Dave Richards AA7EE

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.

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).

July 27, 2012

Using the NM0S Hi-Per-Mite Filter From 4SQRP To Make A Simple 40M DC CW RX

When 4SQRP brought out a version of the David Cripe NM0S-designed Hi-Per-Mite Filter, I noticed that it could be configured to give up to 50dB of gain. Like many others, I’m sure, my next thought was that it would make the process of building a simple direct conversion receiver even simpler. I decided that at some point, I needed to use this little filter board as the audio chain in a direct conversion receiver for CW.  I wanted to see for myself how it would sound.

In the meantime, my preoccupation with direct conversion receivers continued, as I embarked on building a G3WPO/G4JST DSB80. The receiver in that DSB rig uses an SBL1-8 DBM. That particular mixer package is out of production, but I used an ADE-1 and was pleasantly surprised by the receiver. I had some problems with the TX section, so put it aside and started building a ZL2BMI DSB transceiver.  I’d known about this little rig for a while from the pages of SPRAT, but a comment on Twitter from NT7S encouraged me to have a go at building it. The receiver is a simple NE602/LM386 combination with just a single-tuned filter on the antenna input. Of course, the receiver is not THAT simple, as each chip contains more complex circuitry, but the schematic at the chip level looks almost too simple to work.  As with the DSB80, I had problems with my ZL2BMI rig transmitting a sizeable residual carrier, but was surprised that the receiver sounded pretty good. The seed was sown…..

The ZL2BMI DSB rig was actually only the second time I had built an NE602/LM386 direct conversion receiver. The first was about 20 years ago when I bought a kit for the Sudden receiver on 20M. At the time, I wasn’t that keen on it and it soon ended up in pieces. I’m not exactly sure why I wasn’t taken by it, but I think it was a combination of factors. Firstly, I was living in an apartment in Hollywood with no outdoor antenna and was using just a short length of wire indoors as an antenna. It’s also highly possible that band conditions weren’t great, but I just remember not hearing many signals and also not liking the fact that a half-turn of the variable capacitor covered the entire 20M band. Because of this one not so great experience, I have since harbored a bias against using NE602′s in direct conversion receivers, an attitude that I now realize was unfair.

After achieving only partial success with both these rigs but enjoying the fact that the direct conversion receivers in both of them worked quite well, I wanted to build a simple receiver and use the Hi-Per-Mite kit that had been sitting patiently on my shelf for the last few weeks.

Look at the schematic for any NE602/LM386 DC receiver, and they are almost all the same, differing only in whether they use a crystal, ceramic resonator, or free-running VFO for frequency control, whether they have a single-tuned or double-tuned antenna input filter, or whether they use a differential or single-ended audio output.  The circuit is so simple that there is only so much room for variation, but there were a couple of things I had in mind for my mine.  While the receiver portion of the ZL2BMI rig only has a single-tuned filter on the antenna input, I wanted to do what G3RJV did with the Sudden, and place a double-tuned filter there. It’s an easy thing to do, and living in a built-up urban area, I have a lot more RF around me than ZL2BMI does when he’s using his rig in the bush, so a bit of extra bandpass filtering certainly couldn’t hurt. The other decision to be made was the method of frequency control. I was curious to see how the internal oscillator in the NE602 would work as a free-running VFO.  My apologies to you folk who have built umpteen simple NE602-based receivers and have-been-there-and-done-that.  I guess I need to get this out of my system! Another thing that I had seen in other circuits but hadn’t tried was the use of a 1N4001 diode as a varactor. Some call it the poor man’s varactor. Sounds like it was custom-made for me!

So that was my configuration – double-tuned input filter (just like the Sudden) and a free-running VFO tuned with a 1N4001 diode.  From what I’d read, I knew that I should be able to get enough capacitance swing from the 1N4001 to cover at least the bottom 30-50KHz of 40M. Here’s what I ended up with:

There’s nothing original in this schematic  but as you’ll see soon, it did turn out quite well, so I decided to name mine “The Rugster” after my cat, whose full name is Chloe-Rug, but who gets called several different variants of that name, of which “The Rugster” is one.  For the input bandpass filter, I used the vales of L and C that are used in the GQRP 40M Sudden Receiver. The pre-wound inductors in that design have a value of 5.3uH. I decided that I wanted to use T37-2 cores, so I used the calculator on W8DIZ’s site to figure out that I’d need about 36 turns on a T37-2 core to give 5.3uH of inductance. I fiddled around a bit with the values of the parallel fixed capacitors before settling on 47pF for those. I had originally thought that a higher value of around 68pF should work, but that didn’t put the peak of the filter somewhere in the mid-range of the adjustment of the trimcaps.  Some experimentation revealed that 47pF fixed caps achieved this.  Your value may vary depending on what value of trimcap you are using.  For the VFO inductor, I found a design for a 40M DC RX on the internet that was tuned by a 1N4001, noted the value of the inductance, decided that I wanted to use a T50-7 core (for stability) and once again, used Diz’ site to calculate the number of turns needed.

When building receivers, I like to start with the audio chain first. It’s quite affirming to be able to touch the input and hear that reassuring loud hum and mixture of AM broadcast stations in the speaker! So I got out the 4SQRP Hi-Per-Mite kit. Close inspection of the PCB revealed a small problem that was easily remedied. There was a very, very narrow copper trace connected to the ground plane that had been bent so that it was contacting the input pad.  A DVM test confirmed that the input pad was shorted to ground. I didn’t take this picture until I was part of the way through removing the offending trace, but you should be able to see the problem:

A minute or two with a sharp craft knife and that pesky little trace was history. Here’s the finished board:

The only thing that prevented me from putting this kit together even more quickly was my own compulsion to solder the components into the board so that they line up as straight as possible. The holes for some of the capacitors were a bit larger that they needed to be so that if you just plug them into the holes and solder, they’ll be a bit off-square. It’s not a big deal, and most builders won’t be bothered by it, but I just can’t control my OCD tendencies sometimes. Anywhere, I finally got there:

Although with modern low-flux solders, there is no need to remove the flux, it sure does make a board look nicer. I recently started using flux remover on my boards so that I can show off the underside too. At the suggestion of W2EAW, I apply it with a cheap plumber’s flux brush obtained (for 19c) from the local hardware store. Look at that nice clean board!

Trader Joe’s sells Green Tea Mints in a clear-top case that fits the Hi-Per-Mite board well if you’re looking to use it as a stand-alone device. There’s also room for power and in/out connectors (though you might need to cut off the top corners of the board in order to make room for the connectors.)  The tin in this shot had been knocking around my bench for a while, so the clear-top is a bit scratched up, but you can see the potential:

The next step was to build the direct conversion receiver with the NE602 and some of W1REX’s MePADS and MeSQUARES. The most frequency-sensitive part of the VFO circuit was built ugly-style, with the help of a 10M resistor for a stand-off, to help the stability. I don’t have any pictures of the board without the Hi-Per-Mite audio filter/amp added, so here’s the finished board. In the earlier shots, you can see the VFO toroid before I decided to slather it in hot glue in a bid to make it a bit more stable. There is only one component in the final receiver that I missed from the schematic, and that is a 1N4001 diode in series with the 12V supply for reverse-voltage protection.  I’ve blown too many fuses in my shack supply to not use these little fellows in the future :-)

Another shot from basically the same end of the board, showing the input bandpass filter on the left, VFO toroid on the left at the back and the DC input circuit nearest you on the right (reading from right to left, you see the reverse-polarity protection diode,  10uF input smoothing capacitor, 78L08 voltage regulator, and the 0.1uF DC output bypass capacitor. I’ve had those green trimcaps for a long time and it was time they were used. They don’t take solder well (I think I got them from Radio Shack about 10-15 years ago) but it seemed a shame not to use them in something:

Here you can see the varactor tuning circuit built ugly-style just to the left of the VFO toroid. The 3 connecting cables are for the AF gain pot (which is connected in place of R11 and R12 in the Hi-Per-Mite, as per the Hi-Per-Mite instructions), multi-turn tuning pot, and AF output jack. I’ve used lavalier mic cable that I bought from a local pro-audio store. It has 2 inner conductors shielded by a single outer braid and is very flexible, which makes it useful for wiring up small radios.  RF connections (which you’ll see in later photos) will be made with Belden 8216 coax:

At this point, I was very gratified to discover that the receiver actually worked, and seemed to work quite well too. That fact gave me enough inspiration to start building an enclosure to house it in.  The next shot isn’t ideal, as the back of the case is a little out of focus. You’ll notice that the front, and the two bolts sticking up on the right side are in focus, but the back of the case (and especially the bolt at the back left) isn’t. When I realized this, I was too far along installing the board to go back and take another shot and besides, sometimes I just have to control myself and keep forging ahead:

A view of the board installed in the case, with the lid:

A closer view from the top, in which you can see the VFO toroid now doused thoroughly in hot glue.  I’m not sure if it helped the long-term stability at all, though it did help to make the VFO more resistant to physical shock. Another way of securing the VFO toroid in a vertical position would have been to drill 2 small holes in the PCB and use a small nylon tie:

The coax that leads from the 1K log RF attenuator pot to the board is a little long, and that is because the one you see in the photos is a 10K linear pot. It was all I had at the time of building.  I am going to swap it out for a 1K log pot when the next order arrives from Mouser in a few days, and kept that lead a little long in case I need to chop some of the end off when changing the pot out.

Here’s another view of the innards. I am quite pleased with the way this little receiver turned out. You can see one of the 4 brass nuts that are used to secure the lid to the rest of the case. I use brass, as I can solder to it with regular solder. When placing the nut, and before soldering, I screw the 4-40 machine screw through the side of the case into it so I know the nut is in the right place.  If I were to get a little over-zealous when applying solder and accidentally get solder on the screw it wouldn’t stick, as the screw is made of stainless steel:

This is what The Rugster looks like in her case with the lid on:

If you look closely at these pictures, you’ll see that the PCB panels don’t line up perfectly and that the edges are just a little rough.  I am somewhat detail-oriented, but I am not a craftsman by any means, and I know my limits.  I made a conscious decision not to spend a lot of time filing and sanding edges. After making deep scores in the board and breaking it, I ran the edges across a file on the bench a few times and left it at that. Good enough is good enough in this case, and it’s certainly good enough for me.

A view of the underside:

I’ve spent a few evenings listening to it and most of the signals that my K2 could hear, this little receiver could hear almost as well. With the very weak signals the K2 won, of course.  A small part  of this could be due to the fact that the DC receiver, even with it’s 200Hz-wide filter, is still listening to double that bandwidth, as it is hearing on both sides of the local oscillator.  Naturally, I expected my K2 to be the better receiver (duh!) but was really surprised at what an enjoyable experience listening to this one is for such a simple circuit.  As expected, it does overload in the evenings, but all I have to do to get rid of the breakthrough is to adjust the RF attenuation pot back a little from “full gain”, and the breakthrough disappears.   Elecraft use an NE602 in the front end of the K1, and it has an attenuator for the same purpose. I’m not sure whether the breakthrough is from strong in-band amateur signals or from AM broadcasters – it’s actually hard to hear the content due to the narrow bandwidth of the Hi-Per-Mite. One small adjustment of the RF gain pot and it’s gone – then I just bump up the AF gain a bit to compensate. Volume is enough to drive my MFJ-281 ClearTone speaker.  I normally like to listen to CW with a 500Hz note, but because the response of the ClearTone drops off below 600Hz, I kept the Hi-Per-Mite filter at it’s design center-frequency of 700Hz, and the speaker sounds good. EDIT – If the AM breakthrough is from signals in the 550 – 1700KHz AM broadcast band, which I strongly suspect that it is, a simple high-pass filter with the cut-off set somewhere around 2 or 3MHz should work wonders to eliminate this.

The fact that the tuning rate is quite low helps a lot in the enjoyment of this receiver too.  The 10-turn pot covers 6999KHz – 7051KHz giving an average tuning rate of about 5KHz/rotation. The VFO shifts in frequency very little indeed when  the receiver is knocked or moved. Drift is not great, but manageable.  Drift in the first 10-15 minutes is only slightly worse than after that initial warm-up period,  so I’ll give you the results from switch-on, which were 90Hz drift in the first hour,  140Hz drift in the second hour, and 110Hz in the 3rd hour. All the drift was downward – there was no upward drift at all. The drift was steady, and with a little bit of compensation, I think those figures could be improved upon quite a bit. However, for casual listening (and this is all I am going to use this for) it is adequate.

This receiver is certainly not perfect, but considering it’s just an NE602 and an LM386 (oh – and a quad op-amp IC for the filtering), it’s not bad at all.

Thanks Dave NM0S and 4SQRP for this neat little audio CW filter kit.

Here are a couple of videos that I just made of The Rugster. Please excuse the poor video quality – it’s an old, cheap camera:

This one is a little shorter:

July 24, 2011

The WBR – A Simple High Performance Regen Receiver for 40M by N1BYT

A few weeks ago when a sizable order of parts arrived from Dan’s Small Parts And Kits, the plan was to use them (along with the parts from a few smaller orders from kitsandparts.com and Mouser) as the basis for some fun home-brew projects. I got my feet wet by building some small circuits Manhattan style – an audio filter and RF preamp for my VRX-1 DC receiver, and a crystal oscillator in an Altoids tin to check out the MeSQUARES I had just received from QRPMe. These are detailed in recent posts.

Like anyone who builds circuits, I have a mental list of things that I’d like to build which is updated constantly.  Some ideas get pushed to the back of the queue to make way for newer ones, and some stay pretty close to the front for long enough that eventually opportunity and desire collide and magically, it gets made. This is what happened with the WBR regen receiver.

As a teenager growing up in England, I had a one-tube shortwave regen that I built from a kit.  It was an HAC Model DX which used a mere handful of components and used a big rectangular 90V battery for the high tension supply.  It used plug-in Denco coils for band-changing (I think I remember having 3 of them).  I spent countless hours in my bedroom with the high-impedance headphones clamped around my ears, constantly tweaking the regeneration and tuning capacitors and listening mainly to shortwave broadcast stations.  I could figure out which band I was on, but had little idea of the actual frequency.  It really didn’t matter though, because the likes of Radio Australia, Radio Tirana Albania, Radio Moscow, Radio Nederland’s “Happy Station” shows, Radio Prague and many, many more kept me glued to that little set. It was just a small bent aluminum chassis with 3 variable capacitors,  a battery triode, a set of headphones, a coil, 3 fixed resistors and 2 fixed capacitors, but it was pure magic to me.

I think I spent the majority of my teenage years in my bedroom, listening to my record collection and radios. As an adult who recently semi-retired, it feels as if I’ve come full circle.  The chance to spend all the time I want building and listening to radios is an absolute gift, and the WBR Regenerative Receiver (Aug 2001 QST) that I’ve had the pleasure to use these last few days has brought the magic of radio listening back in a big way. It is sensitive, very stable, suffers from no microphonic effects at all, and thanks to the ingenious Wheatstone Bridge tank circuit has very minimal radiation of the local oscillator signal from the antenna port and so no common-mode hum problems.  Oh – and no hand capacitance effects either. I haven’t yet measured the drift, but after I’ve switched it on and let it warm up a little, I can set it to a frequency to listen to a net or QSO, and it stays there. Obviously, there must be some drift, which I hope to measure soon, but I’m not hearing any.  One more thing, not only is there little long-term drift, but my unit is very frequency-stable when subjected to physical shock too.  It’s a great little receiver and has pleasantly surprised me with it’s performance.

I won’t be publishing the schematic for the WBR receiver here, as it’s not mine to publish. It appeared in the Aug 2001 edition of QST, so if you’re an ARRL member, you should be able to download it from their site. The article was also reprinted in the ARRL book “More QRP Power”. There were a few errors in the original QST article and although the parts list has been corrected in the “More QRP Power” reprint, the schematic hasn’t. For the record, when you look at the schematic, R17 should be labelled R7, C22 should be 0.01uF,  C19 (the capacitor connected between R15 and pin 5 of U2) should be 0.01uF, and “the other C19″ – the capacitor connected between R16 and ground, should actually be labelled C20. If they’re going to reprint articles (which I’m very glad they do) I wish they’d make sure that all the corrections are included.

This was the first time I had ever attempted to fabricate an enclosure from PCB material and I’m quite pleased with the results.  I won’t fully detail my construction methods, as I got them from K7QO and WA4MNT, and they both already have excellent tutorials available online on how to make PCB cases. Ken WA4MNT’s PDF tutorial is here, and Chuck K7QO’s is here.

One or two people have asked me how I cut PCB material. If you don’t have a bench shear you can get nice clean cuts by scoring it with a utility blade and breaking it.  After using a sharp pencil to accurately mark the line where I want to cut, I use a metal rule and fresh sharp utility blade to make deep scores on both sides of the board. Expect to go through a few blades, and don’t be shy about replacing them;  cutting with a nice new blade is a pleasure, while an old one will just give you grief and raggedy cuts. Make sure that the cuts on each side are exactly opposite each other. Then sandwich the board in a vice between 2 bits of wood at the score mark thus:

Then, bearing down hard on the whole sandwich, firmly flex the board up and down until it breaks cleanly off.  You can run the board back and forth a few times over a piece of sandpaper, sandcloth etc. on a flat surface to smooth it a little, and you’re done. Easy! If you have a vice, you can use it for the above step.

Work progressed on cutting the panels for the enclosure (the 2 triangular pieces were used later to strengthen the cover) -

and before long I had a drilled front panel, a back panel, a base and 2 strengthening side-struts, which cleaned up with a scotch-brite pad and a little dish soap (for de-greasing). I made sure to rub the Scotch-Brite pad on the board mainly in straight lines as it does give the copper-clad a slight brushed look:

As suggested by WA4MNT in his tutorial, I used an angle iron to ensure 90 degree corners. The one I’m using is actually aluminum angle stock.  One thing that I would have had to learn the hard way were it not for this tutorial is the fact that when you solder the pieces of board together, as the solder cools, it shrinks, and you end up with something closer to 88 or 89 degree corners.  That can really mess up your nice square box, but Ken tells you how to square up your angles. Little by little the enclosure took shape and the thought occurred to me “Wow, I can actually do this!”:

I gave the finished enclosure a final cleaning with Tarn-X and sprayed it with a light coating of lacquer to prevent against oxidation of the copper. It didn’t fully work, as you can see from pictures of the completed receiver at the end of this post.  I’m thinking that I didn’t apply a thick enough coat of lacquer.

I thought it would be fun to fit the controls and stick-on feet to the enclosure to get a feel for what the finished receiver was going to look like:

I have a fondness for quality multi-turn wirewound pots, in the same way that I used to like using high quality air-spaced variable capacitors in my projects. Similarly,  the way that a regular one-turn pot feels when operating is important to me. The pot that I originally was going to use for the AF gain felt a little “scratchy” in the way it rotated, so I bought one from Radio Shack that feels silky smooth.  Even the first BNC connector that I used for the antenna socket (which was left over from an earlier project) was a cheap component, and I noticed that plugging and unplugging the antenna took more force and fiddling than it should have, so I used a new higher quality part (Mouser # 161-9323).  If this seems like a little too much attention to detail, let me explain. The connectors and controls are the way that you, the operator,  interface with the radio.  Higher quality components used here will greatly improve your experience of the radio. What would you rather use – a radio that takes force and fumbling to connect the antenna, has hard to adjust tuning due to the use of a cheaper one-turn pot, and a volume control that turns roughly, or a radio with controls and connectors that operate and rotate smoothly? These kind of things make a big difference. It might seem illogical to spend so much more on connectors and controls than on the rest of the circuit, but given how positively they can impact the user experience, it’s an investment worth making. I was hoping that this receiver would become a permanent part of my station, so spending a little more wasn’t a problem.

You’ll notice in the next picture that I used two 10-turn pots. One was for the tuning, and the other for regeneration. In the QST article, N1BYT details his method of using a one-turn pot with a separate preset to set the regeneration range on the main regen control. I’m sure that this works very well, but I happened to have an extra 10-turn pot available and was wanting an excuse to use as many of these wonderful things as possible, so I omitted the preset (R6 in the article) and used a 10-turn for the regen control. You do get a “whizzing” sound when adjusting the regen if you use a wirewound. To eliminate this, you can bypass between the wiper and ground for both audio and RF with a 10uF and 0.1uF connected in parallel. I’ve left mine as is for the time being, as I quite like the whizzing sound! (Edit: The novelty of the whizzing sound wore off swiftly, so I placed a 33uF cap between wiper and ground at the regen pot. It took care of things nicely.)

Daniel N1BYT’s original design for the WBR didn’t use a volume pot in front of the LM386, relying just on the 1K RF attenuation pot in the antenna lead. I decided that I wanted a little more audio gain in order to drive a loudspeaker, so I used the audio chain from N1BYT’s OCR II receiver, but connected a 10uF electrolytic between pins 1 and 8 of U2 to get the maximum gain of 200 out of the LM386 IC. Running the LM386 with maximum gain, and also fed with a preamp, I wanted to be able to control the gain of the AF stage as well as being able to attenuate the RF input, hence the extra pot. The toggle switch will be used to switch audio filtering at a later stage.

The enclosure build had gone so well that I started hoping that the performance of the receiver, when I built it, would be good enough to justify putting so much time and effort into the case. In retrospect, I probably should have built and tested the circuit first before deciding whether it was worthy of an enclosure.  Luckily it was.

I built the circuit Manhattan style using pads from W1REX at QRPMe.  I talked about the MeSQUARES in an earlier post and liked them so much that I ordered a sheet of MePADS from Rex.  MePADs are Manhattan pads for IC’s, as you can see in this picture. I had already broken off the pad I was going to use in this project:

As you’re building with these pads, if you find that you’ve glued one in the wrong place (I use Super Glue in the gel form), just slip a sharp utility blade underneath the pad and it comes right off. No need to fear a bad circuit layout, as you can change it if you make a wrong move.

The first part of the circuit to be built was the detector and regeneration circuit.  I didn’t have an MV104 dual tuning diode as specified in the original article, so used two back to back MV209′s, which you can see in the following picture, along with Q1 and Q2, the 2N3904 regeneration transistor and the MPF102 regenerative detector respectively:

Now I’ve added a few more components, including the 78L05 voltage regulator, which is hiding behind the grey 2.2uF electrolytic in the foreground:

The completed circuit. At this point, I hadn’t added the 10uF electrolytic between pins 1 and 8 of U2 (LM386).  The 2N3904 audio preamp is just below the LM386 audio IC:

Some more views of the completed circuit.  I knew that I was giving myself plenty of space to work with, but didn’t realize that there would be so much of it left over.  This is rather handy though, as it allows room for some low-pass audio filtering that I want to add later:

All very well and good, but would it work, and how well? The next picture shows the board installed in the case.  I added a 10K trimpot in place of R12 in order to adjust the tuning coverage of R11, the main tuning potentiometer. You can see the orange trim-pot near the front left-hand side of the board. It was only just now looking at this picture, that I realized I had left out C22, the 0.1uF capacitor in the antenna lead:

The same shot from a slightly higher angle:

By adjusting trimmer capacitor C8 and the trimmer resistor in place of R12, I achieved coverage of approximately 6970 – 7311 KHz. The extra 11KHz at the top of the band was important to me so that I can listen to the BBC World Service on 7310.  Vatican Radio broadcasts in English daily at 0250 utc on 7305, so the extra coverage above the top end of 40 allows me to receive that also.  Adjust C8 to set the top end of your coverage, and the trimmer in place of R12 to set the bottom end of the coverage. The tuning rate is a bit higher at the bottom end of the band than the top, being about 50KHz/turn in the CW sub-section.

I spent a few days enjoying the radio in this state before making a top cover for it.  The RF attenuation control doesn’t take the input signal completely down to zero. No doubt this has a lot to do with my decision to use an unshielded piece of copper wire to connect the antenna socket at the back of the rig with the 1K pot.  N1BYT does this in his receiver pictured in the QST article.  I liked it because it’s a reminder of the days when complete radios were wired using this technique, but I think I’ll probably replace that wire with a length of co-ax (after fitting C22 inline with the antenna lead) from the BNC to the 1K pot. That should allow attenuation of the input down to zero (or very close).  (Edit:  I just did, and it does.)

A top cover for the receiver:

The completed WBR Receiver (darn those front-panel fingerprints!) -

The space left at the top right-hand side of the front panel was in case I found a way to hook up a frequency counter to the regenerative detector while in oscillation in order to provide a frequency readout.  I have heard of people achieving this with the WBR by placing a pick-up coil near the main tank coil.  Still not sure whether I’ll pursue this.

Some of the things I’ve heard on this wonderful little radio in the first few days of owning it are the Vatican Radio transmitter in Sackville, New Brunswick on 7305 and Radio Australia (it was great hearing their Waltzing Matilda sign-on melody).  I also heard FO8RZ in French Polynesia on 7001 and immediately switched over to the Norcal 2N2/40 to work him with 4 watts! One of the reasons for building this receiver was in order to listen for AM activity on 40 (amateur, as we all know that there is plenty of broadcast activity there!) and I was happy this morning to hear W6LHQ running 200W of AM from his QTH in Modesto, CA on 7293KHz. This is one great thing about a regenerative detector – it will receive SSB and CW as well as AM. I believe it can receive FM too via the slope detection method, though I haven’t had a chance to try this out.

The Wheatstone bridge circuit in the tank is an elegant way to resolve the problem of oscillator radiation from the antenna that is common in simple regens, without resorting to adding a stage of RF preamplification, and it works very well.  I made no particular effort to match C5 and C6, so either I was lucky, or the circuit is forgiving, because I have encountered no issues with re-radiation. Without re-iterating the summary of this receiver’s performance that I gave in the second part of the 4th paragraph of this write-up I’ll just say this; I was hoping that this regen’s performance would be good enough for me to have it as a permanent part of my station, and it is.

The next step will be to replace the antenna input wire with a length of co-ax, and then build some audio filtering.  At that point, I’ll make some recordings of the audio and post a YouTube video.

I’m also thinking about bread-boarding another version of this receiver to encompass a larger portion of the shortwave spectrum. I have some MV108 varactors, and am thinking that the wider capacitance swing of one of those could give me broad coverage of a large part of the HF spectrum, with a fine tuning control provided by another diode. It would be great to have a regen to give me continuous  coverage of, say, 3 – 10MHz, or even higher.

Lots of fun and experimentation to come. Many thanks to N1BYT for this fine little receiver.

Notes added after above post was written - 

*Current consumption is low too – I measured around 12-13mA in regular use.

*A number of other builders have experienced problems with low sensitivity with their WBR’s.  LA3ZA found that substituting a 0.22uH inductor for Z1, the 1″ length of stiff wire between the center-tap of the coil and the ground plane, did the trick.  In the QRP-tech group on Yahoo, Steve AA7U did some experimenting and found 1uH was the optimum value for him.  Other builders have not had these issues with theirs.  My suggestion would be to build it as in the original design, adding the inductor if it seems necessary.

*As mentioned earlier, I added an audio pre-amp stage to my WBR, as suggested by N1BYT in the original article.  The schematic of the simple stage I added can be seen in this post.

*If I were building this again, I would change the configuration of the LM386 amp stage. Using a 10uF cap between pins 1 and 8 to get maximum gain introduces quite a lot of hiss.  The design that VK3YE used in the Micro 40 utilized a couple of ideas that had been discussed in SPRAT and is much lower noise. I would try using the 2N3904 pre-amp as detailed in this post with the LM386 circuit as used in the Micro 40.

July 1, 2011

First Forays Into Manhattan Construction – Crystal Oscillator and a Peaked Lowpass Filter

With the exception of NT7S’ VRX-1 receiver I’ve had no experience with Manhattan construction. Once the majority of my recent flurry of parts orders came in, I figured it was time to start building a few things Manhattan style, and to start with some simple circuits.

I found a good supplier of PCB material. His eBay username is abcfab. He has a lot of PCB listings – different sizes, thicknesses and materials, and based on the shipment I received from him, it’s good stuff. The cuts were clean and the cut corners were square:

Although I wanted to make my own Manhattan pads, I saw W1REX’s MeSQUARES on the QRPMe site and was curious to try them. They arrive as one sheet of 300 square pads. The material is scored so that you can easily break the pads off with a pair of pliers:

The material these pads are made from is fairly thin (about .03″) and I would normally be concerned about the possibility of shorts to ground with thin pads like this, but Rex has designed them in such a way as to minimize the possiblity of that happening – the square pad of tinned copper on top of the pad doesn’t extend to the very edge – there is a small margin around the tinned area.

The pads are easy to use.  I wanted to put something simple together to try them out, so I built a crystal oscillator in an Altoids tin:

If I were building a more complex circuit in a tin, I wouldn’t build it directly onto the base, as it can get tricky holding the soldering iron at a low enough angle to make the joints. It’s definitely not the way to go if you need a fairly decent component density. I like the MeSQUARES, but wish they were available in smaller sizes too for circuits requiring greater component density. A few days later, I ordered some MePADS which is the same thing, but for mounting IC’s. I’d show you those, but they haven’t arrived yet.

OK, onwards and forwards (or however the saying goes).  I built NT7S’ VRX-1 Direct Conversion Receiver for 40M a while back. It worked well and could hear plenty of stations, but I must admit there was a fair amount of hum.  This is a common problem with simple DC receivers and I’m not sure if it has something to do with my layout, or perhaps whether something I did with the wiring caused a ground loop. (Perhaps feeding the final audio amp with a balanced input wold help eliminate some of it?) After completing it, I put it aside and made a mental note to come back to it one day. Incidentally,  KE7GKM has been making lots of QSO’s with his homebrew station using a VRX-1 on the receive side, so it must be something to do with the way I’ve laid out or wired my circuit. The other thing about simple DC receivers like this that my inexperienced CW ears would benefit from, is some audio filtering.  It seemed like a good excuse to build the active peaked lowpass filter that was designed by W7ZOI and featured on VE7BPO’s site. I built it on a scrap of PCB from Dan’s Small Parts and Kits. While the pieces of board from abcfab on eBay are cut nice and square, the small pieces from Dan’s are not. This is not necessarily a problem if you’re using them to build circuits on, but will require a bit more work if you’re using them to build enclosures. Here’s the completed filter:

A week or two ago, I made a couple of hundred 3/16″  (about 5mm) pads and tinned them all. Now I’m finding that they are rather large, and I’ll most likely only be using them for connection points that have a lot of connections. The trimmer pot nearest you is mounted on these 3/16″ pads. They looked small when I made them, but once I started to use them, they were huge! As the MeSQUARES hadn’t turned up yet, I mounted the NE5532 chip on pads punched from a 3/32″ die (just under 2.5mm).  By bending out the chip leads and trimming them to different lengths, and staggering the pads, managed to do it without any of the pads touching. Pins 6 and 7 were connected together, so I soldered them to one bigger pad. You can get a slightly better view of the way I mounted the IC from this picture:

The two trimmer pots are for adjusting the resonant frequency and Q of the filter (if you take that adjustment too far it starts to ring).

Here’s the board installed on the back wall of my VRX-1:

It is certainly filtering the audio, but of course I still have the hum. Although the filter peaks gently at the frequency set by the trimmer, it is still basically a low-pass filter. I could use something to notch out the 50c/s hum. However, there is so much of it that I think what I really need is to figure out the way I’ve laid out and wired the receiver.

EDIT: I only get hum pickup when an antenna is connected. I just walked out onto my balcony, connected an antenna and the hum was much lower in level than before. I don’t know why I didn’t think of this before, but I live in an old house with substandard wiring and no electrical ground (just the 2 wires at every socket.) I wouldn’t mind betting that if I were to walk out onto the street, the hum would disappear almost completely.

The box that I put the VRX-1 in is one of a really useful line of enclosures from LMB Heeger. Look at this shot of the receiver all cased up:

See the way that 4 little lugs are formed from the top of the case? They stick down just a little and engage with the front and back panel, giving extra rigidity to the enclosure. The aluminum is 16 gauge (0.0508″). The larger sizes aren’t quite as rigid, but the smaller ones like the one above, are, and would be ideal for a VFO or VFO-controlled rig. The above case is 2″ high, the front panel is 4″ wide and the case is 4″ deep. It is LMB Heeger’s Model 143 in their interlocking series, available in crinkly black, painted grey (like the one you see above) and plain aluminum (for that homebrew look). I think Digi-Key carry them, but you can order them directly from LMB Heeger. I’m thinking that a modular station built in these little cases (plain aluminum finish) would look neat. VFO in one, freq counter in another, mixer in another etc etc. Hmmm……

That’s it for now.

The Rubric Theme Blog at WordPress.com.

Follow

Get every new post delivered to your Inbox.

Join 776 other followers