Dave Richards AA7EE

April 3, 2015

An Early Morning Spin On 49M With The Sproutie

This morning, my 2 eldest kitties did a real number on me. The senior was the first. At about 5am, she sat on her food shelf (one of 3 shelves mounted on the wall next to my bed, specifically for the cats to hang out on), next to her empty food bowl and began meowing loudly, while fixing me with an innocent gaze. I was able to ignore this for a good 20 minutes until the next eldest, my blind cat Jingles, jumped up on the bed and also began a “feed me” campaign, which consisted of vigorously rubbing her little furry head against my face. The combined effect of both initiatives was too much to easily ignore so as soon as I had fed them, I found myself sitting in front of The Sproutie and thinking that I might as well make use of the fact that I was up at 5:30am, while night-time and grey-line propagation on 49M would be in full swing.

The choice of 49M for this listening session was simply because it was the coil that was plugged in. I listen to Radio Habana Cuba most nights on 6165 and 6100KHz. The 6165KHz signal, which comes online at 6pm local (0100z) has been rather weak recently, but the signal on 6100KHz from 10pm-midnite (0500-0700z) is a powerhouse. I sometimes record the 6100KHz signal but am quite often foiled in my attempts to catch the penultimate hour of programming, due to RHC’s various foibles. Last night, the carrier appeared on 6100KHz at 4 mins after the hour, followed a further 5 mins later by the audio. My plans to record the 1-hour program in English were thus foiled and by the time it was repeated at 11pm local, I was feeling too sleepy to last the whole hour.

When going to bed, I usually leave The Sproutie on 6100KHz so that I can awake to the sounds of KCBS Pyongyang on the same frequency. It is mainly music, with occasional spoken word in Korean. I hear many of the same tunes during their morning programming, and there is great theater of the mind in hearing their slightly kitschy melodies interspersed with the impassioned-sounding commentary in Korean. I hear the same melodies most mornings, and there is a certain appeal to this somewhat exotic “sameness”. I can imagine the members of the elite in Pyongyang waking up to this kind of “inspirational” programming every morning.

Coffee at the ready, I decided to perform a band scan on 49M with The Sproutie. The idea was to log every station I could hear on the band. The excellent site short-wave.info made it possible to quickly ID most stations, before moving on to the next. I didn’t linger for too long on any one frequency, as the goal was to get an overall idea of band activity, rather than to positively ID every single station heard.

Needless to say, I heard a lot of Chinese :-)  Here’s what The Sproutie and I came up with –

 Freq  Station  Language  UTC
 5830  WTWW  English  1342
 5875  BBC  English  1343
 5915  CRI  Mongolian  1347
 5925  CNR 5  Chinese
 5935  PBS_Xizang  Chinese
 5955  CRI  English  1354
 5975  CNR 8  Korean  1356
 5990  PBS_Qinghai  Tibetan  1358
 6015  North_Korean_Jamming with un-ID’ed station underneath  1401
 6030  CNR 1  Chinese  1404
 6055  Radio Nikkei  Japanese  1405
 6065  CNR 2  Chinese  1406
 6080  CNR 1  Music  1414
 6095  KBS World Radio  English  1415
 6100  KCBS Pyongyang  Korean  1417
 6105  Radio Taiwan International (jammed, but jamming not heard)  Chinese  1418
 6110  PBS Xizang  Tibetan  1420
 6125  CNR 1  Chinese  1422
 6135  North Korean Jamming (w/ music underneath)  1424
 6155  CNR 2  Chinese  1427
 6175  CNR 1  Chinese  1429
 6185  Unidentified station (possibly China Huayi BC. Corp  Music  1431
 6190  PBS Xinjiang  Mongolian
 6195  BBC (jammed, but jamming not heard)  English  1434
 6200  PBS Xizang (or Voice Of Jinling)  Chinese  1436
 6250  North Korean Jamming  1438
 6280  Xi Wang Zhi Sheng (just 100 watts!)  Chinese  1440
 6348  North_Korean_Jamming_with_station_underneath_(presumably_Echo_Of_Hope)  1447

Lots of stations – and loud too, For the majority of the listening session, I had the RF gain on the little Sproutie cranked down to 1/2 or 1 on a 1-10 scale. Another benefit of this band-scan was that I got to fill in a few more calibration points on the dial calibration graph for this coil. The details on this screen grab are a little hard to read but that’s fine, as your calibration graph would be different anyway. Just take a gander at that nice smooth curve though –

Anyway, that’s it. It is now about 9:30am and I am beginning to wish I hadn’t risen so early. However, I blame the cats, and the good side is that I got to take a whirl on 49M before first light. Don’t let anyone tell you that you can’t use a regenerative receiver for serious SWL’ing. If anyone says that their regen doesn’t cut it for SWL’ing, just tell them that it must be because they didn’t build it properly :-)

The Sproutie and a cuppa coffee kept me company early this morning throughout my sojourn on 49M.

February 17, 2015

Extra Coils For The Sproutie Regen, With Coverage Up To 30MHz

Filed under: Amateur Radio,Broadcast Radio,Ham Radio,QRP — AA7EE @ 3:43 am
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Since completing The Sproutie Regen in it’s basic form in August of last year, I have been spending lots of time listening to it, and also some time winding coils for extra bands. On top of that, I wanted to add some extra thoughts and observations on building it and instead of creating a new blog-post every time I wound a new coil, or found something new (such as the fact that it seems to work quite well all the way up to 30MHz), I have chosen to add to the original blog-post. I have also been editing the post a little while adding new material, the point being to improve it and make it as informative as possible to anyone thinking of building it. Also, if I added a new blog-post for every new piece of information, then potential builders would find it harder to access all the info. This way, if you want to learn about, and maybe build The Sproutie, reading the one original post will always bring you up to date.

Since finishing The Sproutie, I have added coils for 2063 – 2670KHz, which covers the 120M BC band, 49M, 31M, 25M, 19M, 16M BC/17M ham, and an experimental coil that covers ~24-29MHz. The coil box is now full, and a picture of it has been added to the original post. I have also updated the coil tables to include full details. The experimental coil for 24-29MHz was wound out of interest, to see if this receiver would work passably at the higher HF frequencies and indeed, it does seem to. I copied SSB on 24M, as well as SSB  and CW on 10M, and local CB’ers on 27MHz. (EDIT on 2/18/2015 – this afternoon, I copied 10M beacons from K5AB in Texas on 28280KHz and WA2DVU in NJ on 28257KHz) I added no padders or series caps to taylor the coverage to a specific band. I didn’t even add a link winding – plenty of RF was being coupled into the circuit from pin 7 of the octal base without it, though a finalized coil would probably include a small loop to couple as much RF as possible, withut overloading the detector or stopping the oscillator.

Blog posts aren’t much fun without photos so, although I just added this photo to the original post, here’s what my coil box looks like now that it’s full up with coils.  If I wanted to add coils for specific ham bands, as well as the remaining BC bands above 16M (and the 22M band), I daresay I could fill another coil box!

The cigar box full of Sproutie coils. The unmarked one sitting above the 25M coil is experimental, and covers approx 24-29MHz.

If you’re getting a bit fed-up with me talking about this little receiver a full 6 months after I finished building it, take it as evidence that it’s a good ‘un. 6 months after building it, I still think that it was a very worthwhile project, and continue to derive much enjoyment from it.  The sounds of Radio Habana, Cuba fill my apartment on many evenings, and on the same frequency (6100KHz), music from KCBS Pyongyang in the mornings, as well as Radio Australia are my main morning staples. I’ve heard Spanish numbers stations, air-traffic control, coastal stations, military communications, and all sorts of weird and wonderful bleeps and bloops that you expect to hear in the shortwave bands. Before I end this post, allow me to say just one more thing that I have said many, many times before. If you are going to build a regen, make sure you pay careful attention to the physical construction. There, I said it. I promise I won’t mention it again :-)


February 9, 2015

Servicing A National PW-D Micrometer Dial and PW Gear Drive

A couple of weeks ago, I posted about some reduction drives I had acquired, including a classic National micrometer dial and PW gear drive, in nice condition. The PW drive has an output shaft that runs parallel to the front panel. Conversely, the drive in which the output shaft effectively runs straight through is called the NPW. I believe that PW stands for “Precision Worm” (EDIT – probably not – see Dave W9BRD’s comments underneath). The PW drive is the original design, and simpler than its counterpart, having only 2 gears – a worm gear and a main split gear, split for the purpose of counteracting backlash. This particular unit came with a quality variable capacitor attached to it which my capacitance meter measured as 245pF, but which I think was a nominal 225pF unit, based on a look at the 1947 National Radio Products Catalog. The dial and drive turned smoothly and worked well. On closer inspection, one of the ceramic insulators in the variable capacitor was cracked and while I was hoping this wouldn’t adversely affect it’s operation, the crack went all the way through and did indeed affect the physical stability of the unit. Nevertheless, the dial and drive were in great condition and were worth it for the price I paid. I will keep an eye open for a stator from a similar unit to replace this one with the cracked insulator. In the meantime, even though the dial and drive were operating smoothly, I couldn’t help wondering if they could be coaxed into operating even more smoothly. I had never owned one of these before, so didn’t know exactly what to expect from it. It was smooth, except for a slight jerkiness when attempting to make very small adjustments to the tuning with the dial starting from a stationary position. It’s picky, I know, but I was looking for an excuse to teach myself how to take it apart, lubricate, and reassemble it. Here’s the condition I received it in – On removing the 4 screws that secured the top of the drive to the main body, the top came off very easily, to reveal a main split gear coated in old yellow/orange grease. Unfortunately, I didn’t take a picture before removing the grease, but although there must have been a thin film on the pieces of metal that came into contact with each other (it did, after all, still operate smoothly), much of the grease had been pushed to the side of the gear and was sitting there doing nothing, having hardened somewhat over the years. The gentleman who sold it to me gave me the advice, “If it ain’t broke, don’t fix it!” but I wanted to clean and re-lubricate it, so I could be sure it was good to go for a few more decades. The other reason was that I wanted to understand how it worked – and not in a “book” way, by looking at diagrams, but by actually taking it apart and seeing how all the parts fit, and work, together. Here’s a rather rough sketch of the PW drive. Hopefully, even my poor drawing will give you an idea of how it works –   There is a great deal of information on servicing these gear boxes, and the National micrometer dial, on this page on the Western Historic Radio Museum website. Look a little over halfway down the page, under the heading “Lubrication and Assembly Of The PW Gear Drive” and underneath that, the section titled “PW-D Micrometer Dial”. This whole page is packed with very useful information on National HRO Receivers, and their restoration. Henry WA7YBS runs the Western Historic Radio Museum website. He ran the “bricks and mortar” Western Historic Radio Museum in Virginia City, NV from 1994 to 2012 and although the museum is no longer, the website is a very valuable source of information about vintage radios. It is a treasure trove. Henry kindly gave me permission to use National’s cut-away photo of this gear drive that I found on his website. It’s a lot more helpful than my drawing –

The National PW Gearbox, before about 1945, when the housing was made of cast metal. Photo courtesy of Charles Hentsch and Henry WA7YBS at http://www.radioblvd.com

I won’t go into a lot of written detail on how to disassemble, lubricate, and reassemble the drive and micrometer dial, as I learned it all from the National HRO page on Henry’s site. I would simply be repeating what is written there. With a gearbox and dial that is already operating fairly smoothly, it is possible to simply work a little extra grease into the parts to add to what is already there. I could have easily done that but once I get started with these things, I like to take them to completion.  I began by spraying WD-40 on the main gear and using a toothbrush to remove the old grease, but I ended up completely removing the worm gear, washers, spring and ball race from the housing, and removing every last trace of the old grease with a combination of an old toothbrush, WD-40 and lots of soapy water. Might as well start afresh. Here’s the partially disassembled gear drive, cleaned of all the old grease. The worm gear is no longer engaged with the main split gear. If it were, the 2 sets of gears in this photo would be much more closely aligned. I have also removed the variable capacitor from the insulated output shaft – I can be quite thick about some things. I knew what backlash was, and also knew that (obviously) an anti-backlash gear was some kind of arrangement that virtually eliminated it. However, I didn’t understand how it worked until I opened this gear box up and saw how it all fitted together for myself. This is why I recommend that you take things apart, play with them, and look at them. That way, you’ll understand how they work and will be able to fix them when they go wrong – Here’s the gear housing, elliptic bearing hub, and worm gear with the washers, spring and ball race assembled on it. Oh, and the end plate. I don’t know what it’s called, so I’m going to call it an end plate – A close-up of the worm gear assembly – The elliptic bearing hub is not actually elliptic. It is just that the hole through which the shaft of the worm gear passes, is off-center to the hub, so that the inner number dial on the micrometer dial is driven in an eccentric fashion. You can see that here – In this shot you can see the conical thrust bearing that is part of the casting of the gearbox in pre-1945 models – A couple more gratuitous shots. I am very taken with the types of mechanics and engineering you find in old radios – and this is in such good condition. It’s hard to believe that it’s around 70 years old. In these next 2 shots, you can see one of the springs that tensions the 2 sides of the split gear, and gives it the anti-backlash action – Henry indicates on his site that he uses Lubriplate 130-A as his main light grease. I was having trouble finding anyone who would sell me a single 14oz container (the smallest available). They all wanted to sell me a 6-pack, whether it was a bricks and mortar supplier, or an online one. Various outlets promised to be back in stock of the individual containers in a few weeks, but I wanted my grease sooner than that. A bit of online research indicated that Mobil 1 synthetic grease was a good (and newer) replacement and was in stock at a local auto parts store. Score! I have since found several restorers of vintage radios who use this grease, a fact which gave me extra confidence. A small pipette/syringe of the type used to administer medicines to cats and dogs, as well as a toothpick, and the judicious use of fingers, proved useful in applying it – Grease was applied (sparingly) to every point where there was metal on metal moving contact.  It was applied to the conical thrust bearing, the worm gear, the inside of the elliptic bearing hub, and the outside of the elliptic hub, where the micrometer dial would make contact. With a toothpick and a great deal of care, I also managed to get grease inside the ball race which of course, is very important. After applying it to the main gear, I put the washers, spring and ball race back on the worm gear, and reassembled the unit, setting the anti-backlash gears to about 1 1/2 teeth of offset, rotated the main gear through several revolutions, and cleaned the excess grease from the sides of the main gear. Once that grease has been pushed off to the side – it’s never coming back. Here’s what the main gear looked like after the excess was cleaned off – Next, the micrometer dial was disassembled and fully cleaned, with the usual generous amounts of WD-40 for degreasing, and lots of soapy water – I really enjoy taking photos of these beautiful pieces of engineering.  I hope it shows how taken I am with them. You’ll notice that the 2 springs that hold the (inner) number dial to the (outer) index dial have a slightly longer “loop” at one end than the other.  The springs fit best when the sides with the longer loop end are connected to the (inner) number dial. You’ll find that out by doing it. The 3 machine screws are for fixing the knob to the index dial. Close-up showing the springs that hold the index dial to the number dial – Sparingly grease the inside of the number dial only, where the toothed edge is. There is no need to grease the index dial. When reassembling, you want to line up the 2 dials so that the machine screws are aligned with the corresponding holes in the number dial, so that you can then screw the knob back on. When in this position, the number 250 should be displaying in the top number slot. You should be able to position the number dial so that 250 is showing. You can rock it back and forth and feel it gently click into place. The number 250 needs to be perfectly centered in the top window before proceeding. If it’s not centered or doesn’t look right in any way, try again. I actually had to remove and reattach the springs before I could get the  number dial to gently but firmly click into place and show the number 250 centrally in the top window. This is what the dial will look like from the back. Notice how the 3 machine screws are aligned with the holes. This only happens when the dial is indicating half-scale (250) – Now, rotate the tuning shaft on the gear box so that the main split gear is set halfway. There are end stops on the main gearbox that make contact with 2 lugs cast into the lid to prevent the main gear from turning more than 180 degrees. In the following photo (which also appeared earlier in this post), you can see the 2 end stops on the left-hand side of the main gear wheel. This is the correct position for when the dial indicates half-scale (250) – For comparison, this picture of an un-greased gearbox shows the gear set to one extreme of travel. Notice one of the end stops at the top of the gear wheel (on the right of the gear) – and notice the 2 “lugs” on the underside of the lid that make contact with the end stops – Slip the micrometer dial onto the shaft and the elliptic hub. It should slip on fairly easily. If it doesn’t, something is wrong. Do not proceed until the micrometer dial easily slips onto the elliptic hub. I fastened the dial onto the shaft with the set screw(s) in the knob, turned it back and forth a number of times and then, because I am particular about these things, disassembled it again and removed all excess grease with my finger. This is what it looked like before reassembly – The fully serviced dial and gearbox operate very smoothly, and should continue doing so for many years. Even better, I now know how to do this. I can pick up more dials and gear drives like this in the future, confident in the knowledge that if they are not operating properly, I can get them to work again. They are very elegantly engineered, with a small number of parts working together to make a very smooth tuning mechanism. They really don’t make ’em like they used to. Oh – and another quick plug for Radio Blvd.  It really is a great site for fans of vintage radios and their restoration. If you enjoy, and get use out of the site, I encourage you to make a small donation to help keep it online. There are links on the site.

The cleaned and lubricated National micrometer dial and PW gear drive, waiting for a replacement variable capacitor unit (spot the errant cat hair on the front of the tuning knob!)

January 27, 2015

Aaron N9SKN’s Sproutie Regen Receiver

I was quite excited when Aaron N9SKN told me he was building a Sproutie. Sometime earlier, I had discovered his website, billed as the “Home Of The 500mA Sidetone Oscillator/Shack Heater” I was searching for information on building HF receivers, and came across Aaron’s build of W7ZOI and K5IRK’s Progressive Receiver, as described in the Nov 1981 issue of QST, and the ARRL handbook for several years. Look at the pictures of the boards that Aaron made for the project in this article. Good stuff!

Aaron’s Sproutie build must have been a breeze for him in comparison. What a beaut. I’m wondering what material he made the front panel from. Is that garolite or something similar, or is it foam-core board? He did a great job of cutting the oval hole for the speaker. I like the “oxblood” color of the phenolic tube bases too – the same color as the Doc Marten’s I wore as a young man :-)


“The Sproutie” as built by Aaron N9SKN. Photo courtesy of N9SKN

Aaron already had a Cardwell 2 x 35pF tuning capacitor in his “junk” box. It was still unused, which is why I put quote marks around the word “junk”. I am often amazed at the number of these lovely old vintage parts that have yet to be used in a project – it’s a great thing for us home-brewers. You can see it in this rear view of the receiver. Note the aluminum plate he installed behind the front panel to minimize hand capacitance effects. I’m looking at those leads from the tube base and the variable capacitors to the circuit board, and wondering if he gets any microphony effects –

Rear view of N9SKN’s “Sproutie”. Photo courtesy of N9SKN

A view of the underside, showing that oval speaker –

The underside of N9SKN’s “Sproutie”. Photo courtesy of N9SKN

Aaron is a hardier soul than me, as he made his own Manhattan pads. Here’s a view of his RF board –

The RF board in N9SKN’s “Sproutie”. Photo courtesy of N9SKN

Nice job Aaron. He posted 5 videos of his Sproutie in action too. Here’s one of them, showing his Sproutie in action on the 25M band. You can find the other videos of his Sproutie on his YouTube channel –

One of the enjoyable things about sharing pictures and descriptions of my activities in this blog, is hearing from builders like N9SKN. It’s great to know there are other people out there building things. I am occasionally tempted to spend my money on a commercially-made HF receiver instead, but I wouldn’t have anywhere near as much fun. Thanks very much for sharing details of your Sproutie build Aaron!

PS – Allow me to rave about Aaron one more time. He scratch-built a K8IQY 2N2/20 Manhattan-style. Major respect!

August 21, 2014

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

NOTE – Many thanks to Aaron N9SKN and Cliff WA9YXG, who pointed out errors in the schematics. They have been corrected, and N9SKN has built a working Sproutie from the schematics in this post so rest assured that if you follow them, you can too.

If you’re thinking about building this great little general coverage regen, I’d urge you not to print out any of this article to work from. The reason is that whenever I make an improvement or addition to this receiver, I edit this article. By working from a printout, you’ll be missing out on any changes I subsequently make. Having said that, The Sproutie works fine as is, so don’t be scared off from building it.

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 on the low operating 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 enthralled 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. The +ve supply line was connected directly to pin 7 of the AD820AN, but this could also be bypassed if necessary. A series 100 ohm resistor with a 47uF or 100uF bypassing to ground should work nicely. You can also sprinkle a few large decoupling electrolytics in the range of 100 – 470uF at various points on the 12V bus directly to ground. A small issue I experienced 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. A 47 ohm resistor placed at the hot end of the 10K AF gain pot provided the necessary isolation (this is shown in the schematic), and I was left with a volume control that operated smoothly, and a variable low-pass filter pot that also operated smoothly. I also added a 0.1uF coupling capacitor on the input (pin 2) of the LM380. The 0.1 and 220uF bypass capacitors on the 12V line were placed so as to bypass the 12V supply directly at the point of entry into the chassis. They were soldered directly on the back of the DC power connector and grounded with a solder tag bolted to the chassis. The 1N4001 diode was also placed at the same point.

Here’s the AF board as I first built it, before changing the 5K resistor on the input to a 2.2K resistor – 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 parallel 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! EDIT – I just did construct a coil for the 16M band (Feb 2015)

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, including the temporary “experimental” coil for 24-29MHz. I didn’t get as far as installing a link winding for this coil, but the main coil was picking up plenty of signal from the proximity of pin 7 of the tube socket to the coil. I have been very pleasantly surprised at how sensitive and stable the set is at these higher frequencies. Soon after winding it, I copied SSB on the 12M and 10M amateur bands, as well as plenty of over-modulated and very loud local signals on 27MHz :-)  Unless you also use the same values of tuning and fine-tuning variable capacitors, and closely copy my layout, your values will be different, but here is the info on my coil set so far. After a little while spent looking at it, it should make sense. Once you get used to figuring out how to wind a coil for a specific set of frequencies, it’s fun.  I have 15 coils so far, with ideas for a few more. I have already filled up my cigar box coil box, and am getting ready to make a second coil box and wind a few more coils. One of the really enjoyable things about a regen with plug-in coils is making coils for new bands. Fun!

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. It is particularly easy to do this on the higher frequency bands. If this occurs, remove a turn or two from the link winding. In operation, 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.

The 15855 – 17850khZ coil stops about 50KHz short of the top of the 16M band, which is nominally 17480 – 17900KHz. However, all these coverage figures are quoted with the fine tuning control set to maximum capacitance. With the fine tuning control, I can tune all the way up to 17900KHz with that coil plugged in.

With the first set of coils I wound for specific bands, I was using significant values of padding capacitance across L1 in order to reduce the frequency swing caused by adjustment of the main tuning capacitor. I noticed after a while that the specific band coils weren’t giving such good sensitivity as the general coverage coils. I have since discovered that it is best to avoid large values of parallel fixed capacitance, as this reduces the Q of the tank circuit at resonance. Adding a capacitor of a few pF to tweak the coverage is fine, but large values (of the order of 50 or 100pF) will reduce performance. If you want to reduce the frequency swing to cover a narrow band, best to achieve it with the use of a capacitor in series with the main tuning capacitor instead. The Q of the tank at resonance seems to be maximized by using as much inductance and as little capacitance as possible. This is more noticeable on the higher frequency bands.

The table for specific band coils is a work in progress. I will add to it as I wind more coils –

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, and were mounted vertically and secured with a couple of dollops of hot glue from a glue gun. In the following picture, the 3050 – 3950KHz 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 –

The coils for the higher frequency bands need less in the way of a link winding, such as 1/2 a turn, which is simply a piece of wire passing through the toroid, but not even being wound around it. For the 16M/17M coil, I found that a 1/2 turn from pin 1 to pin 7 wouldn’t allow the circuit to oscillate, so I used a simple u-shaped loop of wire between pins 1 and 7 placed near the toroid, as in the photo below. The link winding is the green wire. My attempt at using a 1/2 turn link winding for this coil involved a wire from pin 7 through the toroid to pin 1, and this stopped oscillation. However, it’s possible that a 1/2 turn from pin 7 directly through the toroid to pin 4, which is also at ground potential. might allow oscillation while coupling more signal into the detector (it’s a shorter run of wire). I didn’t try it though, opting instead to go for a loop outside of the toroid. Experimentation is definitely key here, and it’s one of the things I had in mind when building The Sproutie. Once you’ve built the receiver, you can still have plenty of fun designing coils for many different bands and amounts of coverage. Here’s that 16M/17M coil, showing the green link winding –

The coil for the 16M BC and 17M ham bands. This one covers 17400 – 18200KHz.

When designing coils for The Sproutie, here are a few things to bear in mind –

Adding padding capacitor(s) across the coil will bring the overall frequency down, and limit the range of frequencies that the main tuning capacitor will cover (as the tuning capacitor is now just part of the overall capacitance across the coil). However, if you place too much capacitance across the coil, the circuit will not oscillate. When making estimates and performing calculations, remember to include the capacitance introduced by the circuit, and stray capacitances. Another strategy for limiting the range of frequencies the tuning capacitor covers is to put a capacitor in series with it (the tuning capacitor).  If I haven’t already mentioned it, the online calculators on W8DIZ’s site are great for figuring out resonant frequencies for tuned circuits involving toroids. The calculator for the T50-7 is here, and the menu to the left of the page has links for the pages for each of the other toroid cores. Each page also tells you what range of frequencies that particular material is good for. However, even after you think you’ve figured out what values you need for the inductor and capacitors, whether you’re going to use padders, series caps etc, you’ll most likely still have to do some tweaking of values until you get the coverage for each coil that you want, based on observation and experimentation. Once you’ve got the exact values you want, make sure to hot-glue the toroid to the tube base. If you don’t do that, you’ll experience instability and microphony. It’s amazing what difference a couple of dollops of hot glue will make!

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 shaft couplers came from different sources. The one on the left, on the main tuning control, is a Jackson Bros part, purchased from Mainline Electronics in the UK through eBay. The coupler on the right was made from all aluminum by John Farnsworth KW2N. He has a small business making these and can also make custom sizes, if you have a non-standard shaft you want to use. For instance he just made a 3/16″ to 1/4″ coupler for my next project. John sells on eBay, but you can also contact him directly through his fledgling website (not yet finished) here. I really like his all-aluminum couplers –

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. I bought several from Gary at Play Things Of Past and used the nicest one. 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 concerned 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.

Note – the National N dials have 3 small rubber/fiber bumpers installed on the mounting plate to prevent the front metal “flange” (the part with the engraved dial markings) scraping on it when the dial is turned.  If your dial is in good condition, none of the main parts are bent, and everything is running “true”, you can remove these bumpers. I did, and the result was a dial that rotated very smoothly. With the bumpers in place, there is a very slight scraping sound as the dial is turned. If you do this, make sure to save the bumpers in case you later wish to re-install them.

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 Sproutie does work on SSB and CW, but SSB reception is trickier due to the need to control the signal input level (with the RF attenuation/gain pot) in order to prevent overloading of the regen detector and pulling of the oscillator, and to adjust the level of regeneration in order to inject the right amount of carrier. If the signal level to the regen stage is too high, the oscillator will pull, resulting in the signal sounding “wobbly” due to FM’ing of the oscillator. This happens very easily, even with moderately strong signals. My preferred method of operating the set when listening to SSB stations is to run the AF gain at, or close to, maximum volume, and to keep the RF “gain” low. Sometimes, I need to keep the RF gain twisted almost to zero in order to achieve a nice stable demodulated signal. When adjusted properly, SSB sounds good on The Sproutie, but it takes a fair bit more work than with a superhet fitted with a product detector. Hams and shortwave listeners who have used older superhets that used BFO injection into a receiver with a diode detector will be familiar with the technique of keeping the AF gain up high, and using the RF gain to control the signal-to-carrier injection ratio. In this case, we are also using the RF gain to prevent the detector from being overloaded and pulling the oscillator. Sound tricky? If you’ve never done it before, it can take time to get used to, but after a while, it becomes almost second nature.

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 very, 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 thicker, and custom front panel with Front Panel Express, and making it a little wider than the current one to allow for a pair of instrument handles to be mounted

-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 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 am 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 realized I didn’t explain that Sproutie is the nickname I gave my 2 1/2 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.

Sprat The QRP Cat aka Sprout aka Sproutie, after whom this receiver is named.

Videos of The Sproutie in action are here.

Thoughts on using The Sproutie to receive SSB are here.

Sproutie is ever-curious, just like her regen namesake is always seeking out signals.

I continue to update and re-write this post, as I make additions and improvements to The Sproutie. I would rather incorporate them here, rather than into subsequent, and separate posts. That way, if you are thinking of making your own version of this receiver, you can get all the latest updated information by reading this one (rather long!) post, instead of checking my entire blog for updates. As of February 2015, I have filled the entire cigar box with coils, with one to spare (that one is plugged into the receiver). The latest coil I made was for 17400-18200KHz to cover both the 16M BC band and the 17M ham band. 16M is just about the highest frequency shortwave BC band that is in regular use so I now have all the SWBC bands covered – most of them with coils specific for the individual bands. Here’s what my coil box looks like now. The unmarked coil sitting at the top of the box is the experimental one for 24-29MHz –

This cigar box cost $5 from a local tobacconist and makes an excellent coil box. The unmarked ceramic tube base sitting on top is the experimental coil for 24-29MHz. 4 adhesive vinyl bumpers stuck to the bottom help to protect it from rough surfaces.

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.

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