Juma RX-1 Direct Conversion Receiver

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Here’s my completed Juma RX-1 direct conversion DDS receiver. If you’re familiar with the unit, you can see I’ve pretty much built it as the designers,
Juha Niinikoski OH2NLT and Matti Hohtola OH7SV, intended.

I only made two changes: I painted the box and I added a CW filter.

The color is “safety blue” and it was a very intentional choice.

First, I like blue. Second, “safety blue” is a specific shade used in the United States for official markings. As such, the color is a standard and anyone wishing to sell paint for use as mandated by the US government must match that shade. I figured that would guarantee all my kits, of which this is the first, will all be the same color.

And finally, “safety blue” is light enough that opaque black lettering is easy to read.

There are 3-4 coats of blue paint on the box, each one lightly sanded with 2000 grit paper before the next.

You won’t find that sandpaper at Home Depot. Instead, look to the car refinishing shops; they use a lot of it. They can probably tell you where they buy theirs but, if you’re nice, maybe they’ll just give you a sheet or two. But if that fails, then I bought mine at a Sherwin-Williams auto refinishing supply store in Phoenix. Maybe you’ll have one in your town.

In case you don’t already know, getting a smooth coat with a spray can is quite a trick1. Lots of very thin coats are needed. But each coat needs to cover the surface and thoroughly “wet” what’s underneath. Here in Phoenix I find that if the work is too far from the spray can, the droplets will land as little balls of paint and stay that way, as little balls. I have to put on enough that adjacent balls touch and then flow together and that takes a heavier spray than I would otherwise prefer.

As in many arts, this can take some practice — and I could use some more.

Rear View

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The back panel is as the designer’s suggest except for my “Built by” legend to indulge my vanity.

The panel markings are decals, created on my PC and printed on special “water-slide decal paper”.

If you don’t have a good drawing program, I will recommend Draw in the Open Office product. It runs on Windows PCs, Macs and Linux boxes and is free. For the best results, turn on the alignment grid and “snap” options.

Because I was painting my boxes blue, I stayed with all black labels. (White lettering isn’t possible unless you have a printer with a white ink cartridge.)

For the decal printer paper, the particular brand I’m using is from Papilio, their inkjet variety. I experimented with both the laser-printed and inkjet-printed papers but found the blacks to be their blackest with the inkjet. Also, the “clear” areas with the laser-printed decals weren’t quite as clear so, while the inkjet decals need an additional sealing step before application, the results were better. What you see in these photographs are the inkjet decals.

Here’s a tip: Use your very best scissors when cutting out the decals. For best appearance, you will have to cut very close to the black. Dull scissors will separate the gel from its backing and, even after application, that raised edge won’t completely adhere. Top quality scissors, on the other hand, will let you trim each decal very closely and, after application, the edge of the decal will be almost invisible.

And here’s another tip: Put two or three copies of each decal to be cut-out on the decal page you are going to print. That way if one gets messed up, you’ll have another one available without having to print another page.

Emtech CW Filter Board in Place

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As mentioned, I added a CW filter.

This particular CW filter is a kit from Emtech, their model AF-1. It is cut into the circuit as you can see below just after the SSB (2.5kHz low pass) filter and switched in/out with the bat switch added to the front panel. Since I’ll be operating CW in the field with this unit, I deemed that a necessity. As you can see, the AF-1’s circuit board fits nicely in the empty space behind the front panel controls: it’s mounted on the single bolt you can see coming through the nut in the lower left-hand corner of the board. There’s a standoff between the Emtech board and the bottom of the box.

Deciding where to cut-in the CW filter was pretty straight-forward. The RX-1 already had an SSB (2.5 kHz) filter and I figured I could just go ahead and leave that one in the circuit to help the CW filter a little. And having used active filters before, I knew they tend to “ring” with too much signal so I wanted a low level signal as input. Placing the CW filter after the SSB filter just seemed like the obvious place.

And it works fine in that location.

One oddness which is probably attributable to the microphonic nature of Direct Conversion receivers, even though the CW filter is “miles” before the volume control, if a received CW signal is too loud and the filter rings, turning the volume down stops the ringing even though the CW filter doesn’t see what the volume control is doing.

Strange. But then again, so it much of life.

Moving on.

Main Receiver Board

CW Filter In/Out Cuts

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This project was my first effort with surface mount parts. Trust me when I say you’re gonna need a magnifying glass and a good one at that, and a whole lot of light. I was lucky to inherit my Dad’s old Circline lamp and with its big magnifier and bright bulb, I’m in heaven. You’ll also need high quality precision tweezers (Walgreens will *NOT* do), some 0.015″ diameter solder, liquid flux and a syringe by which to apply it and finally a soldering iron with a tiny tip. Those really are essentials. Trying with anything less will just be frustrating and wasteful.

The right tools make all the difference.

I’m 61 and assembling surface mount parts is entirely do-able if you have a reasonably steady hand and reasonably good eyesight. Neither has to be perfect. I wear progressives and had to work to move my fingers smoothly in very small motions but yes, it can be done. While it’s true that the parts are incredibly tiny — if you drop one on the carpet, stop immediately, check your pants cuffs (!) and socks before moving an inch, and then use a very bright flashlight while on your hands and knees to find it — and you may find your work time limited by tired eyes and loss of steadiness after several hours. But it’s possible and you’ll feel really good that you can “do this eentsy-teensy stuff” just like the young bucks.

[Imagine the sound of mature Stallion, huffing and chasing off the young bucks in a clatter of hoofs. Yeah, that’s how it feels. Life is good!]

I’m sure I spent a ridiculous number of hours with this kit, far more than anyone who’s done surface mounts before, and far more than my juniors. But then again, I had a great time figuring out whether I should tweeze with my left or right hand while soldering with the other. And I also had to figure out “wrist up” or “wrist down” for the tweezer hand for best steadiness. These little details took some trial and error but, once worked out, progress was sure and steady.

Be sure and go to YouTube.com and look for “Surface Mount”. There are several good videos on the techniques you’ll need to use.

Reverse Polirity Protection

Using Manhattan-Style “Island”

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There was one part in the kit not described in the drawings. Perhaps it was an afterthought. Regardless, it can be a life-saver so I included it — a series diode in the + supply to prevent hooking up the power backwards.

Wanting to attach that part somewhere steady, I super-glued a small Manhattan-style square island of single-sided PC board to the inside back and ran the diode from the input power jack to the island, and a piece of hookup wire thence to the power switch.

So, after many nights of working out techniques, reading stamped values on tiny resistors, positioning single transistors in place and getting everything soldered in place — and double and triple checking again and again — I was done.

Time for the smoke test and … [click!] … hmmm, nothing. No sound, no LCD, nothing.

Check the power.

Yes, There’s power on the inside of the power jack.

What about after the reverse polarity di… [Uh-oh!] … … Did I wire the jack backwards?

Yup, I sure did. Thank goodness (and Jubba and Matti) for that protective diode!

Ten minutes later, and after double-double-double checking what’s hooked where in the power wires through to the board, I was ready to flip the switch again.

[Click!] [Morse code and static] It works!! Hooray!!!

The speaker is super-glued to the inside of the lid with four tiny drops around the speaker edges. For the power and volume involved, this works fine — the speaker cone does not come in contact with the lid at any time.

The headphone jack wiring was fun to figure out because I wanted to be able to have three modes of listening: speaker only, headphones only, and both. Or rather I should call them speaker, computer input, and both because I sometimes connect the RX-1’s audio output to the computer’s microphone input and run Digipan (or fldigi) so I can receive digital mode transmissions. The “both” mode is useful in learning just what the different digital modes sound like while simultaneously viewing them in Digipan (or fldigi).

I used a stereo headphone jack for the monaural output.

The “hot” signal goes to both tip and ring contacts in the jack.

For speaker only output, nothing is plugged into the headphone jack so the contacts in the jack route the sound through to the speaker.

If I insert a miniature plug only part of the way in — just enough so that the tip of the plug contacts the ring connector inside the jack — then there is signal to both the speaker and down the tip of the plug to the computer.

And when the plug is fully inserted, it “breaks” the tip connection and shuts off the speaker.

In operation, I am tickled pink with the receiver’s performance. In my earlier foray into the world of QRP, I operated a Heathkit HW-8 and as many can tell you, they have awful receivers. The RX-1 is, in comparison, a real jewel. Yes, it’s still a Direct Conversion receiver and has the usual microphonic and hum issues but, for whatever reasons, both seem to be nicely minimized.

The unit works well on anything between 9 and 15 volts although the CW filter tends to ring to the noise more at the lower voltages but a careful setting of the volume slightly lower always cures that problem.

I opted for the separate receiver (RX-1) and transmitter (TX-1) because I want to play around with the software. I’m still putting the “tools” together for that and will probably buy a couple of the larger PIC chips so my programming can be a little more expansive. (For example, I want to change the built-in acknowledgement code to use the same pitch as the CW filter centers upon to make it easier to remeber what pitch I want to hear CW at for the CW filter, and use morse “1”, “2” and “3” for the VFO step acknowledgements rather than differently pitched tones.)

The TX-1 kit is sitting on my shelf, soon to be started.

In the meantime, however, I wanted to do more than listen. So I also bought one of the PSK-20 kits from Software Labs and put it together.

But that’s another story.


1 Warm the work and the can of paint before spraying. This will help the paint flow when it lands on the work. Also, coats must be thin (Ken, WA4MNT, instructed me) to prevent a “skin” from forming that prevents or severely slows drying of deeper areas.

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