Significant Other Update: Logic Board

For a few months, I’ve been playing working on a design for a QRP accessory as a way of becoming familiar with both the arduino platform and homebrewing technique. The basic idea was to put everything except a transceiver in one box, so I couldn’t leave anything behind when operating in the field. I wrote up a design overview when I started, and it is more or less up to date. The schematic isn’t necessarily finalized, but I’ve also posted the most recent version.

The first item I built was a relay board, with latching relays to route the signal through a bank of capacitors and inductors arranged in an L-network, configurable on the fly for low or high-Z. The prototype built on vector board  has nice blinky lights to help me visualize how the relays are switching. I’ve also built a power module and RF module (which senses SWR and reads frequency) on copper clad board.

Over the  New Year’s holiday break, I laid out the logic board, which contains the microprocessor (an ATmega368), a real time clock, LCD display, a piezo buzzer, some buttons, and connectors for paddle input and keyer output. The logic board also sports a USB interface to make my life simpler — I don’t think that will show up in the “final” version, which I envision being laid out as two PCBs: one for control, one for relays.  In the prototype, the two boards are joined by a ten-conductor ribbon cable (with RF connections through shielded cable, not added yet).

The two blank areas on the logic board are where the power module and RF module will be pasted in this prototype. For now, I’m leaving them off and concentrating on the programming aspect of the project. I’ve got some ideas about the global operation of the device and its menu structure, but before I really start any detailed coding, I’d like to look through a few similar projects. An obvious place to start is the full-featured CW keyer described by K3NG at  I can’t imagine putting all those features into this project, but I think I’ll learn a lot from reading through the code.

Significant Other Power Supply

Initially, I hadn’t given the power supply for the Significant Other project too much thought: I was more focused on the microcontroller, relays, and so on. After going for maximum efficiency with these components, though, it began to annoy me that it would be very wasteful to use an LM7805 regulator to bring lead acid battery voltage (13.8V) down to something that all the chips and relays could use (5V). The LM7805 tosses out the difference in heat, and while at the low currents that I need that doesn’t amount to much power — certainly, not enough to require heat sinks — it goes against the grain of QRP. If you have to haul a battery up a mountain, you’d like it to last as long as it can.

So, I started looking at more efficient (and lighter) means of powering the unit. The design I selected allows for two options. First, two AA batteries will fit inside the unit. Building them into the case assures that I can’t forget them. One of the goals of the SO project is to avoid unpleasant surprises while setting up the station in some remote location.  Since the unit draws so little current, I’d hope that a pair of AA batteries would last quite a while in field use.

Since radios are made to work from 13.8V sources, this is the other acceptable power input. The unit will be built with dual powerpole connectors, so that even if the battery has a single powerpole, it can be plugged into the unit, which effectively replicates the plug, so the radio can also be plugged into the unit. Even if the radio is greedy and pulls power from the battery causing the voltage to sag, the power regulator should cope with anything down to about 7.5V. If the lead acid battery gets that low, it’s probably toast anyhow.

Getting 5V from a 3V source requires a switching power supply, which could be a problem for a radio project since the switching happens at frequencies in the hundreds of kilohertz range. The LT1302-5 chip that I used in this project does not oscillate at a specific frequency, but is variable, and has the potential to produce RFI over a broad range of frequencies.

I followed the datasheet for the 1302 and built a “typical” supply using available parts. Layout is fairly critical, and I did my best to port their suggested PC board layout to manhattan construction. I didn’t have a 20k resistor, so I went with a 22k. I didn’t have any particularly low ESR electrolytics, so I used ones regular ones, etc. It seemed to work anyhow.

For testing purposes, I ran the power supply with a small load next to my FT-187nd, which was connected to a dummy load with cable that was unshielded for several centimeters. Within the ham bands, the only places I heard hash were on 160m and 80m, and even there, it only seemed to be around a couple frequencies. I had originally built the supply with a 10uH commercial inductor wound on a solid core. To limit EMI, I tried replacing this with an equivalent value hand-wound toroid (45 turns of 28Ga on a T50-2). This brought the noise level way down, and I couldn’t hear it when the antenna run was a couple cm away from the toroid. I suppose I could put the power supply in its own metal compartment, but it’s probably enough to just keep the RF path away from it in the layout.

Getting the right combination of bypass and charge-holding capacitors and discharge resistors is a bit empiric, and I’m not sure I did an optimal job, but I got out the voltage that I wanted. When connected to the oscilloscope, I noticed a periodic ~50mV spike that I thought could be a problem down the line for the microprocessor, so I borrowed a low pass filter from a similar project, the power supply in the Norcal 2030. I again had to substitute a bit — I think the filter inductors came out of an old TV. With that filter in place, the voltage is completely smooth as far as I can measure.

The two power supplies are connected by wire “OR”ing them together. The LT1302 senses 5V distal to a Schottky diode, but putting a diode after the higher power supply means that the voltage prior to its diode must be about 5.3 volts. To get that value, I used an LM317 and selected specific resistor values for its feedback network. The LM317 needs a small load to stabilize, so for the prototype, I threw in an indicator LED that lets me know when the high voltage supply is in use.

When the high power supply is active, it pulls up the LT1302 shutdown pin, which turns off the up-conversion. Without all that switching action, the voltage on the toroid side of the diode should be that of the AA batteries. This means that with the higher power supply active, the diode in the lower power supply is reverse biased and no current flows through it. This should mean that the unit can hot-switch between onboard and external power.

The prototype was a little smooshed because I had originally intended to only build the LT1302 circuit on that piece of copper clad board, and then I added the filter, and finally the 13.8V supply.

The real test of this supply will be whether it makes the other components happy.

Shopping in Brussels: Composants Electroniques et Jeux de Société

Overexposed picture showing conjunction of venus and jupiter above the grande place
Conjunction of Venus and Jupiter above the Grande Place

I had a couple hours on Friday to do a little shopping before meeting up with friends for dinner in Brussels. We had planned to eat near St. Catherine’s, so I took the metro there a bit early. My first goal was Elak’s electronics, which is one of the best hobby electronics stores on the planet, as far as I’m concerned. Part of the store is given over to computer components, but the rest of the store is discrete components: walls of switches, transformers, project boxes, batteries, etc. There is a center counter area where they maintain an impressive assortment of ICs as well. They carry the entire velleman kit line, plus related accessories.

The only drawback to the store is that it is in a corner of Brussels where the streets do not conform to any sort of rectilinear plan. I always get lost trying to find the store, and having a Google map in hand only makes things worse. It is like that part of Brussels does not obey the normal rules of time and space. Sometimes I try to get to it from the De Brokère metro, sometimes from Ste. Cathérine’s, but no matter what, I end up getting spun around and asking directions. When I get there and think about the path I took it all makes sense, but as soon as I leave, the store randomly pops up in some other universe.

External view of Elak ElectronicsAt least I can recognize the outside of the store when I do find it: the wall next to it has a mural with an elephant and a gorilla. The other place that has a reasonable selection of more common components is MB Tronics. When I last visited them, they had a storefront on Chausée de Louvain not far from the Meiser traffic circle, but I believe they’ve moved the store in the last year a couple blocks to the east. MB’s store hours are not quite as fixed as Elak’s, and the store does shut down during part of the summer for vacation, so Elak’s is always a safer bet.

I ended up buying a set of machine screws and a package of assorted ceramic disc capacitors. I’ve bought this screw set before, and had used them up making various projects. The screws are just the right size for most small projects, particularly the kind that you build in an altoids tin. The capacitor assortment is much better than you can find at Radio Shack. The Radio Shack bag-o-capacitors is full of unhelpfully small value components, whereas the Velleman-brand assortment has a full range (in searching the web, I note that they are also sold by Fry’s Electronics stateside). I am sure that these caps are not top-of-the-line low variance components, but they are great for prototyping.

External shot of Wonderland windowOn the way back to the restaurant, I stopped by a game shop, Wonderland, that is only a few minute walk from Elak’s. This store sells primarily  French language versions of “Euro” table top games: Settlers of Catan, Dominion, Carcassonne, etc.  I don’t think that I saw any Z-man or Rio Grande games, but that’s not a criticism as the store wouldn’t have had room for them. The games were predominantly board rather than card, and I wish I had had more time to look through their inventory. Next time I visit the store, I’ll make sure I have more time, and I will also be sure to have more room in my suitcase. They get extra credit for having zombie dice on the counter. While I’m certainly loyal to my local supplier (Area 42 Games), Wonderland may carry some games that haven’t made it over the Atlantic yet.

Winterfest 2012

SA-2040 tuner: two big capacitor knobs, one central roller inductor knob, a turn counter and a knob to select output

At the end of February, the Vienna Wireless Society held its annual hamfest, Winterfest, and this partially accounts for my absence of blog entries in recent months, as I was coordinator for the event. We had more than 100 vendors in our indoor area, another 40 in our tailgating space, and about 700 people were attracted to the event. I could go on at length about the event, and maybe I will at some point, but for now I’d like to post about the items that I picked up at the event. I’ve mentioned my acquisitions to a couple friends and want to show some pictures.

As soon as the event opened, my eyes landed on a Heathkit SA-2040 Tuner. I have an LDG AT100Pro automatic tuner and it does a great job, but for the Collins gear, I wanted a fully manual tuner. The automatic tuner makes excursions in and out of good match, and I just don’t want to subject the finals of the S-line equipment to variable and out-of-range vSWRs. I’d rather map out the setting for the band segments that I use and then manually adjust.

I had, in fact, been looking for a few months at several models of manual antenna tuner on ebay, eham, and the other usual places. The SA-2040 was high on my list. Typically they run over $100, plus shipping. When I found one at Winterfest, I was happy to see that it was less than $100, in good shape, and already had the modifications that I was considering — a knob with a thumb wheel on the roller inductor, a switch to select multiple coax outputs, and a switch that takes the input to ground. I had a lot of administrative work to do at Winterfest, so I bought it as soon as the event opened and stuck it in my car for the day.

An overhead shot of the internal workings of the SA-2040

When I got home, I took a look inside it. Truthfully, when I got home, I crashed on the couch for the day and didn’t get around to looking inside the tuner for a few days, but either way, I did look inside, and saw that it’s in fine shape. The roller inductor and capacitors are heavy duty and in pristine condition, with no evidence of arcing. The modifications look solid, and I couldn’t spot anything troubling. I screwed it back together and then started trying to figure out how I could possibly fit it on my bench.

The SA-2040 does not have an SWR meter, so I needed one. Luckily, I had ordered one several months ago from Ten-Tec as a kit: the 1225 SWR/Wattmeter. I had a good experience putting together the model 1320 QRP transceiver from Ten-Tec, and it has held up well as I’ve lugged it all over the world making contacts. The SWR kit was also a first-class affair — packed well, parts grouped meaningfully in several bags, all parts present with appropriate excess on wire, and a great manual. The wattmeter consists of a metal cabinet, a large cross-needle display, a range switch, and the option of average or peak-reading for both forward and reverse power.

Front view of the TenTec 1225 wattmeter, with meter illuminated in blue

The organization of the manual was excellent, with the usual check it and then check it again double column for checking off completed steps. There was no ambiguity in the instructions, all the landmarks were obvious, and I didn’t need to do any sort of clever interpretation or fall back on the internet to find exceptions, modifications, etc.

Calibration requires no equipment beyond a digital VOM. One trimmer pot controls internal reference voltage, and other pots are used to set the forward/reverse fudge factors for each power range. Having built the WM-2 QRP watt meter, I’d say that this one was slightly easier to calibrate. This SWR meter was a more complex build with more parts and more mechanical connections, but that is commensurate with its additional features (and I’m certainly keeping the WM-2 as well).

So, the 1225 Wattmeter was assembled over the last week and is now inserted inline between whatever rig I’m using (the first coax switch) and whatever antenna is selected (second coax switch). One fun feature of the 1225 is the RGB backlight, which can be adjusted to any color with trimmer pots. I’ve set mine to a dark blue.

TRS-80 model 100 and manualThe other item I bought was not radio equipment. Near the end of the hamfest, I walked by a table and saw a TRS80-Model 100 “laptop” computer. I’ve always thought this computer was way ahead of its time, and that it represented an important milestone in engineering, so when I saw one marked down to $50, I bought it.

This computer is powered either by wall wart or four AA batteries, has a full keyboard, boots instantly, and has a number of I/O ports including an RS-232 and the venerable S100 bus. I verified that this one is fully operational. I’m not sure exactly what I’ll do with it, but I think it was a good purchase.


AKA Interface Considerations

A couple months ago, Ben, NN9S, suggested developing a morse code keyer that would run on android devices (see his write up and video). Since then, we’ve been designing hardware and software to get the job done. The software lives in a repository on the Google Code site, and is moderately functional, but far short of ready for general release. On the hardware side, we’re still batting around ideas. Here is a brief history of the ground we’ve covered with regard to hardware design, and some idea of what’s coming next.

Phone, AKA, paddles and a transmitter - the most common configuration.

We considered all sorts of options such as having the device’s left and right channels run into a standard keyer, using the USB port directly, and some other more exotic alternatives (bluetooth anyone?), but we eventually settled on the idea of having the android device output a sound, which could be used directly to modulate a signal, or could drive a switch of some sort.  The simplest configuration would be the device plugged into the androidomatic keying adapter (AKA), and the AKA plugged into the radio’s key  jack. Presumably, most people would want to be able to also key their rig using a straight key, paddle, or bug, so we assumed that the AKA and some other sending device would connect to the key jack via a Y adapter of some sort.

Android devices encompass a gamut of hardware platforms including phones, pads, netbooks, and dishwashers. Well, soon. At home, my only android devices were a Nexus S phone and an Optimus T phone, so that’s where I started in terms of characterizing the platform. Two essential tools were the android app “FuncGen” and the multiplatform audio editor “Audacity“. FuncGen runs on the android device and can generate various waveforms with all sorts of parameters, whereas Audacity test files (e.g., *.wav files) can be generated on a desktop computer and transferred to the android device for playback using apps like media player or winamp. The phone’s output can also be recorded via Audacity to analyze fine timing events (for those of us without fancier equipment).

Using FuncGen at full media volume to generate a 1000hz sine wave, the Nexus S put out about 1.85V peak-to-peak (654 mVrms) into a 20-100k load, and the Optimus T put out 1.9V peak-to-peak (671 mVrms). The output impedance of the Nexus S was about 14 ohms, and the Optimus T was near 17 ohms. I calculated the maximum power transfer at around 7.9mW per channel for the Nexus S, and 6.4 mW for the Optimus T.

Output voltage at maximum volume into a fixed (20k) load was flat for the Nexus S from 20Hz to 3000 Hz, with only slight attenuation (down less than half a decibel) at 10kHz, and down almost 2 dB at 20kHz. The Optimus was more sensitive to frequency and was ideally flat from 300-3000 kHz, with more marked attenuation at 10kHz (less than 1 dB) and 20 kHz (down almost 5 dB). Both phones demonstrated a log-linear relationship between the number of clicks on the media volume scrollbar and the rms voltage output (e.g., for the Nexus, if full volume was 707 mV, one detent down was 477 mV, the next was 317 mV, 224, 150, 100, 70, etc.) — meaning that the voltage drops off fairly quickly to relatively low levels.

The first hardware design considered was literally a VOX keyer circuit. A quick websearch turned up a nice design by N1HFX. This straightforward circuit is based on a 1458 op amp, which is essentially two 741C op op amps in one package. The circuit was meant to work from very low voltages generated by a microphone, so the first op amp acts as an inverting amplifier, and the second as a comparator. The 741C op amp is a dual voltage op amp, so to run this from a battery, the positive input of the first amplifier is raised to half Vcc by a voltage divider; likewise, the set point for the negative input is a bit above half Vcc. The output from the second amplifier will be a square wave at the same frequency as the input wave, and ranging from about 1V to (Vcc-1V). Since this drive would only be half-duty cycle, some smoothing is necessary before driving a transistor. A simple RC circuit sets the degree of smoothing, and a diode prevents back-discharge of the capacitor into the amplifier. Since the circuit was designed for voice, the RC values were chosen for a relatively long time constant; however for purposes of keying a rig for fast CW or even Hellscreiber transmission, a low time constant is preferable.

Based on the N1HFX design, we prototyped a similar circuit, with the following exceptions – for cost/availability, we used an RC 4558 op amp, which is a workalike to the 1458. Because the phone’s volume is adjustable, and the output is relatively high, we used a fixed resistance ratio for the comparator set point. The smaller RC values used were just large enough to smoothen output wave. We added some bells and whistles including a power indicator LED, and another LED to indicate when the keying circuit was closed. It was probably overkill, but to further protect the phone, we stuck an optoisolator between the rig and the rest of the circuitry. In retrospect, all of these LEDs are nice looking, but consume many orders of magnitude more power than the keying circuit itself, and they might as well be omitted — in our final designs, we just key the rig from the switching transistor.

Another design that we considered was based on a circuit described by Robots Everywhere in their implementation of an earphone jack-based serial port on android devices. Their write up made us wonder if android phones could put out digital levels through the ear phone port — boy, that would certainly make our job easy. As it turns out, some can and some cannot.  The Nexus S can sustain a very flat positive or negative voltage without breaking a sweat: at 20Hz, the output waveform is very square, at 1000Hz, there is some ringing. The Optimus T, however, looks like it’s output is capacitively coupled. At low frequencies, it’s not very square at all — the output capacitor bleeds voltage until the next half cycle. The take home is that around 1000Hz and above, both phones can generate some semblance of a square wave.

The Robots Everywhere design centered on an LM324 single voltage source quad op amp, but used only two of the four op amp subunits. Our next design was aimed at getting away from timing issues related to signal rectification and need for a “filler” capacitor prior to the switching transistor.

Since the phone can generate a stereo output, our next circuit used the left and right channel to generate square waves180 degrees out of phase — effectively get a halfwave rectifier for free by doing it in the software. These waveforms were fed into op amps like in the first design, with amplification occurring in the first stage and inversion in the second stage for each channel. When the positive portions of two channels are then brought together through diodes, the result is just about a flat positive voltage, about about 1V under Vcc (due to the diode drop). This voltage is applied to the gate of a 2N7000 enhancement mode N-channel FET, which has a threshold voltage of just under 3V. The gate capacitance itself further smooths the small defect at the edges of the two square waves. The 2N7000 could be directly connected to the rig, but we again used an optoisolator, since the design required the use of a battery anyhow.

Both the dual and quad op amp designs resulted in some prolongation of tone sent out the phone’s earphone port. The best we got with the dual was about 4-5ms, and the quad op amp design yielded around 3ms extra duration. We could compensate in software, ending the signal earlier, but it would be preferable to avoid that complication. For morse code, this extra delay would probably be unremarkable, as a single code element at 30 wpm is 40 milliseconds, so +/- 10% is probably tolerable. At fast code, it becomes significant, though — at 60 wpm, a single element is 20 milliseconds. Since we had plans to implement Hellscreiber keying as well, with a minimal on-time of 8.163 milliseconds, this delay was not acceptable.

In addition to the timing issue, we realized that it would be preferable to not require a power source for the adapter. Luckily, one solution address both problems — using a transformer to step up the voltage enough to directly bias a switching transistor. This approach was inspired by the interesting work done by the “HiJack” team exploring power harvesting from the iPhone’s earphone port. They designed a miniature power supply using the AC from the iPhone’s audio output as driving voltage. A similar approach was taken by Wolphi, in connecting the iPhone to microphone input for his excellent PSK31 appplication for android. His circuit was, in turn, based on a circuit proposed by KH6TY.  Similar solutions have also occurred to hams in the past developing interfaces for sound-based keying of Hellscreiber, including a design by K9JRI that includes a voltage doubler.

In considering transformers for stepping up voltage, we did try the nearly microscopic one employed by the HiJack team. Unfortunately, it was too far off in terms of impedance matching to function well. We ended up going for the most accessible transformer, a 8:1000 ohm audio transformer from Radio Shack. This transformer gives about an 11-fold voltage step-up, so some losses in rectification are tolerable. In principle, germanium diodes or even synchronous rectification could reduce these losses, but there is enough head room to not worry about it. The currents involves are minuscule, so there is no problem is using tiny (read: cheap and available) signal diodes like a 1n914. A 0.01 uF capacitor does not entirely smooth the wave, but it ensures that the voltage is never below about five volts, and the 2n2222a transistor remains saturated when the transformer is energized. The additional duration introduced by this circuit is around 300 microseconds. I tried adding a voltage doubler per the K9JRI circuit mentioned above; the voltage was marginally increased and the duration increased slightly as well to around 1 millisecond. Since the extra complexity did not add much, I’ve opted to stick to the simpler design, which can be built from easily obtained parts for less than ten dollars (much less, if you mail order and/or buy in bulk).

The transformer-based designs have the benefit of providing galvanic isolation between the android device and the the keying circuit and rig. One downside to transformer-based designs is that they need relatively high volume output from the phone. The design above required that media volume be set within three clicks of maximum on the Nexus S. The dual op amp design worked down to 9 clicks below max, and the quad op amp design worked down to 12 clicks below max.  Both devices tested put out about the same voltage and power, and hopefully most android devices are in the same ballpark, but if there were a particularly underpowered device, or if this circuit was used in some other context where lower output were available, a design with active components might be necessary. The other drawback to

We are now trying to figure out ways to miniaturize the design, and of course, the most limiting factor is the transformer. If anyone spots a suitable transformer let me know. I guess the idea one would be 16 ohm on the primary, and 2000 on the secondary (or 4000 on the secondary, with a center tap — allowing us to get rid of two diodes).

[Update 17 May 2016]

The androidomatic keyer has not had a lot of love lately; Ben and I have been busy with other projects and developing for android requires more sustained attention than we can afford for now. However, I’d still like to see this work preserved. When the GoogleCode repository blinked out of existence, I moved the software archive over to github, where it now resides:

Warming the shack

The power supply, receiver, transmitter and station monitor/phone patch, plus microphone and key
Collins S-Line Station

For several years, I have been putting together a Collins S-line station, and earlier this week, I fired it up for the first time.  This station was the top of the line, when it was produced in the 1960s, and although there have been plenty of technical innovations along the lines of improved frequency stability, image rejection and DSP, these units still sound great.  Now that winter is approaching and the basement is growing chilly, I was more motivated than ever to get this tube equipment on the air.

I have had these radios packed up for quite a few years, not really wanting to ship them all over the world as we moved. Last year, I started putting them back into order. I started by looking through the archive at the Collins Collectors Association and joining their email list. Between the archive and routine access to people who know these rigs inside and out, I came up to speed in no time.  I didn’t see anything too amiss on my inspection of the rigs, aside from some carbon residue near the rectifier tube in the power supply. This is a common defect in these supplies — some arcing occurs from the rectifier tube to the metal cabinet, so I wasn’t too worried about that, but realized that the rectifier socket would need to be replaced by a ceramic one down the road.

photo of the underside of the 32S-3 transmitter
I took a lot of photos during the survey phase to provide some insurance that I'd be able to reassemble the radio after working on it. The underside of the 32S-3 transmitter showing extensive point-to-point wiring. The "black beauty" capacitors are prominent. They and the surrounding silver-colored electrolytic capacitors were replaced.

The most likely items to fail in rigs of this age are the electrolytic capacitors, so I went through each unit and replaced all electrolytic capacitors, plus any paper capacitors and some of the less reliable “black beauty” oil-filled caps. There is some debate about whether it is better to replace preemptively or only when there is evidence of failure, but I leaned towards the preemptive side. While I want to preserve the original engineering, I also want these units to perform well and get some actual on-the-air use.  The other obvious thing that can fail are the tubes. Of course, I can’t just run down to the corner store anymore and use their tube tester. To put that statement in context, these radios are just slightly older than I am.

There are a lot of patch cords that run between the station components, some audio, some RF. These and the power plugs and control connectors had degraded to the point that I wanted to replace them. I rebuilt the connecting cords and power cords using the original plugs and sockets, but for everything else, I visited Joel at RF Connection in Gaithersburg, Maryland. Conveniently, his store is about five miles from where I work, and it is packed with every connector, cable and adapter known to man. He also stocks a lot of the materials used in restoring Collins equipment, including replacement feet. Yes, I replaced the little rubber feet, because they also had degraded over time.

photo of a capacitor deep within a metal slot
This was my favorite capacitor to replace -- deep down in a metal pocket that I couldn't disassemble. It was like soldering a ship in a bottle.

After inspecting the units, I took each out of its cabinet and to greater or lesser extent stripped them down. All the tubes came out and everything got a thorough cleaning. All the pots got a shot of deoxit, and other bits of mechanical tune up were performed. I replaced the burnt out panel lights with long life replacements from D.A. Buska Engineers, LLC.  These LED lamps should last just about forever, which is good because it is something of a hassle to replace bulbs. It involves my wrist bending in ways that seem unnatural. However, I would think twice about using the LED lights again if I did another restoration. Their light is harsher than the original incandescent lamps, and there is some barely perceptible flicker.

After putting everything together came the smoke test. Since these rigs had been off for more than two decades, I approached this cautiously. I tested each unit separately, bringing power up slowly using a variac (auto-transformer). The power supply came up to voltage and all the supply outputs tested nominal. The receiver also powered up, and when I hooked it up to an antenna, it seemed reasonably sensitive. The band calibrator seemed to work, and put the dial right on WWV. I flipped through the different bands, and could hear signals on all of them.

Photo of the upper deck of the transmitter, with the final amplifier cage in the foreground
The top side of the transmitter. The front of the radio and the main tuning dial are in the rear, the final amplifier cage is in the foreground. A bunch of the tubes have already been removed at this stage.

The transmitter was more complicated. When I got to about 80V on the variac, a thin wisp of grey smoke wafted upwards from the power amplifier section of the transmitter. I couldn’t find where the smoke was coming from, but the take home message was clear — I needed some professional help.

Even if I had not hit this snag, I would have sent the radios in for some professional servicing at this point. After such a long period on the shelf, they were in need of a laboratory realignment, which is beyond what I can do with my current equipment. I also wanted someone who really knew Collins equipment to look them over.

I contacted Peter Wittenburg at Collins Rebuilders. He is located near Annapolis, Maryland, which is close enough that I could carefully pack the rigs in my car and drive them up. It was definitely the right call — the radios got the love and attention that they deserved from an expert. Peter swapped out the rectifier in the power supply, fixed a fairly major problem in the receiver’s permeability-tuned oscillator (not for the faint of heart), replaced a few tubes that had seen their day as well as a few other discrete components that were not up to spec.  He ran the rig through its paces and gave me a nice report, that shows by and large, the rigs were seaworthy.

An Astatic microphone
An Astatic D-104 microphone.

While he was performing his analysis and repairs, I picked up an Asiatic “lollipop” microphone with the proper Collins connector at a hamfest. I had previously borrowed a variac for testing, but decided to purchase one for fulltime use in the station. The Collins equipment was made for a time when the line voltage was lower than the present, and higher voltages will shorten component life. The whole station is now plugged into an isobar strip that itself is plugged into the variac, which maintains voltage at 117V RMS. The station is turned on/off with the isobar rather than each unit’s power knob, as those knobs are all but irreplaceable and are prone to mechanical wearing.

Over last weekend, I made sure that my attic antennas were tuned up, presenting less than a 2:1 vSWR across their useful ranges, as the tube equipment is not tolerant of (read: smokes when confronted with) less well-behaved loads. The ionosphere was revved up as high as it has been in the last decade or so, which boded well. I made my first contact on 40m with WB8PPH (“wb8-pumpkin-pie-headquarters”), who was running a special event station for the Circleville Ohio Pumpkin Festival. I had a few more contacts with PA, IA, and IL and then Sweden, England, Croatia and the Czech Republic on 20m, and Cuba, Honduras, Mexico and finally South Africa on 10m. The received sound quality was golden, and I received favorable reports concerning my audio quality as well.

schema of s-line wiring between different modules
Interconnections for transceiver operation.

The rigs can be run in transceiver mode, where the transmitter is slaved to the receiver’s VFO, or they can be run independently, with the both the receiver and transmitter VFOs in action; this is what you would have to do to operate split, for instance, as there is no other RIT/XIT function. Separate operation is also the way to go for CW operation. In transceiver mode, you would hear nothing when zero-beat to the incoming signal (naturally), and the offset in CW mode is 1.3 khz, which is impractically large. To operate CW, I picked a clear frequency and used the “cw-spot” function on the transmitter to send a low-level signal to the receiver, allowing me to adjust them to the same frequency. I did one round of calling (straight key, natch) and had an immediate reply from Northern Ireland and Bulgaria. I had a little difficulty getting the vox-balance right to accomodate semi-breakin operation (some hangtime after transmit so the relays aren’t constantly clicking, but not so much that I miss the reply). According to the overseas stations, there was no problem with key clicks or chirps, so I guess we’re okay on that front as well.

I’ve never been a big fan of sideband, but I got a big kick out of working stations with this rig, and I think I might be spending more time behind the mike this winter.

Controlling DTR in virtual Windows

[This was written a couple years ago, but is archived on a site that I am not maintaining, so I’m duplicating it here as well, to make sure that I have a “living” copy and to put it in the backup stream of this blog.]

When my HP laptop went belly up after four and a half years of heavy use, I replaced it with a MacBook Pro … and thus began my reorientation towards the Mac-side. I made the switch for professional reasons including a desire to have access to a unix command line without needing to run a virtual machine or cygwin on the PC. From that perspective, I am very happy with the Mac, but migrating from Windows to Mac was more difficult in terms of amateur radio-related applications.

My needs are not overwhelming, and I was hoping to find some Mac equivalent for each application that I use. Categorically, I needed programs for logging, contesting and working digital modes. For these purposes, my solutions on the PC were N3FJP’s Amateur Contact Log 3.0, N1MM logger, and MixW, respectively.

It didn’t take me long to realize that the selection of software was much more limited on the Mac side. This is not surprising considering that Macs are relatively expensive and have less market share than PCs. Since many ham software developers are hobbyists, naturally they will develop for the computer that they are using themselves and which would benefit the most users. Don’t get me wrong: the Mac is a well-engineered machine with plenty of horsepower, but even for real time signals processing applications, a cheap PC does the job just as well. I suppose that I could just pick up a used laptop for ham-radio uses, but it just seems more elegant to me to make everything work on the Mac.

For logging on the Mac, there is MacLoggerDx, a very stylish program that has some nice bells and whistles and sells for more than 90 dollars. As such, it’s the most expensive logging program I can recall — likely due to the small market. A major selling point is that it can be extended through Applescript, and there is a community of users contributing scripts. For contesting, I haven’t seen anything on the Mac side that rivals N1MM logger, which has been used for so long by so many people, that it has been honed to a fine edge. For digital modes, some users have developed and shared programs (see several by W7A7 including cocoaModem), but even the most advanced of these suites lack the breadth and stability of their PC counterparts.

After thinking long and hard, my decision was to not abandon the programs that work so well on the PC side, but to run them in a virtual machine. I wasn’t sure this would work, given the need for real-time processing power and external hardware interfacing, but I can attest that this solution is practical. My Mac runs OS X 10.5.5, and for virtualization, I run Windows XP Pro SP3 under Parallels v3.0, with 512 MB memory allocated to the VM. More memory might be nicer, but I can assure you that this minimal configuration works fine. I do not have experience with VMware Fusion, but would guess that it would work in an analogous manner (perhaps someone would like to try this and let me know). As an aside, I have tried running N1MM logger and MixW under CrossOver on both the Mac and Linux platforms, and I could never get that to work entirely. Unlike Parallels and Fusion, CrossOver is not an emulator, but a commercial version of the linux-based wine project.

a Kenwood CAT connector cable DIN connectorMy goal was to integrate operation of a Kenwood TS-450S and a 2.2 Ghz Intel Core2Duo MacBook Pro. The first issue was one of hardware — how to control the rig from the computer. Previously, I bought a CAT cable on Ebay for around five dollars, and it worked flawlessly for years. The circular DIN connector plugs into the radio, while the other end of the cable, a nine-pin male serial connector (i.e., a DB9 connector) plugs into the computer. That was fine for my old HP laptop which sports an appropriate serial port, but it’s bad news for the MacBook Pro which, like many more recent laptops, entirely lacks serial ports. And this is where the witchcraft begins.

Keyspan USB to serial (DB9) adapterThe obvious answer is to buy a USB-to-serial port converter. I picked up the Keyspan USA-19HS adapter (at left) for about thirty dollars on the web. It is a very popular device, and I’m sure it works well for most people’s applications either under MacOS X or Windows, but it turned out to be the wrong choice for trying to combine them. There is a long thread of postings  thread of postings on the parallels support page regarding user frustration trying to get this adapter to work from within Parallels. In theory, there should be two mutually exclusive approaches: 1) Mac-centric — install the MacOSX drivers and then start up the virtual machine. Configure the virtual machine to use the “serial port” that it sees in the Mac environment; 2) Windows-center: within the virtual machine, install the Windows drivers for the device. Then, configure parallels to use this USB device. The latter seems cleaner to me, but fails utterly. The adapter comes with a driver disk that includes a nice diagnostic program which indicates that the port is not working under windows. Trying the Mac-based approach worked better. I found an excellent article by Brian Williams on Mac OS X Hints which detailed his experiences trying to do essentially the same thing. He describes a non-commericial program, serialclient, which makes the Mac-side serial port resource available to the VM.

Following the instructions in the Mac OS X Hints webpage, the MacOS X drivers are first installed, then the adapter is plugged in, and Parallels is launched. Briefly, using the configuration screen in parallels, a serial device is created as a socket in server mode and mapped to a file (e.g., /tmp/serial). A windows session is then launched. To actually enable the serial port, the helper application serialclient is then launched on the Mac side, with appropriate parameters for the connected device, as well as the name of the file serving as the “socket” above. Now, when you hit the “connect” button on serialclient, it links the file on the Windows side to the resource on the Mac side. The port is no longer available to OS X, as it has been redirected to Parallels. At this point, Windows should have a virtual serial port (e.g., COM1:) and you can launch your application.

Homemade optoisolator and one-quarter inch plugAmazingly, all of the above actually works — but there are some caveats. First, this set up occasionally goes down in flames. I’ve had the laptop freeze up a few times, requiring a hard reset. It should be possible to control the port from within Windows, but it is not. Within a DOS shell, the “mode” command should be able to set COM port parameters, but this is not the case. This set up relies on a non-commercial bit of glue, serialclient, that is not supported, was never intended to be used in a general manner, and which might evaporate without warning or break with subsequent releases of Parallels. The deal-breaker for me, though, was that the DTR line does not function correctly.

Since the Kenwood uses RTS/CTS flowcontrol for its CAT functions and does not make any use of the DTR signal, I figured that I could use DTR for keying the rig, getting two-for-the-price-of-one from this serial port. A number of programs, for example N1MM and MixW can be configured to use DTR to key CW.

To use the DTR line, I built an opto-isolated interface according to the schematic provided by WM2U — essentially, a resister, an optoisolator IC and a diode to keep the electrons flowing in the right direction (click on the image at right for detail). I put this board inline, just before the 1/4″ plug that goes into the radio. In my case, since I want to be able to drive the radio from either an external keyer or the computer, it actually feeds into a Y-adapter. The other side of the Y-adapter goes to a K-12 keyer by K1EL and paddles.

An extra wire has been soldered to ground and DTR The shielded line upstream of the optoisolator taps into the ground (pin 5) and DTR (pin 4) contacts of the CAT interface. I had to scrape away from plastic goop to get to these pins. Some diagrams show this connector from one direction, some from the other, so to be sure about orientation, check for continuity between pin 5 and the connector’s shield (assuming it is playing by the rules). The CAT is now a CAT of two tails — one going to the rig as before, the other going to the KEY port.

I tested this set up from the Mac side using the cocoaPTT program to toggle the DTR line, and it worked fine. However, when I set up parallels as above using serialclient as a conduit between OSX and Windows, the DTR line activated as soon as “connect” was hit. Initially, I thought that this was a Windows-related problem, but this does not appear to be the case. No matter what I did, as soon as I enabled the serial port under Windows, my key went down and stayed down. Not ideal operating procedure.

A close-up image of the optoisolator, diode and resistor interfaceFinally, I tried replacing the Keyspan USB to serial adapter with one that I ordered from ZLP electronics in the UK. The ZLP device is much simpler, a short piece of wire with USB on one end, and serial on the other, with no indicator lights or other features. The device was shipped without driver software, but this turned out not to be a problem. Having removed all of the keyspan drivers, I loaded up my Parallels VM and booted the virtual Windows XP session. I then plugged in the adapter resulting in a “new device detected” message from Windows. Windows asked if I’d like to let it search Windows Update for an appropriate driver, and I let it do so. Within a few seconds, I had a functional serial port.

The only thing left to do at this point was to configure programs according the radio’s specifications. For the Kenwood 450, the settings are 4800 baud, 8 data bits, no parity, 2 stop bits. Here’s N1MM logger as an example.

The ZLP adapter works without any need for the serialclient program or other kludges, and it does not have any difficultly driving the DTR line correctly. I’ve used this set-up for a couple weeks now in conjunction with a number of Windows-based programs that use the serial port. So far, no problems.

adapter from EZP: USB to serial (DB9)Even with the overhead of running the windows emulation, the MacBook Pro does not come up short on processing power (e.g., I can be following many simultaneous digital mode conversations at once and otherwise multitask without putting a dent in performance). I have experienced no problems in terms of serial port latency or variations in timing of CW due to processor load. I should also mention that as the Mac is entirely encased in aluminum, I’ve had no problems with stray RF emission from the computer itself.

I’m sure I’m going to catch flak from Mac diehards who understandably want to see native applications developed for the Mac platform, but my conclusion is that when it comes to amateur radio applications, the most expedient way to access a wide library of popular and well-tested programs is to teach the Mac to be a PC. Over time, I am hopeful that the best of the ham-related windows applications will be ported to the Mac and that new Mac-native programs will be developed.

Honey, I blew up the amplifier

Actually, let’s start on a bright note, and then we’ll get to the part involving smoke.

Inside view of the rockmite 40 installed in a mity box
The Rockmite 40

My main reason to build the amplifier was to get a bit more power out of the rockmite. Part of the problem in getting the amp working was also likely low driving power. The basic design of the rockmite uses the prototypical bipolar NPN for the “final” amplification — a 2N2222.  There are variants that use other transistors and get the power above a watt, and there are also some tricks to increase the drive, but the stock rockmite should yield a nominal half watt or so with no mods. So, I took off the cover of the rockmite and poked around, checking all the part values. I had already made one substitution: C8 was decreased in value from 0.1 uF to 0.01 uF to knock the side tone volume level down to something tolerable.

Sure enough, I saw the problem — I had used 47 pF (marked 47J) instead of 470 pF (marked 471) for C15 and C17, which are on the output side. Yes, the “1” and the “J” looked similar. I stuck the right value capacitors in, and power output using a 13.2V supply was 550 m. Not too bad.

Next, I plugged the RM into the Texas Topper. I didn’t power it right away, though, because I was interested to see what sort of insertional loss the Texas Topper introduced when it was not active. Power output was about 400 mW. This probably wasn’t an entirely fair test because the Tx Topper was still on the bench top, with tack-soldered connections to the BNC connectors.

Texas Topper before final install

I removed the extra N4148 from the amplifier, because I figured it probably had enough drive now to work without extra bias. Power output was measured as 2.2W, so about 6dB gain.  While I could live with that (and, in retrospect should have), I was curious what would happen if I bumped the bias back up a bit. The N4148 went back in, raising the bias voltage from about 2.05 to 2.75V. Power output was now 7.7W — 11dB gain. I measured roughly 7Vpp in, 20Vpp out, so roughly in agreement.  Part of the increase in gain might also have been due to switching from using alligator clips to apply power to using a thicker wire terminated on one end with a power pole connector and on the other end with a type N coaxial plug.

So, at this point, I was actually (hah) thinking of introducing a one or two dB attenuation pad, although the idea of burning off power in  a QRP rig feels inelegant. More inelegant, however, was trying to bend the FET forward so the heat sink would fit in the enclosure. When I pushed it forward (yes, with power applied), there was a bit of sizzle and then a bright flash from the LED. I don’t know if something arced before the LED toasted, but I was left with the acrid and no doubt carcinogenic aftertaste of stupidity wafting through the shack.

I inspected the board around the FET and couldn’t see an obvious short. The parts in that area do share some close quarters, and the heat sink is right next to both transformer coils. I took out the LED (shorted now) and yanked the FET. Good thing I had a bag of them on hand….as will soon be even more evident.

With a new FET, a new LED (not quite the same type, but close), and another diode, I was back in business. Everything was fine until I tried to stuff the heat sink into the enclosure. This time, not under power. The problem is, though, at some point, you have to apply power, or the whole thing is just a paperweight. Zot. Sizzle. Flash. Puff.

Yeah. So, at this point, I’m out of FETs and thinking that maybe I need to do something more creative regarding the strained relations between the heat sink and the enclosure. The heat sink should be applied right to the metal tab on the FET, which is the drain. The case is aluminum, and at ground potential, so that particular twain shouldn’t meet.

I tend to only order from Mouser when I get enough items on my “want” list to make the shipping work out, so it might be a bit before I replace the burnt out components, but I’m sure I will in the next couple weeks. The board has held up very well to my repeated soldering/unsoldering, and I really don’t have any complaints about the Texas Topper per se. This is more a mechanical issue at this point — all the electronic bits seem to do a fine job of amplifying. I may, in fact, order another one just to play on another band.

Even without the Texas Topper, doubling the output of the Rockmite should make it more usable. I’m looking forward to rolling both it and the TenTec 1320 out next week for the QRP to the Field Event.

Texas Topper QRP Amplifier

I seem to be doing well enough running the Kenwood B2000 at 5W or the TenTec 1320 at around 4 watts, but I haven’t made many contacts with my Rockmite, which on a good day puts out around 250+ mW, but less when the battery runs down a bit. A while back, I had ordered the Texas Topper (a.k.a. Tuna Topper) amplifier (nominally 5W out) from For $25 it’s a good deal. The design lends itself to flexibility and experimentation, allowing the user to choose whether to use onboard/offboard options, a tuna-shaped round or altoids-shaped rectangular form factor, a range of input powers (using fixed or variable attenuator, if necessary), fast/slow switching, and transceiver or xmtr/rcvr configuration.A round Texas Topper printed circuit board, unpopulated

I put the kit together last week, made an enclosure, stuck it in, connected everything up, and … nothing. Well, not quite nothing. My WM-2 wattmeter read about 50mW output. Not good — the amplifier was doing something, but not in the direction that I had hoped.

The amplifier is well-documented on Chuck Carpenter’s website, which provides a parts list, pictures of the board, schematics and helpful advice. From the circuit diagram, it’s apparent that there are two halves to the pc board — one that controls switching through a relay, and the business end of the amplifier that centers on a MOSFET followed by a filter network.  As far as I could tell, nothing was shorted.  In the “receive” state, signals fed straight through from input to output connectors. When I keyed down the rockmite, I could hear the relay click in, and I was able to verify that the input signal was being appropriately routed over to a 4:1 transformer to feed to the MOSFET.A constructed Texas Topper on the bench top for troubleshooting

I checked DC voltages with a DVM, and verified that the bias voltage (determined by the forward voltage drop across an LED that conveniently also serves to let you know power is applied) was 2.05V. The voltage on the MOSFET’s metal tab (the drain) was about the same as battery voltage, as it should be.  Unfortunately, I didn’t have an RF probe on hand for tracing of RF voltages — the probe was lost in the last move. It would be easy enough to build one (see nice plans on W5ESE‘s site), but I didn’t have a suitable diode on hand and apparently Radio Shack no longer carries the 1N34 in stock. No problem — I have something better, although not quite as portable: an oscilloscope.

The incoming signal was about 8Vpp, and 4Vpp after the 4:1 input transformer. I expected to detect something on the drain of the MOSFET, but all I got was hum (maybe just background).  Probing beyond the MOSFET, I didn’t get much. I was stumped at this point, and starting wondering if I had done something wrong during construction.

It seemed to me that there were two likely places that I could have screwed up — in winding the two bifilar transformers (which, I recall I did while watching an episode of “No Ordinary Family”, so maybe I was distracted), or maybe in installing the MOSFET. I had placed a mouser order at the same time as the kit order, so I had a couple extra MOSFETs to play with. Using static-free everything (mat, wrist band, soldering tip, etc) and low heat, I replaced the MOSFET. No change. The kit comes with 22 and 26 gauge red magnet wire for winding toroids. To be extra careful, when I rewound the two bifilar transformers, I used on strand of red, and a strand of another color. Radio Shack does carry a magnet wire set, which conveniently includes 22, 26 and 30 Ga lacquered wire, and the 22 is gold and the 26 is green. The transformers look much better when wound with two color wire, and it’s easy to verify at a glance that the correct wires are tied together and that they all end up where they should. Again, though, no change.

I tried replacing the MOSFET one more time, as I thought that perhaps I had not had the right load on the amplifier when I tested it the first time, but again, no change.

The Texas Topper laid out for testing on the benchtopAfter  I looked at the data sheet for the FET and noted that the gate threshold voltage is listed as a minimum of 2v and max of 4v. The transfer function graph showed the drain current picking up sharply above 4v. My rockmite has lower output than most, and it occurred to me that I might be at the lower end of this amplifier’s design — not enough umph to drive the FET’s gate. To up the bias voltage, I stuck a 1N4148 diode between the stock LED and ground. This bumped the bias from about 2V up to 2.75V. Result: 1.5W output. On the oscilloscope, the waveform was a bit distorted on the FET’s drain, but smoothed out in the filter and was well formed at output.  Going from 250 mW to 1.5W is somewhere around 7dB gain — not quite the 10 dB gain typical for this amp, but a huge improvement over my rockmite’s usual output.

So, now I am playing around a little to see what happens when I run this system with a fully charged battery and play a bit with the bias voltage. Hopefully, I’ll have a chance to try out the rockmite-on-steroids this weekend.