Projects – Page 2 – Skywired.net

An enclosure for the R2/T2 transceiver

After months of organizing parts, I have finally gotten back to the R2/T2 transceiver project. Don’t get me wrong, the cleaning and sorting is not done, but I felt the urge to do something a bit more… constructive.

While cleaning, I found a box of old electronics junk that had promising cases. Electronics enclosures are expensive. Salvage can be a good way to keep the cost down. I don’t know what this thing once was, but there are military-style circular connectors on the front and back, two fuse holders, a power inlet, and no visible controls.

Opening it up, I found this:

There’s a lot of empty space in there! It looks like it was some kind of power supply. Next to the weighty transformer and big blue filter cap, a circuit board carried 7805 and 7806 regulators, several current-sense resistors and an LM324 quad op amp. It also had a power transistor on board and connected to the big TO-3 transistor on the heat sink in the back.

The board on the other side had a couple of high-voltage film capacitors, some ten-turn pots with their positions set with nail polish, two LM324’s and one RCA 4151 voltage-to-frequency/frequency-to-voltage converter. Down in the lower-right corner, it also held a solid-state relay. I’m a little more stumped about what this board was for. Maybe it was more power-supply logic, or maybe some kind of controller.

Tracing out the wiring harness revealed that 120VAC is run to the front connector, with only a fuse between the connector and the power cord. That could get exciting quickly to anyone not expecting it.

I pulled apart the whole thing, salvaging only the transformer, two ICs, and some fasteners. I tried to salvage all the ICs, but some were corroded into their sockets and could not be extracted without breaking pins. I have not had that happen before.

That’s the final product. I left the fuse holders, the heat sink, and a common ground point in place. They might be useful when this box becomes a transceiver. The front and back panel are 0.125″ aluminum and slide out after a few screws are removed. It will be easy to replace them with new panels for the radio.

The only fixed surfaces in this box on which to mount things are a pair of narrow rails on each of the side extrusions. The bottom is removeable and isn’t set up well to hold circuit boards. I will have to either add a false bottom or come up with a way to mounting the boards at right angles from the sides. Putting the boards flat on the sides, like the original residents of this box, won’t give me enough room, and because the boards are not sized to fit the walls, I would have to improvise some kind of mounting panel or angled standoffs to hold them anyway.

In any event, that problem is solvable. It’s a nice case for what I hope will be a nice radio.

KK7B R2 receiver: lifted pads, a scorched board, and it works anyway

I’m slowly making progress on my KK7B R2/T2 transceiver project. At my last report, I was waiting for replacement capacitors to arrive. They did, and I pulled out my ancient solder-removal iron, a Radio Shack unit from who knows how long ago.

Either my unsoldering skills are rusty or I was too impatient, or both. I managed to lift four pads, completely demolishing one. The other three were salvageable. I’d like the board to be perfect, but having it work is an acceptable substitute.

The new caps went in easily, with only a little fiddling to wire around the ruined pad. Better yet, the excessive bias current I saw before the replacement is gone! The board is supposed to be adjusted to 100 mA current. It used to start at 120 mA, with the bias adjustment set to its minimum, and drift its way up to more and more current from there (going as high as 200 mA before I’ve lost my nerve and switched it off). Now it starts at 84 mA and… drifts its way up from there to 130 mA and more.

OK, so one problem was solved. I can always increase current with the bias adjustment potentiometer.

After scratching my head a bit, I finally noticed one small sentence in KK7B’s articles on the R1 and R2. The audio output transistors need a heat sink, do they? Oops! I dug around a bit in my junk box but couldn’t find anything that would fit. The articles say the audio amp will drive a pair of headphones fine without the output transistors, so I decided to take them out.

I recently got a Sparkfun hot-air soldering station (a Sparkfun Free Day prize!) and thought I’d give it a shot. Sure, hot air is usually for surface-mount parts, but solder is solder, right? Not being sure how to set the airflow and temperature settings, I set both on the high side, put some flux on the output transistors’ pads, and went for it. The result wasn’t pretty:

Oops... A scorched R2 PCB

Yes, I scorched the board. Oops. Between this and the lifted pads, I think I need to work on my unsoldering skills.

Since the transistors are 50 cent parts (TIP29/TIP30), I unsoldered them the easy way: I cut the leads, removed the leads from the board with my conventional iron, then cleaned out the holes with my solder sucker. I didn’t damage any pads this time! (The picture above was taken after all of these steps.)

A little more soldering to hook up a BNC and some other goodies, and the board was alive!

R2 on the bench, surrounded by test equipment

That’s a Tek 191 signal generator as the VFO (variable frequency oscillator), with a frequency counter as the readout. That’s an MFJ QRP antenna tuner in the foreground.

I didn’t build a phasing network yet, so I drove only one VFO input. That causes the R2 to function like a conventional direct-conversion receiver,receiving both sidebands simultaneously. That said, it works. I was able to hear signals, though frankly many were quite hard to copy. I’m not sure what else might be wrong.

The frequency counter spits out a lot of digital noise. I learned to flip it on only to spot-check my frequency. It’s a lot quieter in standby mode.

Did I mention that IT WORKS?

Despite the success, I’ve been struck by a bit of paralysis in moving forward. There are so many choices to make for integrating the radio.

What kind of VFO should I use? Should I design my own or buy a kit?
Which modes should I include?
How much power output do I want?
What power amp should I use? Should I design my own or buy a kit?
Which case should all of this go in?
What microphone, and microphone connector, should I plan for?

Keep in mind that this is supposed to me my fast route to getting on the air, so I’m thinking kits for both the VFO and amp. Besides, with as busy as it has been at work lately, it is nice to sit back and just build something.

It’s not a pretty project any more, but it works! Hooray!

R2 receiver update: Time for new electrolytic capacitors

Crunch time at work has been limiting my basement tinkering, but I recently found time to work on my R2 receiver a bit more. It is pulling excessive bias current, which had me scratching my head. The audio power amplifier bias is supposed to be adjusted so that the whole board pulls 100 mA, but it’s taking 120 mA even with the bias pot set to its minimum. After double-checking all of the component values and verifying all of the bias voltages on the board, I was left scratching my head. Then I remembered the age of the kit. Even while building it, I had doubts about the electrolytic capacitors…

This R2 kit includes 15 Panasonic Z-series electrolytic capacitors. Aluminum electrolytic caps are good at one thing: Lots of capacitance at a low cost. In nearly every regard, they have inferior performance, with high equivalent series resistance (ESR), inductance (ESL), and, yes, a short lifetime. I’m not sure exactly how to translate the lifetime specifications for electrolytics to room temperature storage, but the rule of thumb tends to be that they can tolerate 5 years on the shelf, after which they require a “reforming” procedure. Reforming them at that point can get them to last another 5 years or so. After that, figure that they are shot. Old electrolytic capacitors can have excessive leakage current. In extreme cases, this current is enough to heat them up excessively and they go bang! The capacitors on this board are old enough to vote. Maybe one or more of them are to blame for the extra milliamps.

I had a little trouble finding the perfect capacitors, but then I was looking for performance at least as good as the original. This was perhaps foolish, because I couldn’t find data on the Z-series caps. Panasonic discontinued them in 2000. In lieu of further information, I arbitrarily went for high-quality models from a favorite manufacturer. In some cases, these were nearly double the price of the cheapest options, a breathtaking 23 cents versus 12 cents, quantity one. Seriously, replacements for all of the caps cost all of $2.85, including a few spares for good measure.

My one mistake so far is to forget about the T2 transmitter. If the R2’s capacitors are bad, the T2’s surely are as well. Maybe the spares I bought will end up there.

Troubleshooting is half the fun of this stuff. Who needs sudoku when you can puzzle out a misbehaving circuit?

Building a KK7B R2 phasing receiver

Hi, I’m back! It was a rough week, with a death in the family. As it ended, I found some time for some “solder therapy”. There is something good for the soul in putting together electronics. I don’t know if it’s the scent of the rosin or the satisfaction of seeing a project come together. All I know is it’s good for me.

KK7B R2 receiver, top side

The target of my soldering was my KK7B R2 receiver kit. This is the companion to the T2 I wrote about a few weeks ago. To my surprise, the R2 was easier to put together despite the higher part count. The components are larger, forcing a less dense pin matrix, and the board is separated into sections: mixers and diplexers, audio phase shift, low-pass filter, and audio amplifier. Conveniently, the board uses jumper wires (not yet installed on mine) to connect the sections. The intent appears to have been to make it easier to extend the board by swapping in a different phase shift network or adding alternate filters for CW or contesting, but it will make the board easier to debug as well, since I can bring it up a section at a time.

Keep in mind that this is a vintage kit that is no longer available, but updated versions of the design are still produced by Kanga US.

The board surprised me with its heft. I work mostly with surface-mount digital boards where the fiberglass PCB is the heaviest component. This board, though, weighs 134 g (4.7 oz), which is a lot for 100 cm²(16 in²). The shielded inductors are the culprits. Each black cylinder in the photo is the ferrite shield of an inductor. Each inductor is noticeably heavy for its size, and this board has 10 of them.

KK7B R2 receiver board, bottom side

Once I wash the flux off (it’s not as much fun as soldering), the next piece of the puzzle is to build or buy a VFO. Rick, KK7B, designed a companion VFO for these boards, but it is out of production at the moment. I’m thinking about one of the Si570 VFOs out there. This little chip offers a very low phase noise synthesizer with tuning in steps of a fraction of a Hertz. Having this on a single chip was a pipe dream when the R2 and T2 were designed.

After that, I’ll need to build a power amplifier to boost the T2’s milliwatts up to a reasonable level — at least a watt, maybe as much as 50 W. Finally, I will have to integrate all the pieces into a working rig. Professionally, I do a lot of microcontroller work, so it’s tempting to build a fancy digital control panel with circuitry to integrate every R2 and T2 option imaginable. Instead, I’m trying to keep myself to something simple: one band, SSB only, and a digital frequency readout as the only frill.

I still have to pick the initial band. I’m torn between 20m, my favorite for PSK31, and 40m, which I like for SSB. I’m after whatever DX I can land in either mode. What band would you recommend?

Making a coax feedthrough window for my shack

One problem every ham faces is how to get his signal from inside, where the radios are, to outside, where the antennas are. A while back, I prototyped replacing a screen in my basement windows with a feedthrough panel. The prototype used hobby polystyrene sheets, which were easy to work with but far too flexible for the job. As the weather cooled, I removed it and closed the window, for fear some mice or other critters would find it a convenient way into our nice, warm house. Without a feedthrough, I had no way to use my antenna, so it was time to build a permanent version.

The window I chose is a typical basement window made of glass block, with a small section that opens.

It didn’t take much work to remove the two screws that held the screen window.

My plan was to cut a piece of acrylic (plexiglass) to replace the screen, but a friend one-upped that by giving me a piece of polycarbonate. This stuff is tough. A thicker version is used in bullet-proof windows! It will work fine for the job.

I put the screen on the polycarbonate for size.

It’s perfect!

I marked the size I needed, along with the position of the screw holes, then used a scribing tool and a straightedge to scribe a deep line in the polycarbonate. The idea was to cut it like one cuts acrylic: by scribing it, then snapping it.

(This is what the plastic-scribing tool looks like. I found it in the window-repair section of my local hardware store.)

The polycarbonate was very hard to snap. I found out later that scribing and snapping is not the way to cut polycarbonate. In fact, it can shatter from this treatment. I should have cut it with a saw — I’m told table saws work particularly well.

In my ignorance, I scribed it deeper and deeper, trying to snap it over and over. Finally, it snapped. The material split a tiny bit along one edge (where the scriber had split and I had two parallel lines in the plastic), but in general the edge was clean and straight. I had scribed more than halfway through the sheet before it was ready to snap. Maybe I effectively sawed it after all…

The next step was to make a hole for a BNC bulkhead feedthrough. The tool for this job was a jeweler’s saw with a spiral saw blade. These nifty blades are round and can cut in any direction without turning.

As a template, I used an electrical-box faceplate that had two holes for BNC feedthroughs like this. It came from the trash when an old coax-based (“thinwire”) Ethernet installation was upgraded to twisted pair.

I drilled the hole with my little Dremel drill press. Some Googling of polycarbonate turned up recommendations against using a hand drill with it, because it can break bits if they aren’t held straight. I wish I had a real drill press, but this attachment for my Dremel tool seemed a good choice when I wanted to drill holes in homemade PCBs. It worked great for this job, too. A 1/8″ hole, the largest I had a collet for, was more than large enough to slip the saw blade through.

I also used the drill press to make two holes for the screws that will hold the panel in place.

As I sawed, I took care not to run the blade right against the template. It is stainless steel and would have dulled the blade much faster than the polycarbonate. In any event, I also had trouble turning one corner and needed to clean that up as well.

I set up the Dremel with my favorite burr bit, which looks like it needs to be replaced soon. It went through the polycarbonate like butter, quickly bringing the edges of the hole out to match the template.

I removed the template and tried the BNC bulkhead feedthrough for size. It fit perfectly! My past includes many a panel with misshapen holes that barely fit their intended connector, so this was surprising but welcome. I guess that’s what I get for never using a template before. The feedthrough I used was the former resident of the faceplate-turned-template.

Now, one thing to mention here… I took care to put the nut on the inside of the panel. I used Coax-Seal on the outside connection, to prevent water ingress, and that stuff is messy and a bother to remove and replace. By putting the nut on the inside, I can remove and replace the feedthrough window without disturbing the Coax-Seal. That might be handy when I cut a hole for another connection.

Finally, I put it all together. I pulled the protective covering off of the polycarbonate, installed the feedthrough for real, and screwed it in place of the screen window. It was a little dusty, so I cleaned it before I installed it. Now it is so clear that the coax looks like it floats in space.

The clearance between the internal, movable window and the feedthrough window is enough that I can close the window after disconnecting the internal coax. (I figured that out with the polystyrene prototype.) That’s good for energy efficiency. The panel fits tightly enough that I don’t feel a draft, and I could always add weather stripping, but it’s still only a single-paned window. The moveable window behind it is double-paned.

The picture above shows the Coax-Seal, too. My technique for that, which I learned from the hams at the Case Amateur Radio Club, is to first wrap the connection with electrical tape, with the outer end folded over a bit to make a pull tab. Cover the tape with Coax-Seal, with the Coax-Seal extending a bit past the tape on each end for water-tightness. When you want to remove the connection, you can cut almost all the way through the Coax-Seal with a sharp knife, split the rest open like peeling an orange, and remove it down to the electrical tape. Next, unwrap the tape started at the pull-tab. The tape will take the perpetually sticky Coax-Seal residue with it, leaving a clean pair of connectors ready for reuse.

Jay Eiger was a font of wisdom for this project and also gave me the polycarbonate. Thanks, Jay!

Building a KK7B T2 phasing transmitter

Are you the kind of tinkerer who has a few unbuilt kits sitting around? I mentioned last week that I want a faster way to get on the air than a designed-from-scratch SDR. If I stick to my current course, I may miss the peak of this solar cycle. It turns out that I have had an R2 receiver and a T2 transmitter kit on hand for… well, a very long time. If I recall, I bought them as soon as they were available in 1994.

These are very neat radios. Rick Campbell, KK7B, set a goal of pushing the state of the art in direct-conversion receivers. His series of high-performance receivers includes the R1, R2, R2pro, miniR2, microR2, and microR1. The R2 family are single-signal direct-conversion receivers, and the T2 and microT2 are companion transmitters.

KK7B T2 exciter, top side

The T2 uses the phasing technique to generate single sideband. In this technique, the audio signal is passed through a filter (or a pair of filters) that generate two signals 90 degrees out of phase. These are mixed with local oscillator (LO) signals that are also 90 degrees out of phase. When the mixer products are summed or subtracted, a miracle of trigonometry occurs, producing a clean single-sideband signal. As long as both sides of the system have closely-matched gains and the 90 degree phase shifts are very accurate, the opposite sideband is suppressed by 40 dB or more.

I decided to build the T2 first. Rick’s layout style is very dense, and I wanted to try the board with fewer components first. These are through-hole boards with the component holes falling on an 0.1″ grid in both directions. With only two exceptions, all of the resistors and diodes are installed “standing up”, with their leads 0.1″ apart and 0.1″ spacing to neighboring parts. The T2 board has no silk-screen, making construction that much more challenging. In any event, a fine-tipped iron and small-diameter solder (I chose 0.025″) are the tools of the day.

Rick suggests stuffing all of the parts before soldering any, as a hedge against putting them in the wrong holes. Then he recommends soldering and trimming the leads in rings from the outside in. I tried thaat, and it worked OK. For the R2, I will probably mount some of the trickier components individually, then switch to Rick’s method for the rest.

As you might expect, I had a few problems with solder bridges, but nothing a little solder wick couldn’t clean up.

Although there was no visible tarnish on this 17-year-old board, I did notice that the solder didn’t wet the pads as well as usual. Eventually I thought to start adding flux with the flux pen that I use for surface-mount work, and that did the trick.

Here is my handiwork. I’d like to solder as beautifully as Tom, the electronics tech I work with, but for basement tinkering this will suffice.

KK7B T2 phasing SSB transmitter, bottom side

The T2 needs one more once-over for bridges and cold joints, then I’ll apply power and see what happens!

Links:

Building the AK5388 ADC breakout board

“Honey, the package you’ve been waiting for from Hong Kong is by your computer,” said my dear wife shortly after I got home from work on Friday. Even better, a few minutes later she suggested that I spend the evening in the basement, building up one of my new boards. I have a wonderful wife!

The AK5388 ADC breakout boards finally arrived! I had a hard time waiting for them. First, Itead Studio didn’t ask me to correct the design until the day the finished boards were supposedly going to ship. (They did apologize for the delay — it sounded like there was a communications snafu between them and the fab.) Then I waited five more days for the board to be fabbed. Shipping from Hong Kong to Ohio took ten days. Looking around on the web, I’ve seen shipping times reported from seven to ten days, sometimes going up to as much as 20 days during the holiday season.

All eight boards look great. Other people who tried Itead reported some over-etching and silkscreen problems, but I don’t see any defects on mine. Since Itead now does 100% electrical testing, I have confidence that the boards will all work. I could spot the tiny dimple in each pad where the flying probes touched down, so it is clear that all eight boards were tested.

I went to the basement and heated up the soldering iron. The board went together easily. The 0.80 mm pitch of the AK5388 was downright easy to solder after the 0.50 mm A3PN250 FPGA and other fine-pitch parts I’ve been using at work. Besides, I’ve learned some new soldering techniques lately that helped me solder the AK5388 quickly, but I’ll have to share those in another post. I did use a meter to check all of my AK5388 solder joints, though. There were a few bridges, but they cleaned up without a problem.

The pads for the big electrolytic capacitors are larger than necessary. I used PCB’s default EIA7343 footprint. The pads had plenty of room for the soldering iron, but they could have been smaller without sacrificing ease of assembly. (Did I use the wrong footprint once again? 0805 capacitors seem to be the only ones I get along with…)

Lately Digi-Key has been taking much stronger steps to control moisture uptake by the semiconductors they sell. Instead of just shipping some cut tape in an anti-static baggie, they now seal the chips in an airtight bag with a packet of dessicant and a humidity indicator. I opened the bag this one was in about 4 weeks ago. Not bad so far, considering it was in my not-very-dry basement that whole time. Moisture uptake is important for reflow soldering techniques, but as far as I can tell, it is less significant for hand-soldering.

Now I’m left asking myself what comes next. In my original plan, the next step was to couple the ADC to the FPGA, put a USB core on the FPGA, and build a sound card. Once that works, adding a local oscillator and a quadrature mixer will make everything I need for a PC-based software-defined-radio (SDR) receiver, and this long trek will finally result in a radio.

However, I hear that the bands are great these days, and I’m not sure I want to take the time to homebrew an SDR rig just to get on the air. Maybe I should spend some time on a faster route to a radio, then come back to the SDR. I’ll probably have more on that idea next week.

Until then, keep on tinkering, and as always, your comments are welcome!