Laser-Cut Faceplate for the QST Forty-9er

Thanks to the folks at the Maker Lab at the Harold Washington library in downtown Chicago, I’ve now got a nice clean faceplate for the QST Forty-9er with DDS VFO that I built a few months back. Here’s the final result up front:

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 The panel is just a piece of clear 1/16″ plexiglass that the Maker Lab had on hand, which conveniently is the right width to slot into the grooves in the existing case. The hardest part of the design was taking accurate measurements of the positions of the three holes in the plate, for the Forty-9er’s key and audio lines and the Arduino Nano’s USB port. In truth, I mis-measured the hieght of the USB port, but switching to a slightly higher standoff and adhering the K2ZIA board down with adhesive pads, everything lined up fine.

The process of laying out the design for the laser cutter is very straightforward. The desired shape is simply laid out in Inkscape, a free photoshop-like program. If you use two colors in the image, you can differentiate between the lines to be vector cut (i.e. cut all the way through the material, for an outline or a hole) and the areas to be raster-engraved (lightly etched for a frosted appearance). You can even do vector engraving, where a clean line is cut partway through the material. I set up the machine to vector-cut the outline and holes, raster the letters, and lighting vector-engrave around the letters for extra readability.


The laser cutting process has some trial and error in it; while the lab has some recommended settings, they’re more like guidelines than guaranteed routes to success. It took a couple tries to get the raster settings right to appropriately etch the plastic enough to be visible, without being too deep.


Thanks to the great staff and resources at the Maker Lab, I’ve now got a very presentable front on the rig, and for only a dollar!

Hear you on the air!

73

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Beach 40 Build – Part 4: Low Pass Filter

This post is part of a series: Building the VK3YE Beach 40 DSB Transceiver.

Next on the bench is the low-pass filter, which is mostly responsible for curtailing the harmonics generated by the VFO and mixer. Since limiting out-of-band signals and noise is also useful on receive, the LPF is connected directly to the antenna socket to be useful in both modes.

This LPF is particularly simple, using just three RF chokes and six capacitors. VK3YE’s original low pass filter used a couple components I didn’t have (namely the 820pF caps), so I played around a little in LTSpice to work out a viable filter with the components I had. Here’s what I ended up with:

Beach 40 LPF-06

SPICE simulations show that this filter maintains a similar low-pass characteristic to the original VK3YE design. Moreover, the insertion loss is approximately equal. The match to 50-ohms isn’t great, but is acceptable. And perhaps most importantly, attenuation at the second harmonic is strong at about 42dB:

LPF Spice Simulation.PNG

Constructing the LPF was simple – just six capacitors and three molded chokes that I picked up on a recent trip to California. I laid the filter out on a small piece of scrap copper clad. I enjoy when the layout of a board can highlight the symmetry of circuit itself.

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Before attaching the filter to the rest of the project modules, I did a quick manual sweep of it with my SI5351 board. It’s somehow very satisfying to roll the frequency up past the filter knee and see the higher frequencies just drop off. One word of caution – the SI5351 doesn’t have perfectly uniform output power across all frequencies, so I put one channel of the scope on the input of the filter and one on the output, to observe relative attenuation. Of course, terminating the filter is a must.

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Filter attenuation at 14 MHz, roughly the second harmonic of the desired transmit frequency.

Hear you on the air!

73

 

 

Beach 40 Build – Part 3: Audio Amplifier

This post is part of a series: Building the VK3YE Beach 40 DSB Transceiver.

Today I tackled the audio amp section of the Beach 40. And when I say tackled, I think perhaps the player was already down, and I just had to fall on top of him.

The audio amp configuration I’m currently using is the bog-standard, straight-from-the-datasheet LM386 20x or 200x configuration. A SPST switch puts the 10uF cap between pins 1 and 8 of the IC, or takes it out of the circuit.Currently, I don’t even have a volume control pot in place. Peter VK3YE later modified the LM386 configuration for greater gain and less hiss, which is something I’d like to tackle once I’ve gotten the rig working.

Beach 40 Audio Amp-05

Part of what made today so simple is that I’d already built this audio amp section for another project. I’d been using a bit of scrap copper-clad to experiment with coupling between an NE602 and an LM386, to see if passive filtering would be a useful option for future receivers without introducing too much loss. But since I haven’t touched that project in a couple months and this one’s exciting me now, that board hit the chopping block.

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The old receiver experimentation board. What a Frankenstein!

Extracting just the audio portion of the board was simple enough – now that I’m looking at the pictures and seeing the two big bypass capacitors on the red power line reminds me that I need to think about bypassing the power input at the source, probably with something beefy. I’ve had enough LM386’s turning into oscillators, and I’d rather not subject my ears to that too much if I can avoid it.

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With the VFO, mixer, and audio amp “completed,” I now have the bare bones of the receiver portion complete. Of course, there’s exactly zero filtering, either at the front or the audio portion, so it’s not like this would make a particularly reliable bit of kit. But it’s got enough parts that I can string them together and test.

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While I didn’t exactly succeed in keeping the leads short, using clip leads was just the fastest and simplest way to tie the modules together. RF from the ‘antenna’ (read: long bit of wire strung around the office) is brought into the mixer via the white clip lead at center. The VFO drives the mixer and converts the RF down to audio, where it’s amplified and sent out to the headphones.

To generate a test signal, I used my SI5351-board without an amplifier, with just a short bit of wire hooked directly to the output of the Si5351. I set the output somewhere around 7.150 MHz. Tuning the VFO across the band with the polyvaricon, I found my strong local source no problem. The receiver works!

I can see why Peter later added a fine tuning control to this project – with half a turn of the variable cap, I’m covering around 200Khz of the band, which makes tuning in to an individual station tricky. I do have a rather large panel knob in mind for the cap, which will help, but we’ll see if it’s enough. If I can’t get the VFO drift a little more under control, that fine tuning cap might be necessary to help stay on frequency.

Hear you on the air!

73

The Scout Regen

Last weekend, At the Hamfester’s Hamfest in Peotone, IL, I picked up a little Scout Regen kit from a fellow ham. The gentleman I bought it from was just lamenting to a friend “Nobody builds anything any more…,” but when I walked up to check out the un-built kit, he added, “….except this guy!”

It was unbuilt, but the metal frontpanel already had its decals applied (or perhaps pre-printed?), which was a nice place to start. I don’t have a great track record when it comes to applying decals and labels.

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The un-built Scout Regen kit, sitting atop the variable power supply I also found in Peotone. Also included: a ganged variable capacitor for future experiments.

The overall build took about 2 hours. The Scout Regen Builder’s Manual from the QRPKits website is very thorough, and was easy to follow. The only issue I ran into was a missing 5pF cap for the detector section. I substituted two 10 pF NP0 caps from my stock, soldering them in series before installing them in the PCB. Additionally, after completing the first part of the detector section (basically, everything installed but the coil and the power switch) the current drain had jumped to 20 mA. After finishing the build, the current dropped back to under 10 mA. How odd.

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The completed Regen receiver kit.

Listening with this kit is a lot of fun. Regen control is smooth, but very narrow; I may replace the regeneration control knob with a larger one. There’s all kinds of interesting things in the span of the receiver. Shortwave stations, the 80m and 40m hand bands, I found a couple different “WLO” beacons… really nifty. It’ll be nice to put on in the background while working on future projects.

Hear you on the Air!

73

T/R Switching

A quick post from the day’s experiments: the transmit/receive power switching arrangement for my next project. The scheme is very much like that in the KN-Q7; I wouldn’t have stumbled across this (very simple) setup without an excellent write-up by Andrew Woodfield, Zl2PD.

Here’s the simple schematic:

The power switching arrangement for a new project.

The circuit itself is straightforward – when the “key” pad is left floating, current can flow through the 2k2 resistor attached to the 3904, providing a small base current and driving the 3904 in conduction and powering anything connected to the +12R pad. At the same time, there is nowhere for base current to flow in the 3906, so no current is provided to the +12T pad.

When the key pad is grounded (by a morse key, or other TR switching method), the base of the 3904 is pulled to ground through the small signal diode, and very little current will flow through the 3904 and into the +12R section, effectively killing receive functions. At the same time, a small current will flow through the 2k2 resistor attached to the 3906, allowing it to conduct and powering anything attached to the +12T circuit.

In short: when the key line is floating/disconnected, the circuit is powering receiver functions. When it is grounded, the circuit powers transmitter functions.

I added the little 7808 regulator to this power board, which will be powering some NE602 mixers on receive only. I put both circuits together on a little piece of copper-clad, in something like Island-Pads-meets-manhattan-construction:

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I like how the layouts ends up showing off the inherent symmetry between PNP and NPN transistors. The whole thing looks quite nifty on the bench. I left the input and output pads deliberately large, to accommodate however many connections end up being made.

I did begin construction and testing on the next project, but was stymied by my SLA-battery power source running down (it was down to 7.4 out of 12V by the time I thought to put a meter on it). Rather than switch to the bench power supply (an old computer power supply with the 12V and 5V rails brought out), I took it as a sign to call it a night.

Hear you over the sizzle of solder!

74

DeKalb Hamfest 2016

I’ve been trying to think how long its been since I’ve been to a ham radio swapmeet. Certainly the last one I visited was the Electronics Flea Market at De Anza College with W6KWF sometime during my college days. So it’s been a good long while, and it seemed time to rectify that. And thus, I’m just back from the DeKalb Hamfest. First some details, and then I’ll get to today’s haul. A bit of a long post today, but there’s much to tell.

A brief overview – the DeKalb Hamfest is hosted by the KARC – the Kishwaukee Amateur Radio Club – and overseen by Bob W9ICU. It’s hosted at the Sandwich County Fairgrounds in Illinois around this time (early May) each year. $8 for admission, and easy parking.

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It wasn’t a huge event – maybe 10 or 12 indoor vendors and perhaps three times that many tailgate-ers. The indoor vendors, while they had lots of nice products, didn’t particularly entice me, as I think you could find just as good value on, say, banana-jack-to-BNC tails or audio connectors anywhere online. There was one vendor with a huge spread of every RF adapter and then some, as well as individual toroids for sale, including some big 100- and 240- size ones in unusual mixes. I’ll look for them at future fests.

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RF adapters and connectors for miles.

The tailgate-ers were more my speed, though none of the big-ticket items were really what I was looking for (or in my price range) Several old Heathkit receivers and transceivers for sale ($200-$600), lots of microphones, mobile units, CB radios. A few newer items, but mostly old tube equipment. At least four tables had the same Heathkit GD-1B Grid Dip Meter, which I found curious. Lots of vibroplex keys marked over $100.

But lots of treasures too. Maybe only three or four tables with proper parts bins, but I spent a good couple hours pouring through these to find some treasures. Most table owners were happy to make small talk and chat about their projects and where their goodies came from. I had a really laugh chatting with some of the nice folks running the tables, while piling up the day’s haul.


And what a haul!:

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Here’s the short list, starting at the bottom-left and working clockwise:

  • A pack of five unlabeled compression trimmer capacitors. These turned out to have a range of about 2-25pf.
  • A bag of ~10 small toggle switches with assorted mounting hardware. These range from SPDT to DP4T.
  • A large bag of ~20 rotary pots of assorted values, with a few rotary switches thrown in for luck.
  • A couple RG-58 jumpers with BNC connectors (free!)
  • A large air-spaced variable capacitor – unlabeled, turned out to be about 28-152pF. More on this below.
  • A nicely boxed step-attenuator with 50-ohm input/output impedance. I’d been thinking about building one of these, but running across one of them at the fest (for only $3) was a dream! It has a maximum of 59 dB of attenuation in steps of 20, 20, 10, 6, and 3 dB, as well as a variable attenuator (“3 dB?”) for fine tuning. It has Walter Schwartz’ name on the bottom, who was the nice gentleman who sold it to me. A very cool older ham.
  • A mysterious silver box… more on this below as well.
  • A bag of twenty 47pF NP0 capacitors. For future oscillators.
  • A couple bags of trimmer pots. One bag had five 500 Ohm trimmers, the other turned out to have a mix of 1K and 1M trimmers. I have these in mind as balance pots for diode mixers, but we’ll see where they end up.
  • A couple vacuum-molded packs of trimmer capacitors, 10-. These actually came from the Fry’s Electronics in Downer’s Grove – I couldn’t resist a visit on my way back to Chicago. And I got to help a dad pick out resistors for his ten-year old’s first electronic’s project. Neat!

All of the above for less than $20 total. Hamfest, for the win.


I’m particularly excited about the air variable capacitor. After a big of digging around, I pulled this one out of a cardboard box burried underneath a giant box of tubes:

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Since I didn’t have either a multimeter or an LCR meter with me, I had to make my best guess as to the voltage rating and capacitance of this unit. When I got it home, it turned out to have a capacitance that varies from about 28pF fully open to 152pF fully closed, with stops at either end. The plate spacing (measured with a caliper) is about 0.060 inches, which means it should start to arc-over at around 2400-2600V.

I have a specific project in mind for this guy: building a small transmitting loop antenna. A range of ~28-150pF should be enough to tune a 10′ circumference loop for the 20m, 17m, 15m, and 12m bands, as well as the 10m and 30m bands with a little fudging (this according to the 66 Pacific Loop Antenna Calculator). With some added capacitance in series or parallel and it could possibly be pushed to 10m, 40m, or 80m bands with reduced efficiency. The air-variable capacitor is key here, since it seems the voltages in a loop antenna can peak over a Kilovolt with just 10-20W of output power! In any case, more thoughts on that antenna as it comes together, but for now I’m drawing ideas from AA5TB, Nonstop Systems, KR1ST, W8JI, and VK3YE’s various projects.

In any case, one of the nice things about this particular air-variable is the reduction drive on it. You can see the worm-gear and larger reduction gear in the picture above – it takes 46 full revolutions of the small stud to shift the capacitor full fully open to fully closed. With a difference of 122pF between the two extremes of the capacitor, that’s an average of 2.7pF per revolution. Even using the bamboo-stick tuning method used by KR1ST (to avoid detuning the antenna by getting close to it), if I can manage rotational accuracy to about 10 degrees of the tuning stud, that’s a resolution of .07pF. Not bad!


One of the things I was on the hunt for this morning was a metal chasis or two to mount projects in. As nice as the Virgin Receiver turned out, I was hoping to find something a little more robust for future projects – maybe some old CD-drive casing or a gutted something or other. There wasn’t a whole lot of that sort of thing around, even in the big-piles-O’-parts, but then this silver beauty caught my eye.

I could see a couple of tubes inside, and from the ‘RCA’ and 1/4-inch jacks on the back (an no jack for a key or mic) I had a pretty good guess it was some kind of receiver, but other that, it seemed like a gamble. When I asked the owner of the table what it was, he very gamely said “I have no clue, but for $2, it’s yours!”

Well, at that price, it’s worth it just for the box! And what I found inside is definitely worth more than two bucks to me:

Judged by the two parts labelled “10.7 MCycle Interstage”, I feel pretty confident that this was indeed a receiver of some kind. More than half of the tube-sockets are empty, so I don’t think it’s worth trying to figure out what they were and bringing this thing back to life.

But check out the nifty components! Up front is a 4.5:1 vernier reduction drive, controlling a two-section air-variable capcitor for some kind of tuning. There’s a 30pF NP0 trimmer cap and a coil mounted directly to the air-variable, which is interesting. There’s a couple of 500K log-scale pots, a bunch of high-voltage capacitors, some old carbon-composition resistors, miniature RF chokes… this thing is going to be a great source of parts, as well as a keen chassis for a future project. Perhaps a regenerative receiver? I could see leaving that classy vernier drive capacitor in place as is and working around it…. hmmm….

Hear you on the air, and see you at the fest!

73

Wind, Antennas, and a First Portable Session

This week I had two radio firsts: trying to put together a halfway decent antenna, and a first portable operating session.

I started by putting together a center insulator a dipole antenna. There are many possible designs online, but I ended up making one with materials I’m familiar with. It’s made of a 1″ PVC cross, four PVC caps, three 1/4″ eye-bolts, some nylon lock-nuts, and panel-mount BNC connector. The whole went together in about half an hour. The PVC connections were surprisingly tight as a press-fit, but they and the eye-bolts are secured with lock-tite and sealed with hot-snot just in case. The BNC connector is soldered internally to a couple pieces of hookup wire, which you can see poking out the sides of the cross. The antenna wires themselves are simply tied to the eye-bolts and wire-nutted to the hookup wire.

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The dipole center-support. This picture was taken after the fact, hence the pre-cut Nylon cord wrapped around it. You can see the two copper hookup wires poking out either end.

The dipole center, a 200′ spool of nylon wire, some “stakes” for securing the antenna to the ground (really, 8″ pegboard hooks, on sale at Menards!), some zip-ties, and a couple of rice-filled socks as counterweight all fit handily into this small plastic case I had lying around:

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I live only about a mile from Lake Michigan and the beautiful lakefront park space that runs along the East side of Chicago. I figured with the sunny-but-chilly weather we’ve been having this week, the lakeshore on a weekday afternoon would be pleasant to work in and not too packed with people.

I couldn’t find a suitable table or bench close enough to trees to set up under, but then I spotted the convenient height of the sea-wall between the outer lakefront trail and the lake itself. I couldn’t have been any closer to the lake without getting my socks wet!

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The operating position. There’s a little ledge just on the other side of this wall that was perfect for standing on.

I strung up a portable, homemade 40m dipole (wires cut in advance) as high as I could could into one of the nearby trees – I only got the center maybe 20 feet in the air, but that’s still better than the 8′ the random wire in my home office is. The ends are tied off to stakes near the ground. This made the setup more of an inverted-V than a true-dipole.

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One of the antenna-wires staked off to the ground. You can see the black coax sloping from the tree to the operating position.

The antenna is fed from ~60′ of RG-58, running into a ZM-2 Tuner for matching, and then via a short RG-58 jumped into the Virgin Receiver. I also brought along the ardiuno-controlled, Si5351-VFO I’ve been working on to allow for the ability to change frequencies, as detailed in my last post. If I’m going to spend the time setting up practically in a lake, I thought it would be nice to actually scan the whole band. Since the VFO rig is still on a breadboard, I was quite spread-out over my little stony operating station.

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Visible here: the coil of coax coming from the antenna; the Virgin Receiver, open to allow the VFO to interconnect; the breadboard with the Si5351 VFO on it; two separate 12V SLA batteries; some nice comfy headphones; and the large red cable-coiler I used to store the dipole wires (a bit overkill).

So now, the moment of truth – I hooked up the batteries, connected the receiver and VFO, did a quick by-ear tune on the ZM-2. I’ve never done something like this before – would it actually be better than my office random-wire? Was it all just a jaunt in the park? In short, would it work?

LIKE HECK IT WORKED!

Especially up in the SSB portion of the band, there were some stations that might as well have been on the other end of a phone call. (This, by the way, confirms that the Virgin can receive SSB and AM as well as CW.) In no particular order, I heard:

  • N8KKR, KA9ZXN and others on the Hams for Christ net on 7.263 (330p Central, Monday-Sat). Not my personal bag, but they seemed a very pleasant bunch.
  • W9DCQ and W4LWW having a nice QSO, between the two of them and some other operators I couldn’t quite copy.
  • KG9O and KK4FZI having a friendly chat.
  • Some other snippets of stations.

The listed stations alone include QTHs in Evergreen Park IL, Columbus OH, Middleton WI, Franklin TN, Grassy Creek NC, and Marion IN. None of them terribly far away in radio terms (the Tennessee locations is farthest at ~500 miles), but a great confirmation that the antenna was clear and working. With a height above the ground of less than 1/4 wavelength, the ideal dipole should have a high takeoff angle and be fairly omnidirectional, and that seems plausible based on results.

In sadder news, I seem to have broken some part of the WSPR functionality of my VFO in consolidating that functionality from two separate programs into one. 1W into the antenna and several repetitions netted be exactly zero receptions, either into this antenna or my office random wire. So it’s back into the code I go. While I’m at it, it would really be worth packaging up the little VFO into its own enclosure – having all those wires flapping about is a bit worrisome for transport!

All in all, a tremendously successful, if very chilly, day by the lake. Now that I know what I’m doing, I suspect it would be able to have the station set up and listening in about 10 minutes, and about the same for tear down. With the days getting warmer all the time, I’m sure I’ll be out there again soon.

Hear you on the air!

73

Giving the Virgin Some Mobility

As wonderful as it was to ‘complete’ the Virgin Receiver and have a set of ears to start listening on, more and more I find myself wishing for the ability to change frequency and explore the 40m band a bit. To that end, I’ve been experimenting with hooking my Si5351-Based VFO directly to the Virgin as an easy way of giving it the ability to QSY.

(The VFO, as it happens, has a few more features nowadays, including being able to act as  WSPR beacon and having an auto-CQ mode. The code, as always, is on Github.)

Here’s the receiver as built:

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Jason NT7S dropped a hint in one of his older blog posts that the Ne602 likes to see about 300 mV p-p when being driven externally, and that a 10dB pad could be used to bring the Si5351 down to this level. To that end, I put together a quick Pi-attenuator consisting of one 120-Ohm and two 150-ohm resistors.

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A tiny 10dB pi-attenuator.

The attenuator plugs directly into the receiver where the crystal X1 usually sits, in a little 1×3 piece of female header. The output of the 10dB pad plugs into the side of the header that’s connected directly to pin 6 on the NE602; the ground on the attenuator plugs into the other side of the header, and is therefore connector to ground on the receiver.

The longer red and white wires you can see attached to the pad connect to ground and the CLK0 output of the Si5351. Here’s the current setup, spread out on the bench.

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It’s already quite successful! I can scan up and down the 40m band and pick out CW signals pretty well. SSB signals are faint and pretty un-hearable. Unfortunately, for some reason, my receiver has also turned into an AM radio:

I think some kind of high-pass filter is in order here. There’s always been a bit of AM bleedthrough with this receiver, but given that the AM stations are attenuated by the RF Pot on the receiver, it seems like most of this signal is coming in through the antenna and not, say, bleeding into the audio amp.

Hear you (all around the band) on the air!

73

 

First Received Code! (W1AW)

Today, for the very first time, I copied CW that was on the air! Though my morse is still very poor and slow, here’s what I pulled out:

CH?S_  … W1A?… ..HQ STATIONS INQR____SES EXCITEMENT… AND ADDING TO THE INTRIGUE IS THE … _OR. END OF 5 WPM TEXT. QST 1_ DEC…. AG….

What luck! I’d caught the end of a W1AW code bulletin! Pretty neat for the first code I’ve picked out on the air. Here’s how it happened:

I was noodling noodling around putting my Direct Conversion Receiver into a new enclosure (more on that later). The receiver is rock-bound to whatever crystal is sitting in a bit of female header next to the NE602 Mixer. I tend to leave the 7.030 Mhz crystal in there when I’m experimenting – it seems like big-signal stations seem to congregate down in the Extra portion of the CW band, and that extra oomph is helpful when I’m testing a new receiver with an improvised antenna.

After doing a preliminary fitting of the receiver in its enclosure, and a quick repair (yes, it helps to restore all the connections), was I ever surprised when I applied power and my earbuds leapt to life with clear, slow code! Most QRS (slow code) stations seem to be up in the 7.050 to 7.055 portion of the band. What the heck was this very slow, loud station doing so low in the band?

I grabbed a nearby post-it note and started copying, and pulled out the code above. I know I butchered a bunch of it, even at 5WPM, but it was very exciting to (a) hear un-anticipated CW come over the airwaves and (b) to be able to actually copy some of it!

For my own reference (and others’), here are the W1AW Slow-Code transmission times, both in UTC and CST:

Day CST UTC
Monday 4pm 000z
Tuesday 2pm, 8pm 2100z, 300z
Wednesday 4pm 000z
Thursday 2pm, 8pm 2100z, 300z
Friday 4pm 000z

Hear you on the air! But only if you transmit very, very slowly.

73

On the Importance of Fiddling and Good Power Supplies.

Troubleshooting continues apace on the new NE602-based direct-conversion receiver. As I mentioned in the previous post about it, the receiver develops and unfortunate, LOUD squeal whenever the 10K audio gain pot at sits between the NE602 and the LM386 is advanced past about the 20% position. This is a terrible impediment to reception, so I’ve been working on eliminating this problem.

TL;DR: The receiver is a whole object tied together by its power system. Good power and a good ground are important.

My suspicions centered on the LM386 chip – after all, an audio amplifier with a couple of feedback caps seems like a prime candidate to turn itself into an audio-frequency oscillator.  This, as it turned out, was a red herring – I’d like to publicly apologize to the LM386 for ever doubting it.

The first step I took was to meter the potentiometer’s resistance at the setting just below squealing, and to replace the pot with two resistors that replicated the resistance at this setting. A little fiddling showed that the squeal could still be induced by varying these resistances slightly, which was encouraging – the problem was somewhere else in the circuit, and not a phenomenon of the pot itself.

So I took an entirely different tack to connect the NE602 and the LM386. The 0.01µF cap in series with a 10K pot was derived from a number of other designs, but it seemed worthwhile to utilize the complementary outputs from the NE602 as a means of input to the LM386’s complementary inputs. I stole this linkage directly from the EMRFD. The NE602’s pin 5 is connected to the LM386’s pin 3 through a 220nF series cap with a 10kΩ resistor to ground. Similarly, the mixer’s pin 4 was connected to the audio amp’s pin 2. Both lines were tied together with a 100nF cap.

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You can see the three new blue caps that sit between the NE602 and the LM386, a well as the large white jumper bridging the 10uF cap to pin 8 of the LM386

Long story short – no help on the oscillation-front, but a little more stability in the gain, it seems?

Next, I set about fiddling with the feedback circuitry for the LM386. As the datasheet shows, there are gain configurations from 20x to 200x gain with a simple RC network between pins 1 and 8. My original configuration had a 10µF electrolytic cap in between these two pins. I tried replacing that with a 4.7µF cap plus a 2kΩ resistor, leaving those pins totally unconnected… in all configurations, it was possible to get the audio to oscillate. So no such luck there. I put the 10µF cap back in, and joined it to pin 8 with a small piece of female header, so that I could insert various resistors in that position to adjust the gain later, if need be.

Oddly, both reception gain and AM bleedthrough is increased whenever I touch the leads of the gain resistor. The AM bleedthrough sounds a lot like what happens when you touch one of the un-connected leads of the LM386, but why do I hear CW signals a lot clearer as well?

It was this oddity with the gain resistor that gave me the vital clue. “Well,” I thought, “maybe touching and fiddling with the other components will give me some clues.” I just played around on the board for 20 minutes with it powered on, tying this point to ground, putting that point high through a resistor… interesting things happened, but squealing was still very much a possibility. That is, until I happened to accidentally pull the oscillator crystal out of its holder, and the squealing stopped.

“Now that’s odd!” I thought. “I assumed this problem existed entirely in the audio half of this receiver. Why should removing the crystal, and thereby halting oscillation in the mixer, have any effect?”

A little poking around with a meter and a scope and I had my answer – the little (used!) 9V batteries I was using as a power supply for this receiver were woefully under-powered. Under load (i.e. with audio coming out of the headphones), the voltmeter read about 7.3V, and dropped by about 10 mV per second. Oscillation seems to occur when the voltage  drops too low. (I’m measuring voltage as a proxy for available power, in this case, I think.)

So, after borrowing a nice big 13.8V, 8aH SLA battery from work and bodging together a quick-connect to battery-clip connector… wait for it…

No more motorboating! (And a bunch more audio output to boot.)

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Used 9V batteries just won’t cut it! The big SLA battery provided great power and more audio output.

I still need to boast the overall gain of the system, since it takes a pretty strong signal to get into the receiver, but at least it’s not likely to HOWL in my ears!

Next thoughts: to increase gain, perhaps stealing the alternate LM386 gain configuration from AA7EE’s WBR Recevier, or one of the broadband, ~18dB IF amplifier designs from the original BITX or QRPKit’s variant.