamy shortwave receiver – one that had a front panel fashioned from a sheet of aluminum, and a calibrated tuning dial. It only took me until the age of 50 to finally own one of those only-in-your-dreams kind of receivers.

Which is what this blog-post is all about.

 

 

I’ve had some encouraging success with regens recently. Both the WBR and my modified version of the the WBR, which I built for the 31M BC band, worked well, with no common-mode hum, instability, or any of the other kinds of naughtiness that sometimes accompany the operation of regenerative receivers. There was one main thing about the WBR’s that limited them for me, and that was the fact that they only operated on a limited range of frequencies. After building these 2 receivers, the next logical step was to build a general coverage regen with plug-in coils. I wanted a set that was built solidly, with a reduction drive and a calibrated tuning scale, so that I could prove to myself something that I already knew – that a regen, properly constructed, can serve well as a shortwave receiver rather than just as a novelty, which seems to be the category most people have placed them into these days. I’m reminded of a comment on a discussion forum I saw recently, in which a gentleman was talking about a regen he had built once. It was sensitive, received lots of stations, and gave him much enjoyment, he said, but he never really knew where he was on the band. “Well of course you didn’t!” I thought to myself, “but that’s not because it’s a regen – its because when you built it, you didn’t build it with a calibrated dial. It’s not the regen’s fault you didn’t know where you were on the band – it’s yours!”

Charles Kitchin had a design for a receiver which caught my attention. It was published in the Feb 2010 edition of CQ Magazine and consisted of an oscillating detector feeding a 2-stage amplifier consisting of a low-noise FET-input opamp acting as the preamp. The preamp had a low-pass filter with a variable cut-off point, as well as an extra capacitor in the audio chain that could be switched in to give a nice lift to the lower frequencies, for those times when you have a nice strong signal and want a bit of bass boost. This preamp drives an LM380, which makes for a much lower noise AF amp chain than the default in these types of receivers that employ an LM386. On top of that, there is provision for a line out jack for recording. I was interested – regardless of the front end I used, this could definitely be the AF amp for a “serious” regen!

Before I had even fully decided on the finer details of this project, I assembled the AF amp on a separate board. I wanted this to be a somewhat modular receiver, with the AF and RF sections built on different boards so that if either section didn’t work out, I could try a different one. I wasn’t entirely convinced that the value of Hammarlund tuning capacitor that I was planning on using was going to be ideal, so I wanted it to be a relatively easily swappable part with other tuning capacitors of different value but with the same form factor (of which I own a few). If I was going to go to the trouble of building something like this on a nice chassis, I wanted to give myself the maximum possible chance of succeeding.

Here is the schematic of the AF board. It is a little different from the version originally designed by Mr Kitchin (though not by much) as I will explain -

In Chuck’s original version, the 2.2K resistor on the input was 5K.  The ratio of this value to the value of the 100K resistor between pins 2 and 6 of the AD820AN determine the gain of the stage, and I wanted a bit more. Also, the LM380 was motorboating when the AF gain pot was set to anything higher than half-volume, so I added the 10 ohm resistor in the supply line and bypassed it with a 470uF electrolytic, which stabilized it nicely. Another small issue I experienced with mine was that when the AF gain pot was at absolute maximum volume, rotating the low-pass filter pot caused clicks and “bloops” in the speaker. It seemed to only happen when the slider of the pot had finished traveling over the carbon track and had actually made contact with the metal that formed the hot end of the control. I didn’t know what caused this, and I suppose it’s a bit of a “kludgy” fix, but I reasoned that placing a low value resistor at the hot end of the pot would, in effect, prevent the amp from being driven at absolute maximum volume but that the difference between this level and the absolute maximum volume would be so small as to be barely noticeable. I tried a 10 ohm resistor, which reduced, but didn’t completely eliminate the clicks and bloops. A 47 ohm resistor did cure it though, and I was left with a volume control that operated smoothly, and a variable low-pass filter pot that also operated smoothly. Very satisfying! Finally – about those 2 capacitors marked CT. They are coupling capacitors that help to determine the amount of low-frequency signal that is passed. In N1TEV’s original design, they were both 2.2uF, and I found these values to be a bit high for my tastes, making the audio a bit too bassy. This is all a matter of personal taste of course. I used 0.22uF for CT in both positions and found that it gives a pleasing lift to the lower frequencies. It is not always apparent on the internal speaker (in which case a higher value would be better), but is more noticeable when using good quality earbuds or headphones, and on recordings. Use whatever values work best for you. The 0.1 and 0.01 uF bypass capacitors on the 12V line were placed so as to bypass the 12V supply directly at the point of entry into the chassis. I wasn’t experiencing any problems before fitting them but it can’t hurt to decouple the power supply the very moment it enters the enclosure can it? Nip these things in the bud before they even have a chance to get a foothold, I say :-)

Here’s the AF board as I first built it, before changing the 5K resistor on the input to a 2.2K resistor, and the two 2.2uF capacitors to 0.22uF parts – and before adding the 10 ohm resistor in the 12V supply line and the 470uF capacitor to bypass it. The lengths of lavalier mic cable for the variable low-pass filter and the AF gain potentiometers have already been soldered in place, and they exit through holes drilled in the board. The headphone jack and DC power connector are temporary, for the purpose of testing. The Manhattan pads are of course, as always, W1REX’s MeSQUARES and MePADS -

The front end is a very standard design. It is the same configuration (and indeed the same circuit) as used in the WBR, with the exception that the tank (unlike that in the WBR) is unbalanced. This same arrangement was used in Nicky’s TRF, as featured in issue 70 of SPRAT, and I believe the original circuit was developed by GI3XZM. I wanted this receiver to be usable over a wide range of frequencies, and in keeping with my “modular” approach, wanted the receiver to be as versatile as possible. A plug-in coil system, with both gangs of a dual gang variable capacitor, as well as the fine tuning capacitor, all available at the pins of the coil base, allows for a lot of flexibility when winding coils for different bands. The user decides, when constructing a plug-in coil, whether to include padding or series capacitors for the main tuning and fine tuning capacitors, as well as choosing whether to use one, or both gangs of the tuning capacitor. In this way, with some calculations and a bit of trial and error you could, say, wind a coil to cover a large segment of the HF spectrum, or a single narrow band of frequencies. If, after some listening, I decide one day that I am particularly interested in the 16M broadcast band, I can construct a coil to cover just that one band. Neat!

The J310’s in the RF amp and the detector stage could be any similar N-channel JFET such as the MPF102 or the 2N3819. Likewise, the two 2N3904’s could be most any small signal general purpose NPN transistor. I originally fed the output of the J310 “infinite impedance” detector stage directly into the input of the AF amp board, but quickly discovered that the gain wasn’t enough to comfortably drive a loudspeaker. Had I done a few quick calculations beforehand, I would have realized that. I wanted to take advantage of the fact that the output chip is an LM380, by driving it enough to make a loud noise into the speaker! Adding the single 2N3904 preamp stage after the detector solved the problem nicely. I have built enough of these simple receivers that can drive “a small speaker to a comfortable volume in a quiet room” :-) No more!

As with any circuit of this type, the RF stages, and the frequency-determining part of the circuit especially, should be built with short leads, and stiff wiring. Top quality components will help.  The two 330pF capacitors in the feedback circuit of the 2N3904 oscillator stage should be NPO’s (or C0G’s – same thing), as should the 39pF capacitor. The coils were wound on toroids, and the coil assemblies mounted in octal tube bases. I spent a great deal of time on W8DIZ’ site, using his online calculators to figure out the number of turns required for varying degrees of coverage. Unless you build a receiver with the same variable capacitors, and use a very similar physical layout, you’ll need to do your own calculations, and then be prepared to tweak the final values of inductance and capacitance to get the coverage you want. Incidentally, I used a Hammarlund MCD-35-MX dual gang component for the main tuning capacitor. It was this one that I got a deal on over a year ago. The official specs say that each section has a capacitance of 6 – 31pF, but I also had to make a rough estimate of the stray and circuit capacitance when calculating the required values of inductance and capacitance to cover each band. My fine tuning cap was a Hammarlund MC-20-S, and I had to include the capacitance of that in the calculations too. This is the online calculator on W8DIZ’s site for the T68-6 core. He has similar calculators for all the popular toroids. Very useful stuff.  Note – for some reason, the calculator doesn’t always estimate the correct length of wire that needs to be used. This is easy to work around. Just wind one turn around a toroid measure it’s length, multiply that by the number of turns, and add a few extra inches for good luck (and pigtails).

Here’s a view of the RF board as initially built, before adding the extra (pre-AF board) preamp stage -

Here are the details of the coils wound so far. I have every reason to believe that this receiver can go quite a bit higher in frequency than the 15980KHz which is at the upper end of the range for my highest frequency coil so far. I ran out of tube bases and need to get some more, but did wind a temporary coil which oscillated (and received signals) at around 24MHz.  I’m just not yet sure how sensitive and stable the set will be at that frequency. Your values will probably be different, but here is the info on my coil set so far. After a little while looking at it, it should make sense -

Note – if you wind too many turns for the link winding, you may find that you have to turn the regeneration control nearly all the way clockwise in order to reach oscillation, or you may not be able to reach it at all, as the link winding loads down the oscillator. If this occurs, remove a turn or two from the link winding. It is particularly easy to do this on the higher frequency bands, where the main coil (L1) doesn’t have as many turns. Also, it is easy to overload the detector (as it is with all regenerative receivers). I use my Sproutie with a large outdoor antenna and find that on the lower bands, I usually only need to operate the set with the RF “gain” control set halfway. If I were winding these coils again I would use fewer turns for the link windings on several of the coils (the lower frequency ones).

The coils were constructed in two different ways. The lower frequencies used a larger T68-6 core which I mounted with nylon hardware. I first took a #10 nylon bolt, cut the head off, and epoxied it into the hollow center spigot of the tube base thus -

Before adding the toroid, any jumpers and capacitors were soldered in place (this is going to be the 3050- 3950KHz coil). The soldering’s a bit messy, but it was the first time I had soldered one of these things -

A couple of nylon nuts followed, then a nylon washer, and then the toroid, topped off by another washer and finally, another nut -

The higher frequency coils used T50-7 toroids, which, being smaller and lighter, were mounted vertically and secured with a couple of dollops of hot glue from a glue gun. In the following picture, the 3050 – 3950KHxz coil is on the left, the 14460 – 15980KHz coil in the middle (in a white ceramic tube base), and the 8040 – 10720KHz coil on the right.  The middle and right coil were pictured before the hot glue was added -

Here’s the 14460 – 15980KHz coil with the 2 dollops of hot glue to secure the toroid. I like these ceramic bases and think I’ll use them for all subsequent coils -

For the chassis, I first looked at what was available in off-the-shelf sizes and couldn’t find anything that fitted the bill. Hammond have a good selection of different sizes, but their enclosures, for the most part, use 0.04″ thick aluminum. I wanted something thicker, for a very sturdy structure, so I decided to look into having a custom chassis made. A bit of searching turned up two businesses that manufacture aluminum chassis’ for homebrew tube amp enthusiasts – Dirty Dawg Amps, a US based business who are temporarily out of business due to a fire, and Seaside Chassis Design, who are located in Novia Scotia. Seaside Chassis use a minimum of 14 gauge aluminum for their enclosures. 14 gauge is about 0.064″, which I knew would make for a nice stout case.

Terry was very communicative and straightforward via e-mail about what he could do and what it would cost. I sent him rough drawings, with dimensions, of the chassis, front panel, and mounting bracket for the main variable capacitor that I was hoping he would be able to fabricate. He was able to make all 3 items and on top of that, he would punch all the main holes for me, leaving me just to drill the smaller holes for mounting screws. This was great news, knowing that I would shortly have a solid and well-made chassis on which to build this receiver.

I dropped the ball somewhat and didn’t take a picture of the chassis when it arrived, but here’s what it looked like with all the main components fitted, before wiring it all up. The 2 biggest factors in making this receiver look so grand are the National “N” dial with Velvet Vernier drive, and the excellent chassis. Does this look inspiring or what?

The controls on the upper row are, from left to right – regeneration, the main tuning knob, and the fine tuning. On the lower row, also from left to right is the headphone socket, RF attenuation, the bass boost switch (down = more bass) , the low-pass filter cut-off control, and the AF gain control.

Here’s a view from the back at this point in the construction. Look at that accurately made chassis, front panel, and capacitor mounting bracket. Terry from Seaside Chassis Design did a great job -

Both the RF board (without the extra AF preamp that was built later) and the AF boards installed but not yet wired up. All cables are tagged for easy identification -

Another view of the underside, before everything has been wired up -

The next task was to begin wiring the boards to each other and to the controls. Looking at this view of the underside, I’m thinking that I perhaps could have put a little more effort into dressing the cables more neatly, but it’s perfectly functional. The schematic shows pin 1 of the octal base being grounded but as I was wiring it up, I decided to also ground pin 4 -

Some more views of the underside from different angles and distances. I only twisted the 12V supply lines together for neatness and not for any electrical reason, though it does rather make them look like tube filament wiring :-)  Just to the left of the antenna socket on the right, is the phono jack for the line out. This is such a useful feature. In fact, as I write this, I am using the line out to record KCBS from Pyongyang on 11680KHz. On the other side from the BNC antenna connector, you can see the DC power jack with the reverse polarity protection diode and the RF bypass capacitors. Vinyl grommets were used for all wiring that needed to pass through the chassis. RG-174/U in the form of Belden 8216 was used for the connection from the BNC antenna connector to the board, and lavalier mic cable with 2 conductors and a shield for all other connections to controls (and to the phono jack) -

A view from the top, with a coil plugged into the octal tube base -

The original intent was to mount the internal speaker on top of the chassis on the side using some kind of simple right angle bracket(s). I didn’t ask Terry from Seaside Chassis to fabricate a bracket for me because at that point, I didn’t know what speaker I was going to use. Looking around my room for something I could use, I noticed an unused LMB Heeger enclosure #143 in the size 4″ x 4″ x 2″ – exactly the same enclosure I used for the 31M version of the WBR. I figured that the top part of the box, being a U-shape, could be used as a bracket. If using that part though, why not use the whole box? There might even be some extra acoustic benefits to housing the speaker in a little case, and having it fully enclosed will protect it from dust and small bits of wire, metal filings etc being attracted to the speaker magnet (which happens here in the shack). The sound was a little “boomy” with the case closed, so I stuffed some foam in with the speaker, and it cleaned the “boominess” right up. Although you can’t really see it in these next shots, the speaker case is bolted to, and spaced off the chassis with 4 vinyl grommets to dampen any unwanted acoustic resonances in the chassis. The speaker wire enters the speaker enclosure through a grommet in the side. It’s a small detail, but the grommet is mounted not in a hole, but in a slot in the side of the cover. That way, when I remove the cover of the speaker enclosure, I can slide the grommet out, leaving the grommet still on the speaker wire, and allowing me to completely remove the cover -

There are a few improvements and modifications I’m considering making to The Sproutie but it is now completely functional, and this is how it looks at this point. I must say that I think it’s looking pretty good -

I was lucky enough to obtain a National “N” Dial in good condition and nice working order – not all of them look or operate this well. The tuning knob and reduction drive are an important part of the feel of any receiver, and can do a lot to affect the operating experience so I’ll say a few words on that subject if I may. Before, I do, here’s a clip from a page of the 1947 National Radio catalog. I get a kick from seeing vintage parts in old catalogs, then seeing the exact same thing, in really nice condition, in front of me. It’s a bit like meeting a celebrity for the first time :-)

I was initially concerhend that the 5:1 reduction ratio of the National drive wasn’t going to be high enough for accurate tuning on the HF bands – it was a good part of the reason why I chose the value of the main tuning capacitor and wound the coils so as to limit the tuning ranges to around 2MHz or less. This approach results in more coils, but really helps in creating a regen that can be set to a particular frequency, and from which you can read the frequency (with the help of a calibration graph – more on that later.) This receiver can be set to within a few KHz of any frequency. This is good enough for finding a particular SW AM broadcast station. I can also read the dial setting and then consult my custom calibration graph to find what frequency I am on to within a few KHz. It’s not much by modern standards, but is pretty good for a regen with an analog dial.

The National “N” Dial is marked from 0 to 100 and thanks to the vernier scale located at the top, it can be read to one-tenth of a point. These dials, when in good condition, have a firm yet smooth action with no backlash that makes tuning a receiver like this a good experience. Another thing to note is that these dials were manufactured with CW (clockwise) and CCW (counter-clockwise) characteristics, meaning that as you rotate the dial clockwise, the numbers either go up (CW type) or down (CCW type). This makes sense when you consider that variable capacitors were made as units that either increased in capacity as you rotated the spindle clockwise, meaning that the frequency went down (CCW type), or as units that decreased in capacity as the spindle was rotated clockwise, meaning that the frequency went up (CW type). The latter type is the convention for variable capacitors today. When we rotate our tuning knobs clockwise, we expect the frequency to increase. Back in the days when our predecessors thought more in terms of wavelength, they would have expected wavelength (instead of frequency) to go up with a clockwise rotation of the knob. This particular regen uses a CCW-type variable capacitor, so I married it up with a CCW-type vernier dial. It does take a while to get used to the fact that the frequency goes down when you turn the tuning knob clockwise, but I am beginning to adjust. There seem to be quite a lot of these lovely old National “N” dials around, if you take the time to look. A fellow homebrewer told me that his local electronics surplus store had a number of them in good condition for a very good price (I believe he bought them all!) Hamfests and swapmeets are also a good place to look. eBay is another possibility but the prices asked are a bit on the high side, in my opinion. I got mine from Gary at Play Things Of Past. He was easy to deal with.

One downside to each coil only covering a relatively small part of the shortwave spectrum is that you end up with quite a few of them – more if you decide to wind specialty coils for specific bands. A coil box was definitely in order, so I headed to my local cigar and tobacco shop and purchased an empty cigar box. A trip to the local craft store yielded a length of bass wood, which is a little harder than balsa but can still be cut with a sharp craft knife. I cut slots in the lengths of basswood so they would slot together to form dividers to store the coils in -

The dividers installed in the cigar box, with the coils that had been wound so far (at time of writing this, I now have one more, for the 120M BC band) -

In order to know where you are on the band, you’ll need to calibrate your dial.  I accomplished this by plotting a graph for each coil with frequency on the x axis and dial markings from 0 to 100 on the y axis. For frequency references, you can use a crystal-controlled marker, or off-air signals and an online frequency database such as short-wave.info  Bear in mind that the setting of the regeneration control does alter the received frequency. Probably the best way to standardize your results is to keep the regeneration at or just below the point of oscillation at each dial setting that you take a measurement. The following is one of the graphs I am currently plotting. The original was larger and it is a little hard to read the markings on each axis on this smaller version. That’s fine, as your calibration will be different anyway.  In this graph, look at the line formed by the red dots (the black dots are something different – you can ignore them) -

The variable capacitor I used is what Hammarlund called a “midline” type in which the moving plates (the rotor) were mounted off-center so that the relationship between degrees of rotation and the resulting capacitance was non-linear.  The intent was to keep the relationship between degrees of rotation and frequency fairly linear and by looking at the graph, you can see that it is not bad at all. I also found that once a graph was plotted, I was able to set the dial and return to a particular frequency with a good degree of accuracy and repeatability. When listening to AM stations with this receiver and it’s bandwidth, which is of the order of 10KHz, you can be assured of returning to a dial setting and hearing the station you want.

The set does work on SSB and CW, but SSB can a bit tricky due to the need to inject the right amount of carrier, and the finer control over tuning required with this mode. I noticed that on very strong signals, I’d need to advance the regeneration more in order to demodulate the signal in a satisfactory fashion. Advancing the regeneration changes the received frequency (which is very noticeable on SSB), causing the operator to have to retune the receiver. I didn’t need to use the fine tuning at all when tuning AM stations but with SSB stations, it was a must. With most signals, this need to adjust the injection level doesn’t occur – only with very strong signals. EDIT – As I write this, I am sitting listening to a couple of hams ragchew on 75M. It’s about 2:20am and it just occurred to me that The Sproutie works quite well on SSB. I am finding it more convenient to use the regeneration control for fine tuning, instead of the fine tuning capacitor. The set has been sitting on frequency for about 15 minutes so far with no noticeable drift. I’ve done some casual listening to SSB on 20M but not enough to determine what the drift is like up there. Drift is not noticeable on AM stations at the higher frequencies I’ve been using this receiver so far though (in the 15MHz range). More on this if and when I wind coils for higher frequencies. UPDATE – it is now 3:05am. I have been on the same frequency on 75M for over 45 mins with still no noticeable drift. I’d expect there to be some drift on SSB/CW at higher frequencies, but things are looking good down here on 75M.

While I’m on the subject of fine tuning, allow me to expound a little more on reduction drives. In contrast to the friction drive on the National “N” Dial I used for the main tuning, the reduction drive on my fine tuning is a Jackson Bros 10:1 ball drive which has a small amount of backlash and feels a bit “spongy”. I don’t like it, and am grateful that for my main intended use of listening to AM stations, I won’t need it. I may change this ball drive for either another friction drive, or a different ball drive.  The Xtal Set Society sell 6:1 ball drives manufactured (I believe) by Oren Elliot) that have a more pleasing feel. EDIT – I have since found that making the 2 screws that hold this mini ball-drive to the front panel vert, very tight seems to eliminate the backlash and reduce the spongy feel a bit.  I suppose it increases the pressure on the bearings a little. For the time being, I’ll keep this drive but if I ever change the front panel, that will be the point at which I’ll drill a bigger hole for a more conventionally-sized reduction drive.

For the above reasons, if I were intending to listen to more SSB and CW on this receiver, I would definitely wind coils to spread each entire amateur band over the whole rotation of the dial, and make sure I had a reduction drive for the fine tuning with very little or zero backlash and a better feel (though having said that, the regeneration control works very effectively for fine tuning).

Another thing that has often interested me is the bandwidth of regens. In general, as you approach the point of oscillation, the bandwidth becomes narrower, until it is at it’s narrowest somewhere around that critical point. As you continue to advance the regeneration, the bandwidth broadens out somewhat. I connected the output of a simple noise generator to the antenna socket of The Sproutie, and took screenshots while running Spectrogram, which was being driven from the line out jack of the receiver.  All 3 of these screengrabs were taken just marginally below the point of oscillation (the ideal point for receiving AM). The first one was with the low-pass filter adjusted for maximum bandwidth -

Well, it’s obviously not the kind of brick wall shape we might expect from a good crystal or mechanical filter but if you look closely, the passband is about 25dB down at the 5KHz point and 30dB down at the 10KHz point, That’s not too bad for AM reception, though if you wanted to get really serious about it, a passband of around 5 – 6KHz with a much steeper wall would, of course, be more ideal.

Here’s a grab taken with the low-pass filter pot at the median point, which is looking better -

…and better still with the low-pass filter set to the lowest cut-off point -

It is important to remember that these spectrums represent the response of the entire receiver, and not just that of the front end. SSB and CW signals become higher-pitched as you tune away from the center of the signal, but although the audio frequency make-up of an AM signal tuned off-center does change, the whole signal does not become higher-pitched. Therefore, an audio filter will be more effective at rejecting off-frequency signals for SSB and CW signals than for AM. Nevertheless, the adjustable low-pass filter is very good at cutting down much of the high pitched static that can make simple receivers like this quite tiring to listen to for long periods whatever mode is being received. It makes The Sproutie feel like a “grown-up” receiver!

Here’s The Sproutie, with it’s coil box. I would have felt as if I had died and gone to heaven if I’d had this receiver as a teenager. I’m feeling pretty good about it at my current age of 50 :-)

 

The Sproutie is not completely finished yet (is any homebrew project ever truly finished?) The changes and additions I am considering include -

 

-Designing a custom front panel with Front Panel Express.

-Changing the 10:1 Jackson Bros reduction ball drive on the fine tuning control for something with less backlash and a firmer, less “spongy” feel

-Winding more coils for higher frequency bands. I currently have coils to take me up to 15980KHz, but would like The Sproutie to go to at least 18000KHz, to cover the 16M BC band. I think it will go higher than that, and want to see how high. I did wind a temporary coil in the 24Mhz region. It was receiving signals but I don’t yet know how sensitive and stable it will be in that region.

-Winding coils for specific bands, so that the bands I am most interested in can be spread out over the entire dial, making tuning using just the main tuning control even easier

-I had also considered finding a local cabinetmaker to make a wooden cabinet for The Sproutie, but I’m not too sure about the convenience of sliding the chassis out of the cabinet every time I want to change a coil. If you’re thinking about building a receiver like this, completely enclosing it in metal would be quite a good idea – perhaps with a hinged flap or door on top for coil-changing. Regenerative receivers are quite sensitive, and this one picks up signals from my computer and/or my monitor, which are located nearby

 

It always feels good to build something that works, and The Sproutie certainly does that. It’s a great little receiver for shortwave listening and with an extra tube base, a toroid, some wire and a few extra capacitors, you can add whatever frequency coverage to it you like as you go along.

Oh – and I just realised that I didn’t explain that Sproutie is the nickname I gave my 2 year-old cat Sprout, whose ham radio name is Sprat The QRP Cat. I had already named one of my home-brew radios after a kitty I used to have called Rug, so figured it was time to honor Sproutie in the same way!

 

 

 

November 17, 2013

A Tuned Loop Antenna For The AM Broadcast Band

As a follow-up to the previous post, in which I discovered that the Sony SRF-59, though cheap to purchase, offered surprisingly good performance due to a rather creative and interesting receiver architecture. I did some reading up on external antennas to help pull in weak stations.  Among the Ultralight DX’ing crowd (those who DX the AMBC band with small, cheap receivers) FSL antennas are a source of great interest – they offer good gain and directivity in a small and portable package.  However, I had almost all the materials on hand to build a simple tuned loop and as, typically, I don’t pursue these things in too much depth, figured this would be the way to go.

First off, let’s get to grips with the rather complex schematic of this thing. The SRF-59 doesn’t have an antenna jack, so the external antenna will need to be coupled to the receiver inductively, which just makes the circuit diagram even simpler (at this point, it couldn’t really be any simpler) -

There are many different ways to construct a loop of this type. Big ones give more gain with deeper nulls, but space is at a premium for me and as this was an initial experiment, I decided to go for something modest in size.  You can use a cardboard box, plastic crate, or any number of things on which to wind the turns, but I opted to construct a frame specifically for the purpose.  Hardwood is nice, but I don’t have any woodworking tools. A trip to Michael’s craft store yielded a display of balsa and basswood in pre-cut and finished sizes. Balsa is very easy to cut, but is also very soft, and wouldn’t be very hard wearing in duty as a portable loop antenna.  Basswood is a little harder, but can still be cut with a sharp craft knife, so I decided to try a frame made form basswood. I bought 2 pieces of basswood pre-cut to 3/16″ x 3″ x 24″ and a length of 1/2″ square rod to strengthen the frame. At this stage, I have cut 2 slots in each of the 2 main pieces -

I slotted the 2 pieces together and glued 2 pieces of the square section to them with epoxy, to act as strengthening pieces. The square section was held in place with small clamps while the glue was setting. Here’s the finished result -

I wanted to have a rough idea how many turns would be needed, so found an online calculator for exactly this purpose.  I had a nice air-spaced variable capacitor that had been donated by a friend (thanks Jason!) With both gangs in parallel, it has a capacitance swing of 16 – 705pF.  This frame has sides equal to about 16.5″ in length and using the calculator, I figured that 10 turns, with 0.25″ spacing, should tune the AM BC band. Before winding the lopp, I mounted the variable capacitor -

I split a length of narrow-gauge zip cord in two for the loop. Halfway through winding it, Sprat The QRP Cat bit clean through the wire while my back was turned, so I had to solder a new length on in order to continue winding. She also chewed a small part of the frame while I wasn’t looking. It’s a good thing I love that little kitty!

Here’s the finished loop -

The space between the windings is 1/4″, with a wider 1/2″ gap in the middle. This is in case I later decide to use a rod or piece of square section wood as a supporting mast – it can fit through that larger gap -

Another view of the completed loop -

Of course I was keen to try it out, so I switched the SRF-59 on, placed it close to the loop, tuned to a weak station, then tried tuning the loop and moving the receiver around for optimum coupling. Nothing I tried seemed to work and although I could tune the loop to resonate at the frequency I was listening on, it wasn’t enhancing the received signal at all. In fact, reception was better without it. This was all rather dispiriting and I was about ready to throw the towel in and think about adding a few parts to convert the loop to a novel crystal set receiver when, after taking some shots of it outside on my balcony (the 2 pictures above with the concrete on the floor, and the one below), I decided to set up the radio and try it there. It worked! (All the previous tests had been made in my apartment indoors).

For good inductive coupling between the loop and receiver, you want to orient the loop so that both it’s turns, and the turns on the ferrite rod of the receiver, are in the same plane.  The rod in the SRF-59 runs across the top of the case, so this is how it is oriented (you can also place it inside the loop) -

In the above picture, the loop will receive maximum signal from stations to the left and right of the picture (broadside to the winding) – and it does!  My test was only brief, conducted in the daytime, with signals that were of moderate strength. They were of such a strength that there was some noise and static when receiving them with just the radio. On placing the radio next to the loop and tuning it to resonance, all static and noise disappeared, yielding a more pleasant signal to listen to.  To make operation easier,  when orienting the loop for maximum signal, I rested the receiver on one of the diagonal arms in the frame. If the loop were on a stand, one of the arms would be horizontal.

My loop seems to tune well above the top end of the BC band, but doesn’t cover the bit from 530 to about 600KHz.  A fixed capacitor across the variable should bring the tuning range down a bit.  I’ll fiddle around with it in the next few days. I may also make a recording if the spirit moves me :-) EDIT – I did. See below.

I already had the wire and variable capacitor, so this loop cost me $8.58 in wood from the craft store. The SRF-59 receiver cost me $6.50 inc shipping from eBay, so my complete AM BC band DXing set up set me back a whopping $15.08. I like the kind of fun that can be had for such a small outlay :-)

This afternoon, I went out onto my balcony and made a short recording of KZSF in San Jose.

The recording starts with the SRF-59 receiver without the loop, then I place the receiver inside the loop, which has been pre-tuned to resonance and oriented in the direction for maximum signal. I remove the receiver, and then place it back in the loop for comparison. KZSF is not a DX station from my location in Oakland. It is a 5KW station in San Jose – just 40 miles away. It is entirely possible that I could have found a nearby position from which to get a better signal on the receiver without the loop, but this recording was made to show how a loop such as this can provide a meaningful and useful boost to a marginal signal.

November 11, 2013

AM Broadcast Band Dxing – With A $3.50 Radio!

After finishing the VK3YE Micro 40 DSB Transceiver, I did fool around with crystal radios a little, but didn’t pursue those experiments very far. Perhaps they will continue at some point. However, thinking about crystal radio sets did keep me on the subject of Medium Wave AM Broadcast Band listening for long enough to find out about the hobby of Ultralight DXing, which is the hobby of listening for distant stations (usually on the MW AM BC band) using modest portable receivers.  Some enthusiasts cite a receiver price of $100 and less as a cut-off point, and that seems like a reasonable definition.

It’s a neat hobby, and there is a lot to be heard for the dedicated listener. The fact that it can be done with a modest set-up only adds to the appeal.  In 2007, Gary De Bock N7EKX discovered that a little Walkman radio from Sony, the SRF-59, had very good AM performance, and cost under $20 new. Others acquired their own SRF-59’s and also found that considering that it’s just a small, cheap receiver with an analog tuning dial, it has surprising sensitivity and selectivity. Unfortunately, in order to achieve the best performance, an alignment is recommended, as many of them came out of the factory with less than optimum performance. Earlier models, such as the clear-cased prison issue SRF-39FP, had much better factory alignment as well as a higher quality tuning capacitor, but they cost more.  If you’re willing to pop open the case yourself and perform 2 fairly straightforward adjustments, you can have a sensitive, selective and very portable receiver for the 530-1700KHz broadcast band.

How does a cheap receiver like this manage to provide sensitivity as well as selectivity, with excellent image rejection and almost no birdies? Well, take a look at the one I scored on eBay for $3.50 plus $3 shipping, and I’ll tell you -

The Sony SRF-59 uses a low 55KHz IF on the AM band for good selectivity, combined with a local oscillator quadrature mixing scheme that cancels out images – and it operates from a single AA cell with long battery life too!

This receiver uses a proprietary Sony chip – the CXA1129N. They have not released any data on this chip but after it had been on the market for a while, the basic architecture was figured out. This radio uses a low IF of just 55KHz on the AM band. Yes – that’s not a typo – the IF is 55KHz, which gives great selectivity. Think about those other cheapie portables you have that cannot receive a weak station on a channel adjacent to a local powerhouse. The selectivity on this receiver really helps with those kinds of situations. The problem with such a low IF is, of course, images, which would only be 110KHz apart.  Sony get around this by using a quadrature mixing scheme that splits the LO signal into 2, and phase shifts one of the signals, before mixing them back together. This cancels out the images that would otherwise be a serious problem in this design. What a great idea to implement a scheme like this in such a cheap little receiver! It runs off a single AA cell too – reportedly, the main chip will operate down to 0.95V.  On reading about this, I had to have one, and when I found the above used one for just $6.50 inc shipping on eBay, it was a no-brainer.  It came with the Sony earbuds pictured above though when supplied as new, it comes with a set of light headphones.

Out of the package, it sounded pretty good but I had the nagging feeling it wasn’t receiving as well as it could. Gary De Bock, who has performed many alignments on these units for DX’ers, reported that a significant number of them benefited from adjustment. Although the frequency calibration wasn’t too far off on most, nearly all of them needed some tweaking to the 2 tracking adjustments. Mine, it turned out, did too.

I won’t describe the alignment process in detail, as there is all sorts of info about it documented by more knowledgeable people than me. This post by Gary in the Ultralight DX Group on Yahoo Groups, describes it in detail. Also, this page shows how to disassemble and reassemble the receiver and has some good info too.  Both links open in new browser windows. Here is Gary De Bock’s first review of the SRF-59, published in late 2007.

When I first popped the case off, according to instructions I had read, the board is glued to the back part of the case, so the front part is supposed to separate first. It didn’t happen that way for me – my back part came off first. This image also shows the trimcap that is adjusted for maximum signal at about 1400KHz.  If you don’t have a signal generator (I don’t) you can use a weak off-air signal -

This view shows both parts of the case separated from the board -

The view from the other side -

The other adjustment that needs to be made is shown in the next image. The smaller coil is secured to the ferrite rod with wax. The wax is scraped away (I used a small jeweler’s screwdriver) so that it can slide up and down the rod. Then, with the radio either listening to a signal from the sig gen at 600KHz, or a weak off-air signal at or near 600KHz, the smaller coil is slid up and down the rod until the point of maximum signal is found. Gary recommends to use a small piece of tape or woodworking glue to secure the coil in it’s new position; I smeared the wax that I had previously scraped off back onto the coil and warmed it very briefly with a match to melt it again. In this photo, I had already made this adjustment (my coil needed to be moved closer to the main coil for maximum signal) -

A closer view -

You can, if you wish, adjust the frequency dial calibration too. This process is described in the links I have provided, but it was relatively close in the unit I had. There is a limit to how accurate such a basic dial can be anyway, and it is not too hard to figure out where you are if you use powerful local stations as markers. The fact that US stations are spaced at standard 10KHz intervals helps a lot as well.

I have only spent a couple of evenings listening at home so far. The electrical QRM is quite severe in my place at night. It clears up significantly when I walk out into the street, but standing in the middle of my street at night is not the most comfortable position for a long listening session! So far, I have heard stations up and down the west coast, from Mexican “border blasters” on the Mexican side of the border, San Diego, Los Angeles, Las Vegas and up into Oregon, from my home in the SF Bay Area, as well as stations in the central California valley. This is all straightforward stuff – to be hearing stations up to 500 miles distant, but I’m really looking forward to hearing my first Trans-Pacific (TP) DX. There are quite a few powerful broadcasters in Asia that can be heard here on the west coast, as well as inland, when conditions are good.

Anyway, instead of waiting until I had logged some serious DX, I wanted to share my excitement at this neat little receiver. It has reminded me of the pleasure I used to get from simple radios as a teenager. In fact, I even took it to bed last night and went under the covers with it and a flashlight (to see the dial)! The last time I did this with a radio was as a youngster :-)

The appeal of the Sony SRF-59 for me is similar to the appeal that some sports cars hold for driving enthusiasts. In the same way that basic suspension and a lack of luxury features in sports cars like the early British Triumphs made the driver feel closer to the road, there is not much in the Sony SRF-59 to get between you and the AM band. Having said that, it performs better than it’s counterparts from a few decades ago. I love the fact that a newcomer to DXing could, if he/she kept an eye out for a good used deal, get started with this radio, and a small notebook for a logbook, for less than $10. Excellent! I have already had lots of fun for my $6.50, end expect to have much, much more. Stick this in your bag or shirt pocket the next time you go for a walk or hike (or camping), and you’re guaranteed lots of listening fun.

PS – I bought this radio for the AM performance in such a small, cheap radio (and the novelty of the technology used in such a package). It sounds nice on FM but the reason to own this receiver, IMO, is for it’s AM band.

PPS – This little receiver has quite a dedicated set of followers.  Some people have hooked the board up to air-spaced variable capacitors and vernier drives, with larger cases, knobs, input/output jacks etc.  Others have modded it for different bands. With such a cheap radio, there’s not much to lose if you mess up your mod.

PPPS – Some have commented on how the tuning is too fiddly with the small thumbwheel. I haven’t found this to be a problem – I engage my thumbnail with the teeth of the thumbwheel and find it easy to make small adjustments. If you have very short nails, this might not work for you. I saw a mod in which the rectangular slot for the thumbwheel was widened, exposing a greater width of the thumbwheel.

May 11, 2013

The NA5N Desert Ratt 2 Regen

EDIT If you’re thinking of building the Desert Ratt 2, although the pictures in this post are numerous and quite large, I do recommend reading all the text too, as I have included what I thought were relevant details on the construction as part of my narrative. Also make sure to read the comments and replies.  Previous blog-posts have taught me that readers often ask pertinent questions, so you may be able to glean a little more information from them too.  In fact, just before I wrote this, Paul NA5N made a comment which includes a usefiul piece of information about the 2 x 1,000pF (0.001uF) capacitors in the regen stage.

EXTRA EDIT – Please read the update at the end of this post, after the videos.

I’ve been wanting to build NA5N’s Desert Ratt regen ever since I first found his very attractively drawn schematic for it online. I then found the updated version, called the Desert Ratt 2, and a very good description of how the circuit works – all of these documents available on Paul’s website. What more could an avid regen builder want? Not much, it turned out. Late last year, when N2CX and N2APB dedicated an episode of Chat With The Designers to the Desert Ratt (and to the subject of regens in general), I just had to listen and of course, it fueled my interest in building the DR2 even more. The whiteboard for this particular episode of CWTD is here, and the podcast audio is here.

The WBR was a successful regen for me and while it worked well on SSB/CW, it didn’t seem to quite have the gain with AM stations. This makes sense, as a regenerative detector has to be set below the point of oscillation for AM reception, at which point it has less gain than when it is oscillating (which is where you set it for SSB/CW reception.)  Even so, I had read that bipolar transistors tend to work better as regen stages for AM, as they have higher gain when not oscillating. The search was on for such a receiver, and this was one of the key deciding factors in building the DR2 for me. In fact, Paul has mentioned (I forget where I saw it, as I have done so much reading on this receiver) that the Desert Ratt doesn’t do so well with SSB/CW as it does with AM. My experience with it backs up this assertion, thought it’s a pretty neat receiver for AM.

In particular, I wanted a receiver for covering the 49M SW BC band as although my Elecraft K2 covers a few of the BC bands, 49M is not one of them. There were a few things I found interesting about the design. The use of a phase splitter transistor to convert the single-ended output of the detector to a balanced output in order to drive the LM386 in differential mode was novel. Paul talks about how much RF is flying around inside regen receivers, and how the common-mode rejection of the 386 when used in differential mode can be advantageous in such an environment. I was also intrigued by the detector consisting of 2 germanium diodes – I think I was just looking for an excuse to build something with Germanium diodes again to remind me of my crystal-set building days as a kid :-)

If you look at the schematic of the DR2,  you’ll see that one of the changes in the design from the original DR is that instead of a variable capacitor, it uses 1N4004 diodes as varicaps. I have a bit of a “thing” for nice air-spaced variable capacitors, and I had in mind a nice Millen 50pF capacitor that I picked up on eBay for a very fair price last year. Combined with a 6:1 reduction drive, it made a good combination with a very useable tuning rate for tuning in AM stations.

Anyway, I’m getting ahead of myself here. I did make a few changes to the original schematic for my version, so allow me to introduce my rather wobbly circuit diagram -

The differences between my schematic and Paul’s are as follows -

- I added an RF attenuation pot at the antenna input. After building the DR2, I found that using a relatively short piece of wire indoors as an antenna was causing a lot of common-mode hum.  On top of that, I wanted to be able to increase the signal level into the receiver with the use of my regular outside antenna (A 40M dipole fed with 300 ohm balanced feeder.)  Using the attenuation pot allowed me to use the large outdoor antenna without overloading the receiver.  Use of my outdoor antenna created enough separation between the receiver and antenna that the hum problem almost entirely disappeared.

- Earlier versions of the Desert Ratt included instructions for winding the coil on a plastic 35mm film canister and on an IC shipping tube. The DR2 schematic doesn’t include such instructions, but I wanted to use a toroid, so I experimented a bit and came up with a scheme that seems to work OK.  I used a T68-6 former and the turns info is on my schematic above – a T50-7 would take up a little less space. More about this later.

- I had a few 2-position center-off switches that I wanted to use, so I used one of these for a bandswitch instead of the SPST switch in NA5N’s DR2 schematic. I had originally thought that using the 50pF tuning capacitor with no padding would make the upper limit of frequency coverage too high, resulting in too large a frequency swing in one band, but there must have been more stray circuit capacitance than I had anticipated, as the coverage with no extra padding was about 7.3 – 13MHz. This band became the center position.

- I was attempting to power the DR2 from my shack power supply, which is about 45AH of sealed lead acid batteries with a float charger constantly connected.  This also powers my K2, and the DR2 was picking up processor noise from the K2, as well as a low-frequency “burbly” kind of noise of undetermined origin. The problem went away when I powered the receiver from a separate SLA. but I decided to add extra filtering to the power line anyway.  I found that a 1mH choke as well as a 1,000uF electrolytic almost (but not quite) got rid of the unwanted interference on the power line.  For good measure, I added a 0.01uF RF decoupling capacitor across the power line at the input connection.

- I added an AF preamp stage directly after the diode detector to ensure enough power to easily drive a speaker – even with weak signals.

- The inputs to the LM386 are the opposite way around from the way indicated in NA5N’s DR2 schematic.  With the inputs connected as shown in Paul’s diagram, the LM386 emitted a loud screeching sound.  Swapping the inputs cured this. I was not the only person who had this problem, as I discovered from this post in the GQRP Yahoo Group (you need to be a member of the group to read the post).

-  I left pin 7 unconnected. I don’t understand the way that NA5N has it connected to the junction of the series resistor and capacitor connected between pin 5 and ground in his diagram.  Most circuits that use pin 7 call for a decoupling capacitor direct from pin 7 to ground (usually about 10uF).  This helps reduce large signal distortion, though Paul does say that in this application, it may not do a great deal to help and is therefore optional.  I elected to leave it unconnected.

Now for some pictures.  I didn’t want to spend a lot of time constructing an enclosure, so decided to make a simple PCB L-shaped chassis and build the circuit directly onto that.  With the variable capacitor mounting bracket, it still ended up taking quite a while to construct though. All my projects begin like this, with the main components and control being laid out on the front panel, while deciding on the basic layout -

I’ll spare you the words at this point and apologize for all the pictures that are about to come. If you’re living in a remote area and are still relying on dial-up, then I feel a bit sheepish about the sheer number of images to follow!  I’ve talked before about constructing enclosures from PCB material, so won’t repeat that information here. As well as constructing the chassis from PCB material, I also made a mounting bracket for the variable capacitor and a tuning pointer to attach to the reduction drive with 2 small screws – all from double-sided copper-clad laminate.

I applied several thin coats of lacquer from an aerosol spray.  It was sprayed from a distance, resulting in a light, and stippled coating, which you can see in these pictures. I’d rather apply too light a coat than risk overdoing it. The downside of this is that oxidation will being to affect the appearance of the copper fairly soon. Oh well. The capacitor mounting bracket received a thicker coat. You can see the smoother, shinier finish.

I got the 6:1 reduction drive from Midnight Science. A number of others sell them, and one place that springs to mind is Mainline Electronics in the UK. They are the suppliers for Jackson Bros components (I think they have the rights to manufacture and sell the parts).  They sell on eBay using the name anonalouise.

The enclosure looked a little bit different by the time the DR2 was finished, as the hole for the nylon toroid mounting hardware hadn’t been drilled in the base at this point.

Look at that gorgeous variable capacitor!

A close-up view of the Millen 21050 50pF air-spaced variable capacitor and mounting bracket. This component is silver-plated (the vanes are probably brass), and has double bearings and a ceramic base. It is a very nice variable capacitor, and had never been soldered to before being used in this project. It is at least 35 years old – most likely older!

Boy, was I glad to finish the chassis so that I could start work on wiring it all up.  I decided to build the AF amp first and work backwards, my thinking being that the AF amp would be relatively straightforward. The act of touching the input with a metal screwdriver and hearing a hearty buzz in the loudspeaker would give a welcome psychological boost! If I started by building from the antenna end, I’d have to wait until the entire receiver was built before getting any clue as to whether it was working.

Here’s the chassis with the LM386 amp, the 2N3904 phase splitter, and the 2N3904 preamp built. As has been the case with all my projects since I started using then, I used W1REX’s wonderful MePADs and MeSQUAREs to build the circuit -

Here’s a close-up. The 2N3904 preamp is just below the 6:1 reduction drive, and the 2N3904 phase splitter is to the left of the LM386.  The 100uF capacitor that decouples the supply line to the LM386 straddles it. I read that it is best to ground it to pin 4 instead of to some other point on the chassis to avoid instability, hence the reason for this placement. The other electrolytic that is straddling the chip is the 10uF capacitor between pins 1 and 8 that sets it to the maximum gain of 46dB. The black shielded cable connecting the AF gain pot to the circuit on the PCB is lavalier mic cable.  It has 2 conductors, each of them in it’s own shield, which is ideal for wiring up potentiometers. It is fairly thin and very flexible. I use it in all my home-brew projects. I bought it from a local pro-audio store which recently closed down, so will now need to find another supplier.

In this view, you can clearly see the extra DC supply line filtering that I added, consisting of a 1mH choke in series with, and a 1,000uF electrolytic across, the DC supply. After seeing these pictures, I noticed that there wasn’t very much solder on the joint connecting the choke to the power jack, so I re-flowed the joint and melted a bit more solder onto it.

The power indicator LED’s main function is as a voltage regulator. NA5N marked the various voltages on his schematic for the DR2, and I chose an LED with a forward voltage drop to match those voltages as close as I could.  A green LED in a variety pack I got from Radio Shack had a forward voltage drop of 2.1V, which seemed about right.  The 1N4148 had a forward drop of about 0.65V.

The next stages to be built were the detector and impedance converter/buffer stages.  The description of the DR2 on NA5N’s site gives more info on these stages (as it does for the whole receiver). I couldn’t be sure these stages were working, but bringing my finger close to the diodes resulted in a pleasing cacophony of stations in the headphones – and at a louder level than in doing the same to subsequent stages, so I figured there was some detection/amplification going on :-)

I didn’t know how many turns I was going to use on the toroid, but using the calculator on W8DIZ’ site and an online resonant frequency calculator, I figured that 36 turns on a T68-6 should be a good starting point for the whole winding from pin 3 to pin 6. In Paul’s version, with the coils wound “traditional style”, the tickler winding was about 1/3 of the whole winding.  Coupling between windings is tighter with a toroid than a “regular” coil, so I reduced the number of turns on the tickler. I found that regeneration was occuring at only about 25% rotation of the regen pot, so further reduced the number of turns. Using the turns shown on my schematic at the beginning of this post,  the regen stage moved into oscillation at anywhere between 40 and 50% rotation on the pot, so I left it at that. For the same reason of tight coupling, I used fewer turns on the antenna winding too and because I am using an outdoor antenna, could probably have used even fewer turns.

The toroid was fixed to the PCB with nylon nuts, bolts and washers that I got from my local Ace hardware store.

Here are some pictures of my Desert Ratt 2 with the circuit finished -

The red wires running along the back of the front panel are the regulated 2.1V and 2.75V lines.  I would have run them on the main board but ran out of room due to lack of planning, so went vertical.  Incidentally, although I refer to the 2 regulated lines as 2.1V and 2.75V,  the exact voltages aren’t important.  That’s just what they turned out to be in my case.

The RF amp and regen stages can benefit from transistors with high hfe. I got a cheap Harbor Freight DMM that measures hfe from an eBay vendor for under $6 including shipping.  hfe varies depending on the collector current, but I was doing this mainly for comparative purposes rather than absolute values, so the fact that I didn’t know what value of collector current was used to measure hfe in this cheap meter didn’t matter. It just so happened that my 2N2222A’s tended to have higher hfe than my 2N3904’s, so I ended up using a 2N2222A that measured in at hfe = 203 for the RF amp, and a 2N2222A with hfe = 223 for the regen stage.  The other stages don’t require high-gain transistors. NA5N talks about it in this post on QRP-L from 1999. Bear in mind that he was talking about the original version of the Desert Ratt in this post (just so you don’t get confused when he identifies the various transistors).

I did promise that I’d give a bit more detail on the toroid. Mine was wound on a T68-6 former. The main winding was 30 turns tapped at 27 turns from the top (3 turns from the bottom). The antenna coupling winding was 5 turns.  All turns are wound in the same direction. I used 26 gauge wire, but the precise gauge isn’t important. 26 gauge was narrow enough to easily fit all the turns on the former, yet stout enough to lend some stability to the oscillator, as the toroid isn’t sitting close to the board, and the leads are relatively long. When putting taps on coils, I used to not cut the wire i.e. I would simply make a loop in the wire, twist it, tin the twisted part and keep on winding.  Now I find it is easier to treat them as 2 separate windings connected together. If you can get heat-strippable wire, please do – it makes winding toroids so much easier and more pleasurable.  I wound the first winding of 27 turns, stripped and tinned the end, then stripped and tinned the end of another piece of wire, twisted and soldered them together, and carried on winding the last 3 turns in the same direction (this is important).  The separate antenna winding of 5 turns is also wound in the same direction.  I’m afraid I didn’t write down (or if I did, I have since lost it) the lengths of wire used. I did notice that the turns calculator on W8DIZ’ site (linked earlier in this post) was quoting lengths that are too short for the T68-6 former.  All you have to do is wind one turn around your former, measure that length, multiply it by the number of turns you’re going to wind, add an extra inch or two for the leads and, as we say in England, Bob’s yer Uncle and Fanny’s yer Aunt (meaning – you’re home free!)  When winding toroids, I often find that the first 1 or 2 turns aren’t quite as tight as the rest so when I’ve finished winding, I will unwind one turn from the beginning of the coil, then wind an extra one at the end, to keep the total number of turns the same.  Sometimes I will repeat that exercise a few more times until all the turns are nice and tight.  For this reason, I use enough wire to leave several extra inches at each end.

The next picture shows an anti-hiss filter that wasn’t in the earlier pictures, which I tried and ended up removing due to a low-frequency oscillation it was causing at the higher volume settings.   It was a series 0.01uF capacitor and 4.7K resistor connected from pin 1 of the LM386 to pin 5.   From what I have read, too low a value of resistor or too high a value of capacitor can cause the oscillation. I have seen other anti-hiss filters that used a 0.01uF cap and a 10K resistor, so it is very possible those values would have cured my problem. However, I was near the end of the project and itching to move on, so I just removed it. You can also see the 0.1uF capacitors on the inputs of the IC that have been swapped over to stop the uncontrolled oscillation, and are now crossing each other.  You may not have to cross these caps if you plan your layout accordingly -

Other than the problem with the loud screeching that was solved by swapping over the inputs to the LM386 (my schematic reflects the way the inputs were finally connected), the only other problem I had was with what appeared to be a defect in the 0.001uF (1,000pF) capacitor that leads from the tap on the coil to the emitter of the regen transistor.  I wasn’t getting any regeneration at all but on replacing this capacitor, the circuit broke into a nice loud hiss when advancing the regen pot.

I do have one ongoing issue that I hope someone can shine a light on for me, and that is a loud crackling sound when adjusting the tuning capacitor. At first, I thought a dirty rotor connection was the problem, but it only happens when extra padding capacitance is switched in by the band-switch   With no extra capacitance switched in, the tuning is smooth, but on the lower frequency bands, the receiver crackles when being tuned.  I need to try bypassing the band-switch and soldering the padding capacitors into circuit in case the switch is the problem. I’ll report back when I’ve done further work on this.

Incidentally, the main tuning range on mine covers approximately 7250 – 13000KHz.  Switching in a 47pF capacitor changes the range to 5825 – 8050KHz. I’m a bit limited with my receiver and test equipment here, so haven’t yet been able to determine the coverage of the lowest frequency band.

When first listening to the DR2, I had no idea what frequency I was listening to – only that I was probably somewhere between 5 and 12 MHz. I had no antenna connected (and at this point, hadn’t even built the RF amp stage) but started hearing CW. Lo and behold, it was Hank W6SX 180 miles away from me in Mammoth Lakes, CA. His CW signal was coming through well and in fact, this was the only time I have received CW in a satisfactory fashion on the Desert Ratt. There was no antenna – he was being picked up directly by the toroid.  Any concerns I might have had about the sensitivity of this receiver would have been immediately allayed.

I know the main question that is probably on your mind is – how does it sound, and what is it like to use? How does it “handle”? There are some videos of my Desert Ratt 2 in action at the end of this post. Apologies for the poor video quality, but my only video camera is 10 years old (and has a faulty CCD sensor). You’ve probably read articles about regens that describe the many and subtle adjustments that need to be made when tuning a regen in order to coax maximum performance from it. If you haven’t operated a regen before or if it’s been a while, it does take some time to get the hang of getting the best out of it. As you get further away from the setting of the regen pot where it breaks out into oscillation you lose selectivity and gain, so you need to try and keep the control set just under the point of oscillation. Loud stations can overload the detector, resulting in audio distortion, so it’s worth keeping an eye on the RF attenuation pot too. Also, if the attenuation pot is set too high (too little attenuation), you may get breakthrough from stations on other frequencies. There’s quite a bit going on to keep under control, but if you manage to keep all controls adjusted well, you can coax some pretty decent performance out of the set. I think this is why regens appeal to some people – we are incurable knob-twiddlers!

Stability is easily good enough for AM reception and with a logging scale fitted to the front panel, I don’t think it would be hard to find specific frequencies, as the majority of SW BC stations stick to 5KHz channels. In my casual listening so far, I have heard The Voice Of (North) Korea on 9435 and 11710KHz, Radio Habana, Cuba on the 49M band, Radio Australia on the 31M band, coastal station KLB (South Korea) on 8636KHz, the BBC World Service (forget which band or frequency), China Radio International on 9790KHz, WTWW on 5830KHz, and a number of other evangelical Christian stations (sorry, I tune them out and don’t pay them much attention.)

To sum up, you can definitely have a lot of fun and engagement with the bands on this set.  Being a regen, it is not the easiest receiver to operate, but you shouldn’t let that put you off. The best analogy I can think of is to reference the way that although an older British sports car may not have the finesse and performance of a newer sports model, it’s a lot of fun, and it’s lack of suspension gives you an exhilarating feel for the road that the more expensive cars cannot.

The Desert Ratt 2. A logging scale fixed to the front panel would make frequencies in the SWBC bands easy to find. I must do this sometime :-)

Please note that in the following videos, an MFJ-281 ClearTone speaker was used. My understanding is that this speaker has a slight resonant peak at around 700Hz (helpful for CW) and a relatively restricted overall bandwidth that is good for communications applications. This probably means that it’s not optimum for getting the maximum fidelity from an AM SW broadcaster (not that those stations have a lot of fidelity, but they tend to have a bit more than your average SSB transmission). On top of that, the audio was captured with the built-in mic in my old Canon A80 compact. Please don’t judge the quality of the Desert Ratt 2 audio from these clips. It’s better than this! I’m working on a few audio only recordings that will better demonstrate what the DR2 sounds like, and will put them up in the next blog-post (hopefully within a week or so).

Update – It has been about a year since I built my version of the Desert Ratt 2 and I feel compelled to provide an update. Whenever I first build a project I am often so thrilled that it works at all, that I tend to gloss over any shortcomings, particularly in my blog write-ups. Some of this is due to the possibility that any deficiencies are due to my layout and construction, as opposed to a problem with the circuit design. In the case of my DR2, I am still not sure whether the issues arise from the circuit itself or from my construction, as I have only built one of these. I did, however, want to document what I have observed, as my DR2 has laid on my shelf for the past year, largely unused, while I drag my WBR out and take it for a spin on a regular basis. Here are the issues I have observed -

* There is a lot of scratchiness in the speaker when tuning the DR2. This happens on some frequency ranges more than others, but it happens a lot.  At first, I wondered if it was due to inadequate grounding of the rotor plates but I don’t think this is the case. There is a solder tab for both the rotor and the stator, and the rotor is grounded to the chassis by a direct wire. Also, it is a quality Millen variable capacitor, and it is clean (the oxidation has been cleaned off).  I’m still considering the possibility that it as something to do with my variable capacitor, or the way that I have connected it.

* The set does seem to overload very easily on my outside antenna. Breakthrough from other frequencies is a common occurrence. This got me to thinking about the RF amp stage. The instructions call for picking a high hfe transistor to use in this position but thinking about this, I’m not sure why. Surely the purpose of an RF stage in a regenerative set is to provide isolation between the detector and the antenna, with gain actually being undesirable, due to the tendency of the detector to overload? The more I think about it, the more I think that different configuration for this RF stage would be more appropriate.

* Hum, though not always apparent, does still occur from time to time.

A commenter who goes by the name of Mast does mention that the tank circuit is very tightly coupled to the collector of the regen transistor. I’ll cut and paste his comments here, as I now wonder why I didn’t pay more attention to his input at the time,

“A nice schematic for general use. But the tank circuit is tightly coupled to the collector of the regenerative stage. You will suffer a lot from changing internal stray capacitances of the transistor when setting the regen level. And strong SSB signals will change these capacitances too, causing an unintelligible reproduction of SSB signals.”

At this point, the DR2 has gone back on the shelf while I move on to planning other projects, but I’d be very interested to hear what the experiences of others have been with this circuit.  I know there are folk who found N1TEV’s beginner’s regen to be a little hard to tame – and the DR is based in part on that circuit.  In contrast, both my WBR’s are well-behaved, and have been used regularly since I built them.

July 29, 2012

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.

February 13, 2012

A Free Download in Celebration Of World Radio Day

Today (Feb 13th) is World Radio Day and I noticed from The SWL Post that Myke over at The Shortwave Music Blog is offering a one-day only free download of a musical project that was compiled for release as a 5 CD box set in 2011. Titled “The Clouds Should Know Me By Now: ShortWaveMusic 2005-2010″, the set wasn’t released but for one day only, you can download all these tracks of indigenous music from around the world as heard on shortwave radio.  With the continuing decline of international shortwave broadcasting, this is a treasure trove.  Note that if you’re looking for station ID’s, this is not the download for you. If you’re a fan of international music, especially as recorded off-air, this will be right up your street.

The page with the download link is here. If you’re interested, download it now, as it may not be up after Feb 13th.

Thank-you to The SWLing Post for the tip-off!

EDIT:  The link has already been de-activated. It was a very short window of opportunity.

September 12, 2011

Is Internet Radio “Real” Radio? (Alternate Post Title: My New “Shortwave” Radio)

Filed under: Broadcast Radio — AA7EE @ 7:38 pm
Tags: , ,

Every now and again, I post about something a little off-topic from the subject of amateur radio, though still related to the topic of radio, so please excuse me if the following is not of much interest to you.

A couple of weeks ago, acting on a tip from an old friend and colleague from my days in broadcast radio, I bought an internet radio.  Best Buy were (and at time of writing, still are) offering the Tangent Quattro Internet Radio at a clearance price of $99.99. From time to time I listen to the streams of terrestrial broadcast stations but, as weird as it sounds, listening to them on a computer just doesn’t feel like “real radio”. I’ve been wanting a stand-alone device to listen to online streams for a while now.

I’m reminded of the debate that raged among hobbyists when digital photography was still new. Some claimed that digital photography wasn’t “real” photography because the images weren’t captured on film and printed on papers with silver-based emulsions. I didn’t get that argument at all because it was the message (i.e. the image) rather than the medium that was more important to me, and digital just seemed like a much more capable medium to act as a carrier for the message. Similarly, although I do have an attachment to the medium of radio, I’ve come to realize that over 50% of my enjoyment of shortwave broadcast listening was for the actual content – and there is plenty of easily accessible content on this internet radio.

Apologies for the lack of a more set-up shot, but I didn’t want to stop listening.  The little black box on top of the radio is my flash recorder that plugs into the line out on the radio for recording programs, and unrelated (though very topical) you can see the various connectors from the CC-20 transceiver beta board poking out to the left. The telescopic whip sticking up at the back is for reception of local FM stations if you want to use the built-in FM tuner:

The sound is really good, thanks in part to the bass port on the back of the radio:

The shortwave broadcast bands have not been what they were for quite a few years now, and many international shortwave broadcasters continue to scale down and cease their operations altogether – particularly transmissions to the developed world. Unless you like listening to endless religious broadcasts, the pickings here on the West Coast are pretty slim. In sharp contrast, I have 15,349 stations immediately available on this internet radio to choose from – and that doesn’t include the fact that if a station isn’t included in the list on my receiver, I can add it as a separate stream. Oh – and it does podcasts too – I already have Soldersmoke programmed in. I remember when I was a much more active SWL being able to listen to the foreign services of many countries on the shortwave bands, but wondering what their domestic broadcasts sounded like. Now I wonder no more because not only can I listen to the foreign services of stations like Radio Tirana, Albania and Deutsche Welle, there are even more domestic broadcast stations to choose from. Even the low bit-rate streams from less developed countries give more consistent and better quality reception than the shortwave bands ever have.

Of course, all these streams are available to listen to on a computer, but the convenience of a dedicated stand-alone box to listen to them makes the process of “tuning in” a whole lot simpler.  In the morning I hit the power button and after the internet radio has logged onto my WiFi network and buffered enough audio, it automatically begins streaming from whatever station I was listening to last – just like a regular radio. Sure it takes a minute or two to fire up, but so did vacuum tube radios in the old days :-)

I’ve found stations with fabulous music from Senegal, Mali and Algeria, to name just a few. There is a News/Talk station in Jamaica that is very entertaining, as I have never heard serious social issues discussed in heavy Jamaican patois, interspersed with reggae songs and streamed with the production quality of many great reggae songs recorded in Kingston in the ’60’s. That is to say, the audio is poor quality but somehow, it adds to the aesthetic of that particular station. I own a number of good music compilation CD’s from Radio Nova in Paris, France and have finally been able to listen to the station (it’s quite good and plays an interesting musical mix).

Best of all though, I’ve been listening to a lot of BBC Radio. From the news and current affairs of BBC Radio 4, to the comedy and drama of BBC Radio 4 Extra, to the musical shows of BBC Radio 6, it’s a reassuring reminder that first-rate broadcast radio is not dead. Anyone who listens exclusively to the commercial stations on the AM and FM bands in the US could be forgiven for having given up on broadcast radio. In my humble opinion, the medium of radio is just too good to trust to commercial forces alone. When you place broadcast radio in the hands of people who just want to use it to make money, you end up with stations that broadcast to the lowest common denominator. Whether it’s talk designed to infuriate listeners and increase ratings rather than inform and educate, or music stations that play the same tired small list of songs out of fear that they might play something different and interesting that would cause listeners to tune out, the end result is mass mediocrity.

Thanks to Randy K7AGE, for letting me know that just after I bought my internet radio from Best Buy, that they lowered the price for a few days to $79.99.  I called them up and was able to have the price difference refunded to me.

Is internet radio real radio? In my opinion, the answer to that is yes.

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