T-R Time Machine

This past January, I became aware of a new event celebrating the vintage rigs popular in the 50’s, 60’s and 70’s, the Novice Rig Roundup. I love the tube rigs from that era, and absolutely had to put together a station and join in.

I already had on hand a few transmitters capable of the job: two QRP 2-tube rigs based on a classic MOPA circuit (you can find links to several on this page), and a Knight T-60 kit built transmitter. The T-60 was particularly appropriate, as it was designed to operate at less that 75 watts input, making it the perfect Novice transmitter.

I operated all of these transmitters under crystal control, and after a week of trying to make contacts, gained real respect for those ops that learned the ropes that way. For someone who was licensed well after solid state transceivers were the norm, operating a separate transmitter and receiver, constrained to a single frequency, was a real challenge.

Back in the day, ops would call on their crystal frequency, and then listen up and down the band for a reply. In the modern world of automatic zero-beating and narrow passband filters, being stuck on one frequency just doesn’t work all that well. After a week of not too much success, I was determined to add a VFO to my vintage station. Operating with a separate VFO, in addition to a transmitter and receiver, does complicate things a bit.

Separate transmitter/receiver stations always involve some kind of switching arrangement if a single antenna is used for both components. These can range from a simple toggle switch connecting the antenna to either, to automatic RF-sensed switching which allows for full break-in operation. (Full break-in, the ability to hear the receiver between the elements of the Morse code characters, is what we are used to in modern transceivers, and there are a number of commercial, kit, and homebrew solutions to provide this kind of switching for separates.)

Typical switching devices, called T-R (transmit-receive) switches, usually provide the following functions, in the correct sequence
On key down:

  • mute receiver
  • switch antenna to transmitter
  • key the transmitter
  • provide sidetone (optional, but handy)

On key up:

  • un-key the transmitter
  • switch antenna to receiver
  • un-mute receiver

Adding the VFO to this mix increases the complexity a bit, because of some of the characteristics of VFO operation. Unlike a crystal oscillator, which does not provide a signal until the transmitter is keyed, a separate VFO can run and be keyed independently of the transmitter. The VFO can run continuously, or be keyed on and off in synchronization with the transmitter. Both methods offer advantages and disadvantages.

Allowing the VFO run continuously, and just keying the transmitter, will undoubtedly produce the most stable oscillation. The VFO will drift less, and show no start-up instability on each keyed Morse element. However, there is an odd side-effect to this mode of operation: something that was known as “backwave” back in the day. Suppose you hear a signal in your receiver and want to reply, by tuning your transmit frequency match exactly (this is called “zero-beating” the signal.) With the VFO running, you will hear your oscillator in your receiver (if it is not muted) and the tone, the backwave, will blot out his signal. You won’t hear him.

Also, if you are relying on listening for your transmitter in the receiver while keying as way of providing a sidetone, the backwave will either obliterate your keying, or make it sound weird. So all in all, the “let it run all the time” approach is not too desirable.

The alternate approach, that of keying the VFO in synch with the transmitter, is the one that was most often used. In this scenario, the VFO running only during the times you are actually transmitting a Morse element – sounds ideal, right? Except there is an issue that arises in this case: chirp. The VFO, when transitioning from off to stable running can exhibit slewing of the frequency. This “bird-whistle” effect is called chirp, and is the hallmark of a poorly run station.

Operators would attach a “C” to your RST signal report to indicate there was chirp on your signal, and if it was really bad and you were noticed by an Official Observer (OO) station, you could be the recipient of a dreaded “pink slip” notification to clean up your act. Chirp on your signal is to be avoided at all costs. I’ve had some direct experience with this phenomenon. I had a Heathkit HW-16 transceiver coupled with a HG-10 VFO. The thing chirped like a cage of finches. It was one of the reasons I finally sold the rig.

There are a few things that can be done to minimize or eliminate chirp with synchronized keying. One is to run the VFO on its own power supply. Another is to allow the VFO to stabilize before keying the transmitter, and to un-key the transmitter before the VFO. Some tricky timing is required. This approach was first explored in tube circuits in the 1950s, and paved the way for modern transceivers with internal T-R switching.

Living in the 21st century and having access to cheap and efficient microcomputers, it is easy to shift the keying into the future, by introducing delays between keying the VFO and the transmitter. This is the idea behind my VFO-friendly switching system, the T-R Time Machine.

T-R Time Machine, front panel

T-R Time Machine, front panel

The external event of closing the code key drives a chain of events that put the signal on the air lagging the keying by a few milliseconds, but always keying the transmitter only when the VFO is in stable oscillation.

The sequencing works slightly differently depending on whether you want to be able to hear signals between the elements of your sending. This mode, called QSK (one of those Morse code signals meaning “I can hear you if you interrupt”), will wind up transiting between transmitting and receiving several times a second as you send. Non-QSK mode (for want of a better term,) keys flips the antenna and keys up the VFO, and leaves you in that state while you send. The station remains in transmit mode for a fixed “hang time” after the last Morse element sent, before flipping back to receiving. The T-R Time Machine can manage either mode.

In non-QSK mode, the T-R Time Machine starts the VFO, delays a couple of milliseconds, and then starts keying the transmitter in time with the external keying. The VFO runs continuously, but the receiver is muted so you do not hear the backwave. After the hang time has elapsed since the last keyed Morse element, the station is sequenced back to receiving. The hang time is set to be just a bit longer than the typical pause between phrases, to minimize the amount of switching.

The first morse element is robbed of a millisecond or so of duration, but at 25 WPM that amounts to about 3% shorter, and only the first transmitted element is affected. It is completely unnoticeable on the air.

QSK mode time-shifts your keying by a couple of milliseconds, so the keying the transmitter lags the keying input. On key up, the VFO runs for a millisecond or so past the unkeying of the transmitter. The effect is to slow the output keying very slightly, but again it is unnoticeable on the air.

My Arduino sketch re-uses some code I had written for the Digital Fist Recorder project, but adds a new concept to control the keying: a coding device called a Finite State Machine. Using this approach, you model the process as a series of states. Each state performs some logic on entry, and based on current conditions, decides how to transition to the next state. Organizing the code this way makes it very easy to maintain. The switching tasks, things like key down, VFO start, transmitter start, etc. lend themselves nicely to representation in the code as discrete states, and the resulting code is quite clear to follow.

I have not yet posted the code to GitHub, but intend to do so, and will post an update here when it is availble. As always I am releasing this code as open source under a GPL license.

Building the TRTM naturally divided into to two phases: the easy part and the hard part.
The easy part was building the boards containing the Arduino and switching components. The TRTM consists of several pre-assembled components:

  • An Arduino: I used a standard Uno R3 board, but also had implementations running on a Pro Micro clone
    (the sketch for TRTM is small, and will run on just about any Arduino board)
  • A dual relay board
  • Two Key-All HV boards
T-R Time Machine, interior view

T-R Time Machine, interior view

In addition, there are two voltage regulators which I built using ICs and Manhattan style assembly. One regulator drops the input voltage to a regulated 9 VDC to run the Arduino. The second provides a regulated 5 VDC to run the relay board and the Key-All modules. In early prototypes of the TRTM I ran into difficulties sourcing enough current from the Arduino to run the outboard switching components; this version sidesteps the issue.

The hard part for this project was fitting all the components, and the large number of external connections into an enclosure. I went through three iterations before settling on the version shown here. The project required:

  • three antenna jacks,
  • four phono jacks,
  • external power pole mounting,
  • four indicator LEDs,
  • a speaker (the speaker is mounted in a hole on the bottom of the enclosure, under the main board and fires down),
  • a panel mounted pot for sidetone volume,
  • two switches, one for power and one to select QSK mode are also on the front panel,
  • as well as two input jacks for a key. The two jacks are wired in parallel, and provide either 1/4″ mono or 1/8″ stereo plug inputs, since I have keys wired both ways,

Drilling all those holes in the right place proved challenging (you can see the extra holes in the boards, and on the bottom of the enclosure where I had to move things around.)

An additional feature in the latest enclosure is the complete isolation of the switching circuit from the Arduino, to avoid RF feeding back into the computer and causing issues. I accomplished this by mounting all of the switching connections (the antenna and phono jacks) on a panel made of polycarbonate plastic. This material is inexpensive, and easily machined with ordinary hand tools.

The plastic plate is mounted on the back panel of the aluminum enclosure through oversized holes, so the switched grounds are completely isolated from the enclosure ground. Actual switching isolation is provided by the use of relays, and the Key-All units which are opto-isolated from the control circuits.

T-R Time Machine back panel

T-R Time Machine back panel

This project was difficult to complete. In addition to all of this drilling and screwing was the difficulty I had getting the LEDs to work. Yes, I was not able to light an LED using an Arduino…

It plagued me for weeks, until I finally sat down and carefully stepped through the code to discover that I had never initialized the digital pins used for the LEDs as output pins in my sketch. Silly code bug, but it really drove me nuts.

T-R Time Machine with working LED indicators

T-R Time Machine with working LED indicators

So I finally have a very nice, working, VFO-enabled T-R sequencer ready to go. Unfortunately, I still haven’t gotten a VFO to work properly with my Knight T-60, but that will be a story for another day. Until then,

73
de N2HTT

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In the loop.

It’s Memorial Day weekend, and the weather in the Northeast is finally summer-like. We were at the upstate QTH over the weekend expecting to be doing some work on the place, and so combined the holiday with a few vacation days – only to discover that the delivery of some construction materials had been delayed. We had no recourse but to relax and enjoy ourselves instead.

Making the best of it, we sought other recreation, and I of course turned to ham radio for diversion. With  the nice weather back, I love to operate out-of-doors. We have a nice deck and a big back yard, so my operating expeditions rarely take me more than a few feet from the house, but are adventures nevertheless.

The only problem for me on this particular weekend was the CQ WW WPX CW Contest, held 0000Z, May 28 to 2359Z, May 29. I regularly participate in sprints and weekend CW events, but of the sort where 15 WPM hand-sent CW is typical (take a look at the SKCC and NAQCC  sprints for example.) I’m not that comfortable with the rapid fire, high-speed CW of “real” contesting. I will listen to a call about 10 times to get it, send an exchange in my plodding fist at QRP power, and if the other op is patient, he will reply. Or ignore me altogether, which is often the outcome. So I decided that for all intents and purposes 80, 40, 20 and 15 meters were not available over the weekend, and I should probably play on the WARC bands. Daytime conditions for the high bands were actually not that great, so it was pretty much going to be that 30 meters was the band of choice.

My normal setup for QRP operation is to used an end-fed halfwave wire cut for 40 meters, with quarter wave counterpoise. I use a 4:1 balun directly at the termination of the wires, and use a tuner connected to the balun by about 6 feet of coax. This arrangement works well, and with the tuner covers 80 – 10 meters easily. But, with the bands uncooperative, and some time on my hands, I decided to try something new, and build a full wave loop for 30 meters.

Using the classic formula 1005/(frequency in MHz), and shooting for 10.125, the middle of the 30 meter band, I came up with loop length of 99.26 feet. I have two 20 foot fishing poles that I use as portable masts, so I figured that a long rectangle, 10 feet high, would be easy to support with the poles, with the horizontal bottom wire well off the ground. I just had to take up 79.26 feet with horizontal legs, which meant a horizontal run of 39.63 (top and bottom wires.) Putting up the poles about 40 feet apart seemed very do-able, this idea could work!

Hard to see 30 meter loop

Hard to see 30 meter loop

I have tried to build full loops in the past, with no success, and have discovered some gotchas to avoid. The first is don’t try to build a portable loop out of one continuous length of wire. I have tried this, using all kinds of clever plastic spacers made of poly containers to hold the corners of the loop, and I have never gotten it to work. The damn things slide around no matter what you do. Even duct tape doesn’t help, not to mention that it adds a kind of down-in-the-heels look to the resulting antenna.

Nope, this time I constructed the antenna using five lengths of wire, constructed to come apart into two pieces. I cut the two 10 foot verticals, and the 39.6 foot top horizontal wire, and joined the corners with crimped ring tongue terminals. The ring provides a place to tie a short length of twine and fishing tackle clip, which fits over the top section of the mast. This construction takes care of the top and two sides, which are stored as one piece when the loop comes down.

Top corner of the loop showing ring terminal crimped in place.

Top corner of the loop showing ring terminal crimped in place.

The I cut the bottom horizontal wire in two, and made a 4:1 balun for the center attachment of coax. I crimped a ring tongue to the ends of the two verticals, and the ends of the bottom horizontal. These terminals accommodate #6 hardware, and a 1/2 inch #6-32 bolt and wing nut attach the horizontal wire to verticals, completing the loop.

I made the 4:1 balun using a pill bottle, and 12 turns of 24 gauge speaker wire forming the bifilar winding. Recipes for 4:1 air core baluns abound on the web, so I am not going to link any here – a web search will result in dozens to choose from. I wrapped the windings and connections of my balun in very attractive violet 3M electrical tape which I got at the local home improvement store. They make this tape in a variety of colors to go with any decor.

Pill bottle 4:1 balun, wrapped in attractive violet electrical tape

Pill bottle 4:1 balun, wrapped in attractive violet electrical tape

The second thing I learned from prior loop-building experience: do not use twisted-pair wire as the radiating element of a loop antenna. I know, when you say it like that it seems kind of obvious, but at the time I tried this it was a surprise when the resulting loop didn’t behave well at all.

I had the luck a while back to obtain a 1000 foot spool of twisted pair telephone wire for $2 at a flea market. This is really nice 24 gauge solid insulated wire, but to use it for antenna purposes you have to separate the strands. The good news is I got 2000 feet of antenna wire for $2. The bad news is that it took over an hour of excruciating untwisting to separate a hundred feet of the stuff. I use the Method of Two Cardboard Bobbins as you can see in the photo below; once you get more than 4 feet separated the stuff goes all over the place and tangles maddeningly unless you wrap it on something.

A large quantity of cheap twisted-pair, and the Method of Two Cardboard Bobbins

A large quantity of cheap twisted-pair, and the Method of Two Cardboard Bobbins

I started building the loop late Saturday, and it wasn’t until the middle of the day Sunday that everything was set up. I checked the resonance of the loop with an antenna analyzer, and found that it was perfect – at 9.200 MHz! I guess the 1000/frequency formula is intended to be a starting point, and you adjust from there. My loop was resonating almost exactly 10% low. Looking at the formula, you can see that changes in frequency are proportional to changes in length, which meant that the loop was 10% too long, which was just about 10 feet. This is where the construction-in-pieces approach paid off. I decided to leave the verticals 10 feet long, and remove the excess from the horizontal wires. This worked out to be 2.5 feet at each end of both horizontal wires. After a few cuts and crimps, the loop was put back together with the poles 5 feet closer together. This time, the measured point of resonance was spot on at 10.125, with about 100 kHz of bandwidth < 2:1 SWR, more than enough to cover the entire 30 meter band, no tuner required. Now on to the real test – an actual QSO.

I have lots of small QRP radios fit for the job, one of the best being my kit-built KX1. I have this rig all tricked out for portable operation, complete with a homebrew clipboard to hold everything, and Li-ion rechargeable batteries for a little extra kick.

KX1OnClipboard

KX1 portable operating station, on homebrewed custom clipboard

This clipboard station works really well; I shamelessly stole the idea from a very nice commercial product from SOTABEAMS; mine is an inexpensive fiber clipboard with some notches and rubber bands.

Unfortunately, the HF bands were pretty moribund during the day, Sunday. Although 40 and 30 meters perked up around sunset, it was very late before I had a chance to try operating, and I decided that I did not want to go sit under a tree in the yard in pitch blackness testing my new antenna.

Monday, Memorial Day, was very nice weather, and although we had a brief thunder-storm in the afternoon, things cleared up beautifully after that, with cooler temperatures and less humidity than Sunday. I kept my eye on Band Conditions, and just around sunset, about 8:30 PM local, 30 meters indicated as wide open. I soaked myself in bug repellant and ran down to the tree where the coax was parked with my KX1 kit, hooked up and took a listen. I didn’t want to spend the time to drag a lawn chair down there, so I just stood under the tree.

Almost immediately I heard Carl, WB0CFF calling CQ from Belle Plaine, Minnesota. At first there was a little QSB, but then he came in very solid. I was standing there under the tree with the board in my left arm, keying with my right hand. My keying was a little shaky, as the it was hard to hold the board still while standing up; it took a little time before I got used to the arrangement. We had a nice, but short QSO; Carl gave me a 559 report for my 3 watts into the loop. As it turns out, Carl is an SKCC member, so we exchanged numbers as well. It all worked perfectly.

It was quite dark when we were done, and I decided to not hang out in the yard. Earlier in the day, we had seen either a badger or porcupine run across the yard

North American Porcupine

North American Porcupine

(a porcupine would be more likely, but it really looked like a badger.)

By Yathin S Krishnappa - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=24504952

Badger By Yathin S Krishnappa – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=24504952

In either case, I didn’t wish to run the risk of meeting our visitor in person, especially connected by a run of coax to a fragile construction of poles and wires. As far as I’m concerned, the project was a success.

So it was a really nice weekend, beautiful weather, a little sun, some bug bites, one QSO, and a new portable antenna. And, come to think of it, my first and only CW QSO to date completed standing up.

73,
de N2HTT

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A tale of two VFOs

I am always a little surprised at the constant learning experience afforded by this hobby. My assumptions are usually incorrect, but the discovery of what is actually going on is always entertaining and instructive. Consider the story of the two VFOs.

At the end of my last post, I was pretty convinced that my refurbished HG-10 VFO was not going to cut it in my Novice tube station. The station was keying oddly, with transmitter output wandering up and down, although the output from the VFO was steady. At this point, I was keying the VFO and the transmitter simultaneously, using the old two diode trick. I had more ambitious plans about keying, but this comes a bit later on in the story.
About the same time as the discovery of weird transmitter keying (by the way, the T-60 is totally well-behaved when using a crystal instead of the VFO), I happened across an eBay listing, only hours old, for a Knight V-44 VFO in good condition.

This, if I haven’t already mentioned, was the VFO that sparked all this VFO interest. I had seen one at auction that went for a really silly, high bid. That’s when I started looking for HG-10s. Anyway I jumped on the V-44 at its very reasonable price, and sat back waiting for its arrival. During that wait, I got my hands on some V-44 documentation, and studied up.

The v-44 has a self-contained high voltage power supply, and uses a really clever design of putting the power supply at the top of case, with the tuned circuits below, so the heat from the supply won’t add to thermal drift. This gives the cabinet a kind of cool “portrait” rather than “landscape” aspect ratio. It’s rather charming.

Knight V-44 VFO

Knight V-44 VFO

Since it contains a built-in high voltage supply, I knew I would have to re-cap it. I also found it interesting that the manual warns the V-44 not be used on or near grounded surfaces such as a metal table – adding a three-wire plug and a fuse was a must. So the laundry list of repairs was:

  • new filter cap in the HV supply
  • three-wire cord, with chassis ground and an inline fuse.

This was not too tall an order, and while the VFO was still in transit from its former owner I ordered and received the necessary cap from Mouser. I was all set.

The timing of the arrival of the V-44 was about a week before I would be traveling to the upstate QTH (site of the novice station) for about ten days. During this time I would be there on my own, leaving plenty of time for ham radio adventures. So it seemed really important to get the repairs done before this trip. Really, really important.

Of course, there was always the normal myriad of responsibilities: family, work, social… so it seemed that bench time was a scarce commodity during this lead up to the trip. I allowed myself to fall victim to the worst (and possibly most dangerous, but happily not in this case) foible that affects amateurs of any stripe: I rushed the job. Big mistake.

I started with the cord replacement, which was straightforward enough. It involved replacing the old coax output cable, which looked like RG-8x or similar, with skinny
RG-174, which made room for the new 3-wire power cord without needing a new hole in the chassis. Adding the in line fuse was not a problem either, there was enough room in the under chassis to squeeze it all in. The problem arose when I looked for a convenient stud to attach the third grounding wire. There wasn’t any.

Now it’s close to 11:00 PM on a week night, getting near bedtime, but if I could button up this cord replacement, I could take the VFO upstairs, plug it in and just give a listen. I was so close… Okay, no problem, I’ll just drill a 1/8th inch hole in the chassis, put a bolt and nut in, and Bob’s your uncle – chassis grounding. Without giving it much thought, (obviously) I chucked in a bit and started drilling.

These old boat anchors are built tough, no flimsy soft aluminium chassis here – nope this was good heavy gauge steel and it was taking some effort to get through. I was putting some pressure on the bit, and just as some part of my hindbrain was screaming “No, no lighten up your going to pop through…” suddenly the bit popped through. Right into the main tuning tank coil.

In that instant, the thought that I should have put a little wood block in there behind the hole (there was ample room) flashed through my head, way too late to be useful. I think I said “oh, darn” loudly a few times. Maybe that wasn’t exactly what I said.

Carefully modified main tank coil

Carefully modified main tank coil

I surveyed the damage. It was pretty bad, but not cataclysmic. If I had shattered the coil form, it would have been game over, but all I had accomplished was to destroy the windings. This, I thought, could possibly be fixed. I went to bed.

The next day, I pondered my options. Obviously, all I had to do was wind a new coil on the ceramic form, with the same inductance as the one I had destroyed. No problem, I’ve wound lots of coils. I just had to look up the inductance of the main tank coil in the manual. Every coil in the circuit was called out by value in the parts list except the main tank coil. It was just referred to as Knight Kit part number 152014, “coil, oscillator, tank”.

Okay, this wasn’t going to be that easy, but I wasn’t about to admit that I had murdered my beautiful VFO with a power drill. I’ll just count the turns, and compute the inductance.
There then ensued over the next day or so a great deal of careful, methodical turn counting. Of course, with roughly 30 gauge wire, and a chunk of the wire actually missing (bits were cut by the drill and fell off) it was actually impossible to get an accurate count. I even tried taking a macro photo of the damaged coil, and using a photo editor putting little tick marks on the image every ten turns so I wouldn’t lose count. In the end I had an estimate plus or minus 5 turns, not quite close enough for engineering work, as they say.

Okay, plan B occurred to me. The dial calibration instructions called for adjusting two trimmer caps separately, one for 80 meters, and one for 40 meters and up. This meant that the tank coil, which was obviously a fixed inductance, had to resonate the main tuning capacitor plus the each trimmer at two different frequencies, which are known. Since the trimmer caps are identical, I just had to find an inductance that could resonate both frequencies within the range of the trimmers, with the main cap set at the calibration point.
(Footnote here: I mistakenly thought both ranges were calibrated to the same point on the main tuning cap. This is not true. This error may explain some of the new weird behavior I saw later on. But at this point I was feeling awfully clever.)

I then embarked on a course of feverish calculation: inductance by wire diameter by gauge and number of turns, resonant frequency at various values of capacitance – finally I zeroed in on the target value, and I started to wind my new coil.

I use the penny and quarter technique. I laid out 10 pennies in a row, and 8 quarters (I needed 81 turns total). After winding each turn, I moved a penny to a new location on the table, gradually building a new row. When ten pennies were moved, I moved a quarter to represent 10 turns. Lather, rinse, repeat. It’s a foolproof method of guaranteeing that you don’t mis-count, and it only costs $2.10.

penny and quarter technique, afer turn 23

penny and quarter technique, afer turn 23

But alas, I did not nail it on the first try. Two thirds of the way through, I was running out of room on the coil, and realized I had used the wrong, too large, wire gauge. Okay, start again with the right gauge, only to run out of room again. Re-compute number of turns for smaller gauge wire and start again. Lather, rinse, repeat.

Finally my coil was done, but there was another nagging issue. The original coil had been treated with an application of “coil dope”, a thin varnish. Reading on the web seemed to indicate that using coil dope was a good idea, especially in oscillator circuits, as it minimizes drift by stabilizing the turns of wire against movement or vibration. But where to get some? You can buy it mail order, but it is expensive and I didn’t want to wait. So I looked for recipes of home-brew coil dope and discovered that it is dead easy to make. Smelly, inconvenient, and extraordinarily flammable, but easy.

You make coil dope by dissolving styrofoam packing peanuts in toluene. Toluene is a highly volatile, flammable, toxic and generally nasty solvent use as paint thinner, which can be had at any hardware store in inconveniently enormous quantities. I needed about an ounce, and was only able to find it in a gallon container. At this point, I’m all set with regard to future toluene needs.

The packing peanuts are easy, right? I mean one always has scads of the things from incoming packages in boxes in the basement. Right? Well, we had recently recycled every last one of the little buggers at the local retail shipping store, there was not one to be had on the premises. I had to wait until I got to work the next day to get some at the office. Sensibly, our IT guy has never thrown out a styrofoam peanut since we moved into that office space. You never know when you will need some.

Making the coil dope was done outside, despite the still chilly late winter temperatures, and even at that I nearly passed out from the toluene fumes. But finally, after dissolving an amazing number of peanuts in two ounces of toluene, I was rewarded with a maple-syrup-like consistency, evil-smelling fluid. Voila, coil dope!

Artisanal Coil Dope

Artisanal Coil Dope

Using a cotton swab, I covered the coil in a light coating which dried in seconds, and looked pretty official when it was all done. And I still have 2 ounces – 1.5 milliliters of coil dope left for future endeavors.

Rebuilt main tuning coil

Rebuilt main tuning coil

The finished coil popped right back in place and I quickly soldered back the two connections. I went through the calibration procedure, and everything seemed okay, but the story wasn’t over yet.

I took the VFO on my trip to the upstate QTH on schedule the following weekend, and installed it in the Novice station. It looked good, and I fired it up on 80 meters. I was using the technique where you turn the VFO on (not keyed) and key the transmitter. This gives rise to a phenomenon called “backwave”; if your receiver is not muted you hear the VFO tone behind the sound of your keying. It sounds odd, faintly disturbing, like listening to the receiver with it and your head in a garbage can – but I was determined.

V-44 in Novice Station

V-44 in Novice Station

A little tune up, and a couple of quick CQs. Not hearing any response I headed off to the computer to check Reverse Beacon Network, to see where I was heard. Okay, lots of stations were picking me up on 80 meters, definitely a good start. Next, on to 40.

I tuned everything up on 40, and gave it a go. Lots of good reports on 40. And, lots of good reports on 80. What? Yes, it looks like I was a candidate for the Worked All Bands Simultaneously award. Definitely not a good thing. Subsequent tests with a dummy load showed that I was getting strong sub-harmonic signals that were getting through the output tank in the transmitter. Not a good thing at all.

I knew that the oscillator in the V-44 generates strong harmonics from two fundamental frequencies: a 160 meter band fundamental generates the first harmonic which provides 80 meters, and an 80 meter band fundamental provides harmonics used for 40 meters and up. Something was definitely out of kilter, perhaps output filtering, which looks kind sketchy from the schematic, or maybe my home-brew tank coil resonating someplace it shouldn’t. More research is needed, but for now sadly the V-44 is on the bench.

In light of this revelation, I went back to playing with the HG-10, and in doing so made a remarkable discovery. If you don’t use fancy two-diode keying arrangements, the HG-10 works fine with the Knight T-60. Take away the diodes from the keying circuit, and the funky instability goes away.

The old 2-diode trick for simultaneous keying of the VFO and transmitter

The old 2-diode trick for simultaneous keying of the VFO and transmitter

I know from prior experience that the T-60 is notoriously hard to key with anything other than a plain key that shorts to ground. It has a fairly large dropping resistor in the keying circuit, which has the advantage of presenting a low DC voltage to the key when it is open, removing any shock hazard. The downside of this arrangement is that if there is any significant resistance in the key, the rig won’t key. (I had this happen with a soviet military key, which had 20 ohms resistance internally in some kind of built in filter.) And apparently, the T-60 doesn’t like diodes in its path to ground. With a regular key, the HG-10 drove the T-60 perfectly well, I was mistaken in the assumption that the low drive was at fault.

The HG-10 is now part of the Novice station, and the V-44 is back in the shipping box, awaiting further diagnosis to figure out the weird sub-harmonic behavior. I may re-visit the winding of the tank coil. I may find a “for parts” V-44 and replace my homebrew coil with a real one. But for now the V-44 is sidelined and the humble HG-10 is doing the job.

THe HG-10 takes its place in the Novice Station

THe HG-10 takes its place in the Novice Station

So my Novice station is now VFO enabled, with backwave, using the HG-10, but I really wanted a setup where the VFO was keyed (no backwave), no chirp (a common side effect of a keying the VFO), and a sidetone with receiver muting would be nice. This gave me another idea, involving 21st century microcontroller implementation of 1950’s circuit, but that’s a story for next time. Until then,

73
de N2HTT

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Rockbound no more… almost.

The saga I am about to recount is too involved for a single blog post, so this is part one of two.

After enjoying a month of the SKCC K3Y operating event in January, I was really excited to learn of a new event, only in its second year, that took place the last week of February. The Novice Rig Roundup, a week-long exercise in operating those beloved, chirpy, antique rigs of our childhood, was just too appealing to me to pass up.

Granted, I never had any of those rosy experiences to wax nostalgic about, as I wasn’t licensed until middle age, and then firmly in the solid state era. I never held a Novice license, but I really do like those old tube rigs, and so enthusiastically dived in.

I had decided to operate QRP, using either of the two homebrew MOPA transmitters in my arsenal. I have two QTHs, and one QRP tube rig at each location: the W1TS Simple Transmitter at one, and the QRP Blowtorch at the other. I also have a classic “Novice” transmitter, a Knight T-60, which operates QRO at about 40 watts out; at 60 watts input it definitely would have qualified for use by a 1960’s Novice.

It turns out the third week of February was very busy for me with only a little operating time available, and I managed a total of two (yes, only two) contacts over the entire time. And, one of those was with the T-60, as I abandoned my normal QRP operations for some serious muscle.

The issue, it turns out, is that operating crystal controlled on a single frequency is really hard! It seemed like there were only two possible scenarios:

  • when the crystal frequency was quiet, calling CQ endlessly with no response, or
  • frequency was busy with someone else calling CQ. Work him once and you’re done

Both QSOs I made were with folks who happened to be calling on my crystal frequency. I know that in the good old days, hams would call on their transmitter frequency, and then listen up and down the band for a reply. QSOs were full-duplex, on two frequencies. With modern transceivers, this practice is long gone. We are all used to carrying out our conversations in half duplex on a single frequency, often with narrow bandpass filters engaged. It is possible to reply to a calling station only a few hundred Hertz off, and never get heard – believe me, I know.

So although I had fun, and am looking forward to next years NRR event, I found myself yearning for a VFO. I had at one time an external VFO setup – a Heathkit HW-16 transmitter/receiver, paired with a Heathkit HG-10 VFO. I never really liked using the HW-16 with the VFO. It chirped like crazy; I recall making one QSO with it that I cut short after a few overs because the thing was chirping so badly. Eventually I sold the HW-16, and the VFO with it.

I have read that the HG-10 was more prone to chirp when it is powered from the transmitter. There is also a problem with keying the VFO and the transmitter simultaneously, as there is a little key-up instability in the VFO. So I had all of this in mind as I started to look for a VFO for my station.

The Heathkit HG-10s are readily available on eBay – at any given moment there are at least two at auction. I would certainly prefer to find something else, but those HG-10s are out there, and because they are so plentiful, the prices remain relatively low (for eBay, anyway.) While searching the classifieds and eBay, I came across a Knight kit V-44 VFO. This is the companion to my T-60, so naturally I was very interested. After researching the V-44, I was even more interested, as the engineering looked pretty good. Unfortunately, several folks were bidding on that unit, and the price went nuts. It sold for about four times what I was willing to pay, not including hefty shipping. Back to searching for an HG-10.

I quickly found one that looked functional, but not super-good cosmetically, for a reasonable price. While I was waiting for it to arrive, I decided to build a standalone power supply for it, trying to head off the chirping. Also, a standalone supply would allow me to use it with any rig.

Back when Radio Shack was last going out of business and closing stores, I picked up a pair of robust filament transformers on clearance for just a few dollars each. I knew you could use these, back to back, to create a 120v isolation transformer. The HG-10 documentation describes a simple, non-regulated high voltage supply that can be used to power the VFO, based on a single isolation transformer.

HG-10 PSU

HG-10 PSU as described in the manual

The time had come to use those filament transformers; I built the PSU.
It is a simple half-wave rectifier design, with two filter caps. I added a three prong grounded plug, and a bleeder resistor across each filter cap.

PSU under chassis wiring

PSU under chassis wiring

My version of the schematic looks like this:

HG-10 Modified PSU

HG-10 PSU, modified for back to back filament transformers

and I measure about 160vdc at pin 4. The finished unit looks very neat:

Finished HV PSU

Finished HV PSU

Once the HG-10 arrived, it was time to open it up and check it out. Wiring job looked okay, and I could tell that the unit was configured for grid block keying via pin 8 of the octal plug.

hg-10_under_chassis

hg-10_under_chassis

This is a very common configuration for these units, as this was the setup needed to connect an HW-60 or HW-16 transmitter. I wanted cathode keying, since the keying would not be provided by my transmitter. This involved removing a jumper, moving a connection from the octal socket, and changing a resistor. All of this is well documented in the HG-10 manual, which can be found from many sources on the web. Since I was powering my unit from its own PSU, I had to add a dropping resistor ahead of the 0B2 regulator tube – again the manual provided a graph for computing the value needed, and I guesstimated I’d need a 1 watt 1k resistor based on the 160 volts I measured from my PSU.

While researching the HG-10, I came across mention of a problem with 80 meters, and a modification published by Heathkit late in the production of the HG-10. The RF choke supplied for the plate of the output tube is too low a value, resulting in low output and a clipped signal on 80. I checked the output from mine, and sure enough, there is the distorted signal:

Clipped waveform on 80M

Clipped waveform on 80M

While the Heathkit memo suggested replacement with a larger value choke, apparently the folk remedy for the problem is to swap the cathode choke (1 mH, too large) for the plate choke (200 uH, too small). The smaller choke works fine in the cathode circuit. Instant fix, without having to track down hard to find antique parts.

RF Chokes before the mod

RF Chokes before the mod

RF Chokes, interchanged

RF Chokes, interchanged

After making this change, the 80m output looked much better.

Output waveform on 80M after mod

Output waveform on 80M after mod

With all these changes made, and the PSU I was ready to go. A rough measurement of the output from the HG-10 showed about 7v out at 80 meters, and about half that on 40 and above. The unit nominally produces about 5v out, so these seemed about ballpark.

Cathode keying requires simultaneously keying the transmitter and the VFO. The simple way to accomplish this is to place a diode ahead of the connection to each device, above the key. This allows current to flow from each device to ground via the key, but no current can flow between the devices – the diodes block this. You need a voltage drop across the key that is at least as large as the forward drop across the diodes, about 0.7v, but with tube equipment this is not likely to be a problem. I built this interface to split the keying

Tandem keying interface

Tandem keying interface

Finally, hooked the VFO up to my W1TS Simple Transmitter, tuned up and… really low output. Slightly better on 80 than on 40, but not really enough to be useful. The output was just too low.

The following week, I tried it with the T-60. On 80, I was able to tune up the transmitter, but on 40 it was just not stable, the signal would jump up, then drop, then come back. I think there just isn’t enough output from HG-10 to drive any of my transmitters.

So quite a bit of work, a lovely, working VFO, but the wrong one for my purposes. I am still rockbound. But then, an amazing bit of luck which I will tell you about in part 2.

73,
de N2HTT

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Snowy day, new glue.

We’ve had the first snow of 2016, and it was a doozy. “Winter Storm Jonas“, as it was named in the media, was a nor’easter that roared up the east coast dropping record amounts of snow. We got our share, but being north of the storm track, not so much as to be catastrophic. A solid foot and a half of light powder, easily shoveled, no big deal really. Most importantly, even though we had some serious gusts, my new mast survived the storm handily, and my antenna is still in operation. Quite a relief, actually.

Mast still up after Jonas

Mast still up after Jonas

The arrival of the storm did have an impact on how we spent the weekend. Planned travel was cancelled, bread, milk, extra cat and dog food purchased, and we hunkered down to enjoy a quiet snowy weekend. Aside from occasional forays out to clean off the cars and the front steps, we were all left pretty much to our own devices. Ah, blissful unstructured time!

I had been planning to start the construction of my first space charge tube regen – the unexpected down time seemed ideal. Unfortunately, I was not as well prepared as I thought. I’ve decided to build this first version using a wooden “orange crate” construction, with thin slats and solid ends; spaces between the slats will accommodate the tube sockets. I figure this will give me some wiggle room as I lay out the circuits, without the profound emotional commitment demanded by making holes in an expensive aluminum chassis. Earlier in the week I had raided Home Depot to find nice oak slats and boards to build my crate, I was all ready to go.

The first step was to determine length of the end boards by laying out the slats and tube sockets. Oh drat! No sockets! I thought I had sufficient sockets left over from the construction of the W1TS Simple Transmitter, but as it turns out, the ceramic sockets I had left can’t be mounted easily between slats. The fiber kind, with a molded-in metallic ring is the type I need. Not a one remained.

Also, an inventory of variable caps came up short. The nice 150 pF one I had intended to use got drafted into my crystal receiver, and a rummage through my box of variable caps failed to turn up a replacement. I was seriously unprepared.

These shortcomings could be easily be remedied by a trip to eBay or Antique Radio Supply, but that would not help me this weekend. No, no regen this weekend.
There was plenty else to do, shovelling, operating, catching up on correspondence and paperwork, but I really was in the mood to play at the bench a bit as well. As I have been listening with the crystal receiver quite a bit in the evenings, I thought it might be a candidate for some improvements.

Attached to the antenna (new diode)

Attached to the antenna (new diode)

I have been using alligator clip leads for the antenna, ground, and headphone connections, and this is less than ideal. The signals would suddenly disappear as I jogged one lead or another – very makeshift. And I really have been listening to broadcast AM with this radio. You can actually DX broadcast AM with it – I heard WFED AM (1500 kHz) from Washington DC quite clearly the other evening. It’s sort of ironic, I never listen to broadcast AM with any of the “real” radios I own, but I am delighted to sit through auto parts store commercials when they are captured with just a coil and a diode.

Anyway, some improvements were definitely in order, and I just recently came into possession of an interesting new glue, that seemed to be ideal for the tasks at hand. In conversation a while back, my sister-in-law mentioned this new glue that she had heard about, but not tried yet. It is cured with UV light, sets in about 4 seconds, and has very high bond strength. It was one of those “as seen on TV” things; we don’t watch the right TV apparently, I had never heard of it, other than the stuff the dentist uses to make those costly repairs in your mouth.

She had bought some on Amazon. Intrigued, I asked for a link on Amazon, and she responded by saying she’d drop the glue off with me – I could try it and let her know how well it worked. Sounded good to me.

There are several varieties of this glue on Amazon, varying in price from about $10 – $20 for small tube, say about 100 ml, and an LED UV light used to set the glue. I think the brand I got was one of the $10 varieties. [Looks like the price has gone up a bit at the time of this writing]

Reading the package, I came away with a somewhat mixed impression, as the glue was described as “safe and non-toxic” but the instructions suggested gloves and goggles for handling, and a quick call to 911 before abandoning all hope should you accidentally ingest the stuff or get it in your eye. I decided to take the route of full protective gear, and I threw in a pair of UV goggles for good measure; I’m completely paranoid about cataracts.

The first task I tackled was to mount an SO-239 socket on the back of the receiver’s base board, thus eliminating one set of dodgy alligator clip leads. This would be tricky, because the edges of the board are not finished square, but have a very nice rounded edge. Not a good platform for traditional glue, because of the lack of contact area. I suspect that before UV glue, I would have tried hot glue, and the joint would have been

  • messy
  • not very strong, and would pop off the first time I connected a cable.

This is exactly the kind of challenge that UV glue excels at. It has almost no “sticktion” – you can’t put a dab down, place parts together, and let go. They will simply fall apart. But if you can hold the parts in place and hit it with the UV light, 3 to 4 seconds later, the stuff is rock hard. No tackiness at all. And perfectly clear. The glue is a fairly stiff gel, pretty much stays where you put it, but it will start to run under its own weight after a few seconds. It works best were you can use it to fill a void, by applying several applications and curing each one.

I laid down a bead of glue, held the connector in place, and zapped it. Wow, instantly rock hard. I filled the voids and zapped again. Immediately I had a connector that would stand up to attaching and removing the antenna cable, no problem. Pretty amazing. The glue cures clear and glossy, and is very hard to see, but it does fluoresce brightly under the UV light, so you can see clearly where it is.

SO-239 attached with UV glue. (Glue is fluorescing)

SO-239 attached with UV glue. (Glue is fluorescing)

Next, I wanted to add a socket for the 1/4 inch mono plug on the end of my high-Z headphones. I had scavenged such a socket from a ham fest special I am parting-out, but it was the kind that mounts in a 3/8 inch panel hole. No panel, what to do?

I made a little mini-panel out of single-sided PCB material, soldered to make an “L” shape with a second piece. Using the UV glue, I cemented the bottom of the “L” to the underside of the board, and then filled in the voids on both the top and bottom with additional glue passes. Again, rock solid, and luckily so, as considerable force is necessary to plug in or remove the phones. Another instant success.

Front panel attched with UV glue (Fluorescing)

Front panel attached with UV glue (Fluorescing)

By this point I had dispensed with the gloves. This stuff is easy to control using the applicator squeeze bottle; unless you expect to really have to tussle with the job at hand the gloves probably aren’t all the necessary. But I’m standing by the UV goggles – you can’t be too careful when your eyes are concerned.

So now with antenna screwed on, and headphones plugged in, everything was nice and solid and the rig was working FB. For some reason today, the strongest station I could hear was a French language call-in show. Quebec maybe? Endless hours of entertainment. But I was still not quite satisfied…

No more alligator clips. (Well, only one.)

No more alligator clips. (Well, only one.)

The receiver was working at top form, but it seemed to be lacking something. An aesthetic improvement perhaps? Armed with this amazing glue, I faced the rampant desire to glue more stuff to this radio.

My wife maintains an art studio on our property, she is primarily a painter, but creates puppet shows, writes and illustrates children’s books, and often creates small assemblages and sculptural works. Suffice it to say that her studio contains a wealth of odds and ends, and she is very generous with her materials when it comes to decorating radio projects. I think she feels that they are under-decorated in general, and will do what she can to correct that unfortunate state.

I asked for, and was graciously granted, two “frozen charlottes” from her collection. These small doll figurines date from the Victorian era, and were very popular toys for children. There are lots of internet material on them, you can look it up. Several years ago, when we first set up her studio, I had found a huge bunch of them from Germany on eBay, and given them to her as a “studio warming” gift. We might have the definitive collection in the northeast.

I thought that two charlottes, representing the etherial spirits, fixed at either end of the coil tube would not only be attractive, but enhance the operation of the receiver.

Frozen charlotte, guarding the entrance to the coil.

Frozen charlotte, guarding the entrance to the coil.

Unfortunately, I did not make quantitative, before and after measurements of signal strength, so this report needs to be considered anecdotal at best – however I swear those French-speaking callers were at least a half S-unit stronger after applying the figures. And I love the way it looks.

Crystal set, decorated.

Crystal set, decorated.

Get this glue away from me, before I really hurt myself.

73,
de N2HTT

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A complete digression.

Happy New Year.

Now that the holidays have passed and time has started to free up a bit, I’ve found my way back to the bench and started to resume experiments on a space charge tube regen, And while I actually have some interesting progress to report on that front, I’ve been sidetracked into completing a completely unrelated project by yet another random sequence of events.

This one started, as they often do, with an eBay purchase. I probably shouldn’t be telling you this, but I will sometimes bid on a lot of several crystals, if there are at least a few I can identify as being on useful ham frequencies. If the average price per crystal is attractive I’ll go for it.

Out of band FT-243’s are also useful because the holders can be filled with modern HC49/U or similar crystals. I sometimes find crystals at frequencies just below the ham bands. These can (in theory, I have never actually attempted it) be ground down to raise their frequencies a few hundred Hertz, to values that fall in-band.

Of course, I get a lot of fairly useless stuff as well. There may be a few interesting holders, or items that might have antique interest, but for the most part the rest is of no use and collects in a bin on a shelf in the basement.

This particular lot contained a curious item: a sealed detector crystal for a crystal receiver set.

The mysterious Philmore Fixed Crystal DETECTOR

The mysterious Philmore Fixed Crystal DETECTOR

These holders contained a hunk of galena and a cat’s whisker (short, stiff, wire probe) sitting on a “hot spot” on the galena, potted in a plastic holder to permanently secure the detecting action needed in a crystal set. This was a modern convenience compared to the tricky business of finding a “hot spot” that would detect. The really cool thing about these crystal receivers is that they produce audio with no additional input power. All the energy necessary to produce the audio is captured by the wire antenna.

It was a curious artifact, and I put it on a shelf on my operating desk, as a radio good luck talisman; but everytime I spied that crystal dector sitting on the desk, I wondered to myself whether it still worked. I would not have gotten much past the wondering stage were it not for the fact that I happen to have a pair of high-impedance headphones – a critical component in any crystal receiver experiment.

High Impedance Headphones

High Impedance Headphones

These phones were given to me by my elmer, W2WTV Gordon (sadly a silent key now for many years). I recall visiting him one Saturday while he was cleaning up in his shack, and him handing these phones to me, saying gruffly “Here take these, you might want to build a crystal radio someday.” I held on to them ever since, not knowing when if ever I would want to build a crystal radio, but they just seemed like something you shouldn’t part with, because they’d be difficult to get a hold of if the need ever arose. I measured the DC resistance of these to be about 2200 Ohms.

So almost against my will, I started to research crystal radio designs. Some of these babies can get very elaborate, after all at one point they were state of the art. There are dozens of designs out there, and I started to filter through them. Patterns began to emerge.

I decided on a variation of the simple “oatmeal box” receiver. Easy to build, it requires a big air wound transformer (hence the oatmeal box) connected to the antenna, a resistor, a variable capacitor, a diode detector, and high-impedance phones.

I was all set, I had all that stuff on hand. I decided to go a little uptown from the oatmeal box, and wound my coil on a piece of 2 inch PVC pipe. Also, I used one of those cheap, ecologically responsible bamboo cutting boards for a base. I think these things are great for any “bread board” project, the small ones cost about five dollars in the produce department of the local supermarket. Easy to drill, and good looking. What’s not to like?

This past weekend, I got to work. Not that there weren’t about twenty more pressing things I should have been doing, but by this point my curiousity about the detector was approaching obsession. First step: recompute the number of turns needed on my piece of pipe, by reverse engineering the oatmeal box design.

Using an online air wound coil calculator, I estimated the inductance of the oatmeal box secondary to be about 500 microHenries, and then calculated the number of turns I would need on the pipe for the same inductance. Using 28 AWG gauge wire, it worked out to be about 100 turns. Sitting down to binge watch old PBS shows, I started winding.

I won’t bore you with the details of winding the coil. I know a lot of guys don’t like winding toroids – this was far, far worse than any toroid. 28 gauge wire is impossible to handle. It was a nightmare. Finally I got the coil done, and measured the inductance. More than twice the calculated value. This thing wasn’t going to resonate any where near the AM broadcast band.

Re-purposing some nice 18 AWG gauge enameled copper wire liberated from my wife’s studio, I wound a second coil of 60 turns, tapped as indicated in the design. Much better experience all around. I don’t recommend using teeny wire for this kind of project.

Taking much more time than it should have, I laid out the components on the cutting board. I didn’t have the specified 47 kOhm resistor, so I used a 51 kOhm instead. Nothing about this design struck me as being all that critical, so I figured close was good enough. With everything wired up, I brought my creation up to the shack, and hooked to my antenna.

Attached to the antenna (new diode)

Attached to the antenna (new diode)

The antenna is a 135 foot doublet, fed with ladder line, with a bunch of stuff between the wire and the shack, like a big balun and an automatic tuner. But since rf does demonstrably get in through this pathway, I figured it might work for the crystal receiver. I hooked the center pin of th PL-259 connector to the top of my coil, and the shell to the ground side. Hooked up the phones, and spun the dial….

Profound silence.

I tried all the taps. No good.

Okay, back to the design docs.

Schematic for my version.

Schematic for my version.

After carefully reviewing the small schematic for the rig I realized I had flipped the sense of the secondary. Could phase matter? Well, just to be sure I switched two soldered connections, and now my construction exactly matched the schematic. Back to the shack, hooked up the alligator clips, and…. nada.

Okay, well it could be the phones – I had never tried them in any other circumstances. Or, it could be my antique crystal detector. I popped the old unit off, and stretched a brand new 1N34 diode across the posts. Connections hastily reconnected, tuned the cap on one tap, then the next… wait, what was that? Yes, faintly, distantly, but unmistakably – salsa music!

I ran downstairs and got my son to come up and listen. He put on the phones, concentrated for a moment, and said “Sounds like a Spanish language station?”

Yes! Success! A soft, vague whisper, but reception nevertheless! My receiver works, and the question answered: the antique crystal detector is a curio only, dead as a doornail.

A little further experimentation reveals that the phase of the coils does matter – switching the antenna and ground connections completely killed the signal. Playing with the receiver late in the evening, I was able to hear four or five distinct stations, but none as strong as my Spanish station, which turns out to be WEPN, ESPN Deportes on 1050 kHz AM in New York.

Switching between taps improves selectivity and reduces sensitivity as fewer turns are selected. I probably could get better reception with a good earth ground and a more direct connection to the wire, but those experiments will wait for some other time. For the time being, I am at peace with crystal radios.

Now about the space charge tube regen receiver. I have found an interesting design that I think will adapt nicely to space charge tubes, and I have prototyped the input RF amp with encouraging results. But this will be the subject of another post. Until then

73
de N2HTT

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A key repair.

Back at the beginning of November I happened to be in the local Barnes & Noble bookstore on a Sunday afternoon, when I noticed signage for the “Mini Maker Faire” event B&N was holding the following weekend. My initial reaction was: “not that interesting”. I had attended the big Maker Faire at the Hall of Science last year, it was fine, very focussed on 3D printing and robotic oriented stuff. For me, not worth a repeat visit.

But then, as I was leaving the store, I saw the phrase “signing up new presenters every day” on the announcement board. That piqued my interest: would they want a demonstration of a ham radio “maker” project?

Left to my own devices, I would have left the store still pondering this question, but for my wife’s prodding to go ask a manager. I did, and they were interested. In fact, the store personnel I spoke to mentioned that although they had reached out to local schools looking for presentations, they had gotten very little response. I got a call the following Monday from the store manager, made my pitch, and was put on the schedule for the following weekend.

The next thing to do was to reach out to my local radio club, PCARA, and see if there was any interest in participating in the presentation and supplying collateral material about Amateur Radio, and our club. I got an enthusiastic response, and checking back with the folks at B&N got permission to display and hand out ham radio materials – I thought this was pretty generous of them.

The project I had to demonstrate was a pretty good fit for a ham radio “maker” project.

DFR front panel

DFR front panel

Called the Digital Fist Recorder, it is an Arduino based device that records and plays back CW sent with a straight key, cootie key, or bug, exactly as sent – thus exactly reproducing the “fist” or sending style of operator.

DFR under the hood (arduino stack on left, keying circuit on right)

DFR under the hood (arduino stack on left, keying circuit on right)

I had written an article on the project for QST Magazine, which had just come out in the November issue, so the timing was perfect.

In the presentation (done at three sessions from Friday evening to Sunday afternoon), I touched on

  • some background on amateur radio
  • the history and use of morse code in radio
  • a demonstration of the DFR, complete with a key that audience members could try out
  • a discussion of the “maker” aspects of ham radio, showing kits and scratch built projects

The presentations went well, I gave out several handouts and lots of interest was generated.

CW Demonstration, B&N Maker Faire (photo by M. Pritchard, NM9J)

CW Demonstration, B&N Maker Faire (photo by M. Pritchard, NM9J)

As part of the demonstration, I knew I would have people wanting to try out the Morse code key, so part of my preparations included selecting a key that would stand up to being used by folks not familiar with the art of radio telegraphy. I have a lot of keys, and I am rather fond of all of them, so I put particular effort into selecting a key that would be robust enough to withstand any unintentional abuse. After much deliberation, I chose my D-117/K5 “Chinese Army Key”.

Chinese K5 key

Chinese K5 key

It is very heavy, solid steel with hard brass and steel contacts. While not my favorite for daily use, one gets the impression that you can bang the hell out of it without complaint.

It performed marvelously, survived without a scratch, and acquitted itself well in the hands of several young kids who learned to send their first names in Morse Code.
And it reminded me of another Chinese key that I own, one that I liked very much, that had met its demise at my own hand about three years ago.

The other key was a K4. This key was manufactured for the PLA in the 1960’s, and was readily available on eBay and at Morse Express for very reasonable prices. In recent years the original source of the surplus keys has dried up, but they are still made at the same plant in Changshu, and are still available today but at much higher prices.

The key featured a very heavy cast iron base, shrouded in a highly chromed steel jacket. A steel lever, supported by robust posts, pivoted on jeweled pivot bearings, and featured large silver contacts. It has a very good feel, and stayed put on the desk.

I liked this key so much that I was using it in the November CW Sweepstakes three years ago when, while rearranging things on the operating desk, I knocked it off. The key hit the ground lever first, and shattered the bakelite pedestal that supports the lever. I couldn’t bear to discard the parts, so I just put them away in a box at the time. Playing with the K5 made me nostalgic for the K4, checking current eBay prices for the key made me consider a repair job.

Examining the parts of the broken K-4, it was apparent that the breaks were clean and tight-fitting. Not too much shattered material had been lost in the break. The key was a likely candidate for a glue repair.

Broken bakelite pedestal

Broken bakelite pedestal

I wanted a glue that would have extremely high cured strength, and some ability to fill in the broken areas. A two-part epoxy seemed to me to be the most likely candidate.

The epoxy should have a very high cure strength, and since the key had been broken for three years, I didn’t think a rapid curing glue was needed. I went searching the adhesive department of our local Home Depot, pondering over the many two-part epoxies available, comparing cured strength, cure time and color to select the most appropriate.

As it turned out I didn’t find a slow cure epoxy that would work; the only one they had was opaque white in color, definitely not a good choice. Second runner-up was Gorilla Glue two-part epoxy, with a 5 minute cure time, which is clear and boasted the highest cured strength. The short cure time was not an advantage in this case, as it meant that all the positioning and clamping had to be done quickly.

On the same visit, while cruising the tools section, I found a new item that I can’t recommend highly enough – DeWalt now makes tiny trigger style clamps, for only about $5 each. These are perfect for clamping small repairs, and I picked up two to supplement the other small clamps in my armamentarium.

I knew that the success of the repair depended on absolute flatness of the glued bakelite block, and good end-to-end compression of the break while the glue set. To achieve this, I used clamps at 90 degrees to each other – two to hold the base flat, and one to compress the glue joint. I needed a flat surface to clamp to. As it turns out, I recently bought at a thrift store a picture frame fashioned from two thick blocks of plate-glass. I intend to use this for some crystal grinding experiments, but in the mean time it made the perfect support for the glue job.

Preparing to clamp glue job

Preparing to clamp glue job

To avoid the embarrassment of gluing my repaired bakelite part forever to the glass block, I covered the block in plastic cling wrap. I figured that even if the plastic wrap clung to the glued joint, it would be easy to pick off. Or at least easier to pick off than the thick glass block.

The last concern I had was the inevitable bit of glue that squeezes out of the joint and sits on the surface, creating a bead that follows the path of the break. A little research suggested that using acetone or mineral spirits once the join was clamped would remove the bead. I had acetone on hand, and am pleased to report that this trick worked perfectly.

The epoxy user's friend

The epoxy user’s friend

The procedure was:

  • wrap the glass block with clingy plastic
  • pre-position the broken bits and clamps
  • mix the glue and apply
  • clamp the joint – starting with one end of the piece down to the block, then all the parts together, then the other end to the block. This involved a little fiddling to get it all snug
  • with acetone on a paper towel, clean up the glue bead
  • leave the whole thing to think about it for at least 24 hours
Glue job at 23 hours 59 minutes... wait for it..

Glue job at 23 hours 59 minutes… wait for it..

That last step is the hardest, but you really need to allow the epoxy to rest undisturbed for a good while to reach full cure strength. Anyway, the key had been on the disabled list for three years, so what was the rush?

After the curing time was over, the bakelite base came off the block easily, and it was no problem to pick off the clingy plastic. I then re-assembed the key, and had to deal with the matter of the conical spring.

Unfortunately, the original conical spring was lost. Whether I lost track of it when the accident occurred, or it got away during captivity in storage is not clear, but it is definitely gone. A quick check of internet sources for conical springs shows that:

  • you can get any spring dimensions and springiness you want from spring suppliers
  • they won’t sell you less that 25 or so minimum order

Not wanting to invest that kind of money in a world class conical spring collection, I looked to other venues. Morse Express sells replacement springs for Nye-Viking keys; I ordered a couple and although they are very nice, they are much too small for the K4.

I then took a look on eBay for either springs, or busted keys with the springs still attached. I found an auction for three springs that were a bit larger. These turned out to be usable (it’s what you see in the photo), but I am keeping my eye out for other possibilities in the future.

The K4 key, after repair

The K4 key, after repair

All in all, the repair was a great success. The key handles just as it did before the disaster, and I’m really pleased to get it back in service.

73
de N2HTT

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