Sunday, September 3, 2017

Huffing Lacquer

After building up the preamp for my Crosley 86CR, I decided that it was time to finish that radio completely.  You see, I repaired, stripped, grain filled and prepped the thing for lacquer over two years ago, and have using it in a partially assembled state ever since.

I like working with lacquer because it gives me lots of somewhat valid excuses for procrastination:  It's too hot, too cold, too humid, etc.  Since we've had temps in the upper 60s to low 70s, with low humidity, for about a week, I just couldn't put off breaking out the HVLP sprayer and going to town.

Unfortunately, I didn't photo-document the process... I never intended to write it up... but here we are.  I'll do my best to paint a word picture.

Here's what I started with in 2015:

It was actually in decent shape, though there was some veneer damage to the dial panel and a few dents from having things dropped on or banged into it.

To repair the veneer, I removed the dial panel and removed a section of veneer from a location that is hidden behind the side panel and surgically grafted it into the spot where the exposed veneer was damaged.  It took a few hours, but turned out well - the repair is all but invisible.

To work out the dents, I employed the XYL's iron and steamed them out.  This was easy - just lay a moist cloth over the dent, heat it with the iron and the fibers of the wood expand and the dent disappears.

After these and a few other minor repairs, I completely stripped the cabinet using Citru-Strip.  I like this stuff because it doesn't stink, so I'm able to do it indoors.  The trick to using it is to get a feel for how long to leave it on before scraping it off; too soon or late and you make extra work for yourself, but find the sweet spot and you'll have naked wood in no time.

Once stripped, it's time to sand.  This set has some fairly beefy veneer, so I was able to get quite aggressive with it in some spots.  It's not rocket science, start with a coarse grit and keep working it and progressing to finer grits until it's smooth.

But, even when it's smooth, you still have to fill the pores, which is my least favorite part of the job.  Ever see a refurbished antique that, when viewed from different angles, looks like it's had glitter splashed on it?  They skipped this step.  Again, it's not rocket science, the stuff goes on sort of like a slurry and you rub it into the grain, then scrape off the excess by dragging something like a credit card sideways across the grain.  The more you're able to scrape off, the less sanding you'll have to do in the next phase.

More sanding, this time with 400 and then 600 grit until the surface is as smooth as possible.  Light should reflect from it when it's done.

Then it's out to the garage for sprayin'.  Wear a respirator, the stuff ain't good to breath.  Before doing any color coats, I spray a very thin "spit coat" of lacquer over the entire cabinet.  The purpose is to seal the grain and all that grain filler so that it stays put.  If you don't, it'll all come out when you apply masking tape - ask me how I know...

Once that dried (overnight), I taped off the field and shot the trim with a couple of coats of heavily tinted lacquer. I had to play with the amount of dye to get the color right.  It's good to have lots of scrap boards from different species to experiment with.  Lacquer dries to the touch fairly quickly, and I removed the masking tape as soon as I was able to do so.  I've had bad experiences from leaving it on longer than necessary, so once I'm able to handle the piece, it's gone.

Then came the color coats, where the whole cabinet was shot with several coats of dyed lacquer - again, the correct color and shade was determined by experimenting on scrap, and then the "formula" noted so that I'm able to duplicate the mix.  I wasn't counting, but I'd guess I shot 6 color coats over two days before being satisfied that I had enough coverage.  I should mention that I use gloss lacquer for these coats; I think it covers better, but that's just me.

Once the color coats have cured (24 hours or so), I wet sanded the cabinet, first with 400 and then 600, before shooting the top coats.  I did dye these, but not as dark as the color coats.  As before, I wasn't keeping track, but I probably shot 5 or 6 coats of satin lacquer, then let it cure for a couple days before bringing it indoors for assembly.

Which is where we are now.  I apologize for the poor quality picture, the lighting in the shop is terrible for photographs.

 Yet to be installed is the speaker box (more on this at another time), grille cloth and decorative grill.  It's close to midnight, so I'm done for today!

Wednesday, August 30, 2017

Noodling The Next Rig: What If?...


There's a good chance that anyone who reads this has at least heard of Tony Parks and his Softrock SDR kits (www.fivedash.com).  If not, check 'em out - they're fun to build and an inexpensive way to wet your beak in the world of SDR.

I've got a number of his older (V6.2) transceiver kits built for different bands and can't say enough good things about them, so this is, by no means, a criticism:  While they work fine and do everything well, the operating experience is, as Bill, N2CQR would say: Very appliancy.  

This got the wheels turning: Why not use the SDR as the "back end" (modulation/demodulation and audio) of a "traditional" transceiver?  The idea's been festering in my mind for a few years now, I suppose it's time to lance that boil.

As with any other project, if you want any chance at bringing it to a successful conclusion, you need to start with a plan.  At this stage, I know what I want:

- 80 - 10 Meter coverage.
- Power output > 20 Watts.
- Front panel touchscreen display.
- Rotary controls for audio and tuning.
- Integral processor (Rasp. Pi) for SDR.
- Arduino Pro-Mini for user interface and system control.

So, I took those requirements and drew the above block diagram to use as a road-map.  As I said before, the only way to eat an elephant is one bite at a time.  This is the basic recipe I'll use for making elephant sandwiches. 

Moving forward, I'll determine the specs for each block, which will lead to more noodling before I even think about melting any solder.  Sure, it's fun to just jump in and make shi... stuff up as you go, the problem is that projects started that way tend to either go unfinished or don't work right when they are.

Tuesday, August 29, 2017

An Adequate Preamplifier for the FM band.


I've been trying to focus on cleaning the shack, but keep falling victim to spontaneous construction.

The other night, I fully intended to get some organizing done, so I went down to the shack and fired up my trusty old Crosley 86CR so that I could catch the Tigers game while going about the task at hand.  The 86CR is an interesting old set in a couple ways:  It covers the current FM band (was manufactured in '47), but the dial is calibrated in FCC channel numbers rather than the more familiar frequencies of 88-108 MHz.  It also uses an odd double conversion scheme, 5825 and 167.5 KHz, on the AM and SW bands.  

Apparently, the 86CR wasn't intended to be used from the basement of a home at the fringes of a metropolitan area, as it lacks an RF amplifier ahead of the mixer.  So, as I moved around the shack while listening to the ball game (on FM), the signal would fade in and out.  Irritating.  Gotta fix that, but don't want to put much time, effort or money into it.  

I call the circuit that I whipped up "The Adequate Preamplifier".  It's nothing particularly special or unique, costs about a buck, and gets the job done.


 The circuit uses a pair of J-310 JFETs in cascode.  Some folks like to think of this as being similar to a dual-gate FET, which I suppose it is, but when I look at it, I see a common-source amplifier (Q2) direct coupled to a common gate amp (Q1.)  Tomato, tomahto.  

The input network provides some semblance of a bandpass filter.  The LC network is comprised of L1, C3 and the gate capacitance of Q2.  With the antenna - a couple feet of wire dangling behind the set - connected as shown, there's enough load on the circuit for it to provide gain across the FM dial without retuning.  

The output transformer, made from a broken TV balun, provides balanced outputs to mate to the old Crosley. 

Performance?  I made no effort to quantify anything - it's got gain and doesn't oscillate, and the voices of Jim Price and Dan Dickerson are coming through the speaker loud and clear.  Performance: Adequate.

The next step will be powering it from the Crosley and hiding it below deck.  It doesn't draw beans for current, so maybe I can power it across the cathode resistor of the 6V6...

At this rate, I doubt the shack will ever get cleaned.

Tuesday, August 22, 2017

Improved Mightier Mite!

All homebrewers have tremendous egos, and I'm no exception.  But, if you've got half a brain and N6QW points out a potential problem with your design, you damn well better listen!

Pete mentioned that he'd had problems with key clicks in one of his rigs where he keyed the final amp stage as I was doing, and suggested that I might be better off keying the driver stage.  I hadn't noticed any clicks while testing, but I figured I'd better give it a close look.  When I did, I still didn't hear "clicks", but the keyed waveform was definitely not optimally shaped.  I was also having problems with my "Discreet Transistor Keyer", which wasn't at all happy trying to source an amp of current.

So, I followed Pete's suggestion and modified one of the prototypes by adding a PNP on the supply-side of the driver transformer's primary and played around with some RC values (using stray parts floating around on the bench) and came up with a combination that works fairly well.  The revised schematic is shown below:


I think I mentioned this in an earlier blog entry, but you'll notice that I'm kind of anal about version controlling everything. 

I don't do that for show - I do it because I've got a crappy memory and it helps me keep track of things.  It's a discipline that my friend and Engineer Extraordinaire Bill Smith (normal person: no call letters) drilled into me, and I freely to admit that it's saved me a hell of a lot of time that would have been otherwise spent chasing my tail.

So, my advice for anyone pursuing the experimental method of anything is: Keep lots of notes, rev everything and maintain a history log.

73 de Steve N8NM

Sunday, August 20, 2017

A Mightier Mite from Michigan


Pete, N6QW, has recently posted an excellent blog entry where he talks about (what else?) building stuff and the importance of starting small and progressing to more complex designs as one builds skill and confidence.  The Michigan Mighty Mite, a single transistor CW transmitter, is an excellent place to start: It uses a minimal number of parts and is easy to duplicate; if one follows directions, it's almost hard to NOT make it work.  But, once you build one, where do you go next?  There's a quantum leap between a MMM and something like a BiTX, and, let's face it, it's certainly possible to make contacts at a fraction of a Watt, but it's not easy - especially for someone without much experience in doing so.

That led me to come up with a "next step" project, something a bit more advanced than a Mighty Mite, that'll put out a reasonable amount of power, while still using a minimal number of common, inexpensive parts - all of which are available from outlets like Mouser or Digi-Key.  So, I present to you: The Michigan Mightier Mite.

While this rig uses only three transistors and one IC, it is capable of delivering a fair amount of power to the antenna; each of the prototypes ran about 7 Watts out of the requisite low-pass filters on 40 and 80 meters while using a 13.8 Volt supply - more than enough power to make contacts on the QRP watering-hole frequencies.

The theory of operation is straightforward:  The rig is comprised of four circuits: Oscillator, Driver Amplifier, Power Amplifier and Bias Supply (for the Power Amplifier.)

The Oscillator (Q6) is your typical Colpitts crystal oscillator; R1 and R3 set-up the operating bias for the transistor, while C4 and C5 provide feedback to get it oscillatin'.  Output is taken off of the emitter through C6.  I tried a number of random crystals in the prototypes and the oscillator took off nicely at frequencies between 3.5 and 14 MHz.  It could possibly work at higher frequencies; my only 10m rock wouldn't oscillate, but that could be a bum crystal.

The Driver stage (Q2) uses a BD-139 transistor to goose the output level from the oscillator to the several volts needed to drive the gate of the Power Amplifier.  R3 and R4 set up the operating bias, R5 provides some degenerative feedback to aid in stability (C8 bypasses R5 so that the signal gain isn't reduced) and T1 matches the output impedance of the Driver to the input impedance of the Power Amp.  Q2 will need a heat-sink; in my prototypes, I mounted it directly to the surface of the PCB and this seems to be sufficient.

The Power Amplifier (Q3) uses the ubiquitous IRF-510 to deliver QRP+ power to the low-pass filter through impedance matching transformer T2.  It is absolutely essential that this transistor get's as big of a heat sink as practical, those little TO-220 sinks are marginal, but no need to get spendy: Find a defunct PC and liberate its CPU heat sink - those are more than adequate, and usually free.

 U1, an LM78L05 Voltage Regulator, provides a reference for the '510's gate bias. This bias is adjustable via R6, which, initially, should be adjusted for 0V.

Construction and testing should take place in the following order: Oscillator, Driver, Bias Supply and, finally, the final.  Only after one stage is complete should you move on to the next.

The only adjustment, and this is critical, is the PA bias.  It is absolutely essential that you start with the pot set so that the center pin is at ground potential.  Adjustment is simple - power up the rig, close the key and slowly advance the bias until you reach a peak in the output power, then STOP!!!  Going beyond the peak will cause the IRF-510 current to rise quickly, ultimately destroying the transistor.  Get a few extras - they're cheap.  If you have an ammeter, the rig should draw about an amp while transmitting - a little more or less is OK.

Something that may look odd are all of the .1uF and other caps that go from the 12V supply rail to ground.  RF does weird things, so it's good practice to "bypass" the supply line near each active device, especially the IRF-510.  This shunts any stray RF infecting the supply line to ground, hopefully preventing unwanted oscillations.   I might be lucky, but my prototypes were all unconditionally stable.  Your mileage may vary, but solving those problems is part of the game, so get used to it!

Once you have the rig running, you can "customize" it to suit your station; one possibility is to use a double pole switch for SW1 and use the second pole to mute your receiver, but the most fun modification is to try running a higher voltage on the IRF-510; depending on the transistor, it's not unusual to be able to drive these to 15 Watts or more with a 24 volt supply.

Even if you're not a CW fan, this is a worthwhile project and not just an acedemic exercise; with a few minor changes, this transmitter can be converted into a linear amplifier strip for use with an SSB rig.

73!

Tuesday, August 15, 2017

New Life to All American Five - Part 3: Patient Survives.

Where we started two days ago...

Done.  


The rest of the repairs were comprised of changing out all of the paper capacitors with modern mylar film components.  This is a pretty boring process and I'll spare you the blow-by-blow description, suffice it to say that I replace one at a time (unless one cap is blocked by another - in that case, two at a time...) and test the set after replacing each one.  This isn't so much to see if the new part improved performance as much as it is to catch any mistakes - I must have been paying attention while working on this one because I didn't screw anything up.

The cosmetic "restoration" was primarily just cleaning 70 years worth of crud off of everything and replacing the speaker cloth.  The veneer and original lacquer finish weren't terrible, so I opted to refresh rather than refinish it.  That's the nice thing about lacquer - you can clean, repair and "reflow" it - what I like to call a "scuff and blast".  I prefer to do this whenever possible, not only because it's a hell of a lot less work, but it usually looks more appropriate when some of the scars of age show through.

Here's a short video clip of the set in action:

video

So, that's that.  As you can see in the background, my bench is utter chaos and I really need to spend a few days reorganizing the junk.  Don't think I'll be blogging about that - but my plans for my next project are starting to gel.  I'll drop a hint because I'm kind of excited about it:  It'll be a combination of traditional radio technology and software definition - stay tuned!

73 - Steve N8NM

Monday, August 14, 2017

New Life to All American Five Part II - Filter Capacitor Replacement.

Easily the most common failure point with vintage radios are the electrolytic capacitors, primarily those in the power supply.  In anything over 40-50 years old, it's virtually guaranteed that the filter capacitors have either failed or are soon going to.  While I generally abhor the "shotgun" method of replacing anything that could possibly be bad, I do replace filter caps before powering up any piece of ancient electronics. 

This set uses a fairly typical arrangement for All-American Five tube sets: A single cylinder containing three electrolytics - Two 40 uF 160 Volt capacitors to filter the power supply and one 20 uF/25 volt capacitor used to bypass the cathode of the audio power amplifier tube.  I've heard that there are vendors who can supply original-style replacements, but the original can be "restuffed" with new replacement parts for about the cost of shipping of the original-style replacement.  So, being cheap, that's what I'll do with this set.

This is the "before" shot of the capacitor mounted on the top of the chassis.  Note the phenolic insulator at the base - this must be re-used and care taken that the capacitor housing doesn't short to the chassis as it actually "rests" a couple volts below ground potential.


The view from below.  This is where I scribble notes detailing what goes where before I warm up the iron - I'm not going to remember how it's wired, and tracing from a .pdf scan of a photocopied schematic is a drag.


The cardboard cover of the capacitor is typically held to the aluminum cylinder by a blob of tar.  This was the hot-glue of the day!  



   
To get at the guts, I carefully undo the crimp where the cylinder is attached to the base. 

  
The dried-out capacitor elements can usually be removed by pulling it out by the terminals using a pair of pliers.  Some fight harder than others, and you have to be careful not to damage the terminals as we're going to reuse them.  In this cap, the guts were held to the cylinder with another tar blob - pretty typical.


The new caps are installed to the original terminals.  In order to wire them, drill small holes in the base plate and route the leads to the underside.  The original wires extending from the terminals are aluminum and won't take solder.  With a bit of care, the holes/wires can be all but invisible. 

I secure the caps and wiring with a little tape, just in case someone ever decides to operate the radio in a paint shaker.

The guts are stuffed back into the cylinder and the cylinder is re-crimped to the base.

Bottom view.  The crimp isn't pretty, but it's not going to show.

All the wires get reconnected according to the notes taken earlier.  Notice the yellow capacitor with the green leads - This was connected to one of the terminals on the electrolytic, and since I intend to replace all of the old paper caps anyway, I replaced it while it was halfway disconnected.

And the "After" shot.  Looks the same as the before - which is exactly what I want!

This is where I stop, visually inspect everything and - using an isolation transformer - apply power to the set and check performance before proceeding to replacement of the paper caps. Most of the time, the set will come to life somewhat - there are bound to be other problems.  These should be noted before continuing.  This set was pretty typical, it played well at first, but slowly crapped out over time.  Troubleshooting revealed a bad rectifier tube - no big deal, got one coming!  Meanwhile, I cobbled together a solid-state replacement so that I'll be able to continue refurbishing it while waiting on the tube.

This looks like a good stopping point - More to come!

 

Sunday, August 13, 2017

Giving New Life to an All American Five - Part 1.

I have a weakness for old radios.  I don't know how many I have - easily more than 100 - but I still don't consider myself a collector because I don't have the desire to accumulate specific makes or models.  Nope.  I just like working on them and, when something interesting comes up at the right price, I have a hard time walking away.

Earlier today, my wife and I met friends Sean, WX8L and Jeff, KF8XO and their wives Andrea and Karen for a trip up to Port Huron where there was a ham radio "trunk sale" being held along the shore (seawall, actually) of the St. Clair River.  While it wasn't a "full blown" hamfest, there were some cool items, including several boat anchors, being offered up for sale at reasonable prices.

While I was able to resist the siren song of the R-390A (already have one), NC-300 (Wife would kill me) and Mosley CM-1 (that one was tough to resist), I couldn't pass up the grimy little Silvertone 7054-J with the $12 price tag, especially when the seller said I could have it for $10.


For whatever reason, this little set "spoke to me", so while I'm pondering on my next homebrew project, bringing this fellow back from the dead will give me something to keep myself busy (and keep this blog active.)  



So, where do you start on something like this?  I like to start by cleaning the heavy dust and crud from the chassis, and, as you can see above, this set had more than it's fair share.  I'm not out to detail the chassis - just get it to where it's not as gross to work on.  a few minutes with a semi-firm bristle brush makes a big difference:


While cleaning, I noticed that the #47 dial lamp was open, so I replaced it.  Low hanging fruit; one less thing to deal with later.

This set has push-button presets that operate mechanically, and, as you'd expect, the mechanism was gummed-up with 70 year old grease.  Another low-hanging fruit: cleaned, adjusted and re-lubed the mechanism, including the tuning capacitor. 

The most common failure item in old radios is not, as most people assume, the tubes.  Nope, it's the capacitors.  In a radio this age, it's a virtual certainty that the electrolytic capacitors in the power supply will be shot, so I checked the value of the originals and made sure I had suitable replacements on-hand.  


The "can" in the above picture contains the aforementioned power supply caps - it's a "three-in-one" affair: The can contains two 40 uF, 150 volt caps and a 20 uF, 25 volt cap.  Pretty typical - what I'll do is "restuff" this can with modern replacements (47 uF/160V and 20 uF/35V.)  You can usually go a bit higher in capacitance (within reason) and can always go higher in working volts, and those will be fine replacements.

But, it's 2300 hours and I'm not going to get into that tonight.

73 - Steve N8NM

Saturday, August 5, 2017

Discreet Transistor Keyer - Part 5: Stick A Fork In It - It's Done.

The DTK sitting atop the SR-16.

After spraying the scrap metal enclosure with a coat of paint (to match the SR-16), it doesn't look bad at all. 

Getting reacquainted with the "TO" style of keyer hasn't been as daunting as I'd expected; I'm getting better at it now that I've learned to simply slow down and not "get ahead" of the keyer; by that, I mean wait until the break after a dit or dah before pressing the paddle to start the next (unless sending a series of dits or dahs, in which case you just hold the key closed as with a "Curtis" style keyer.)

In the meantime, I've discovered a new digital mode: FT-8, and have become somewhat addicted to it.  The mode is available in the WSJT-X version 1.8 release candidate that's available at: https://physics.princeton.edu/pulsar/k1jt/wsjtx.html

If you're familiar with JT-65, FT-8 is a lot like JT-65 after too much espresso.  Transmissions take place every 15 seconds as opposed to ever minute, so the pace is much quicker - almost contest-like.  The trade-off is that the ultra-weak signal performance isn't there and QSOs with signals much lower than 15 dB below the noise floor often require repeats to complete.  Still, it's loads of fun!

73 - Steve N8NM

Wednesday, August 2, 2017

Discreet Transistor Keyer Part 4 - Building the "scrap metal" enclosure.

In my last post, I mentioned Master Homebrewer Pete, N6QW's recent blog showing how he builds beautiful enclosures using an inexpensive bending brake.  Pete, a true craftsman, did a marvelous job, as any craftsman who takes pride in his work would.

Now, I'm going to show you the other side - how to quickly bend up a simple enclosure from a piece of scrap 22 ga. aluminum.  Because for some projects, simply being good enough is good enough!

I started with a couple of scrap "rails" that I had left over from another project and the keyer's circuit board:
Laying the bits on the bench, I took a couple of measurements and determined that the box would be about 4 1/2 inches square by 1 1/2 inches high.  The easiest enclosure to make (in my opinion) is the simple "clam shell", where you have  top and bottom panels that slip over one another to form a box.  Since I already had the rails for the front and rear, this one will be easy because I only need two bends in each panel. 

For the bottom, I laid out the dimensions on the scrap aluminum sheet.  Since the dimensions are 4 1/2" square and I'll need about a 1/2" "lip" on the left and right sides, I cut the piece to 4 1/2 x 5 1/2" using a pair of shears.



In the last picture, you can see the lines drawn on the soon-to-be bottom panel that show where the bends will be, um, bent.  Now, it's off the the "back room" to do the bending!
Lined up and clamped in the brake.  For a small bit like this, a single vise-grip is enough to hold it in place.  I use multiple C-clamps when working with larger pieces.

One side bent - square bend in seconds, try doing that the way the old handbooks tell you to. 


And repeat for the other side...


Nice!  
The top is formed the same way, except that I left a full-height (1 1/2") "overhang" on each side instead of the 1/2" used on the bottom panel.
The components, ready for assembly.



When I'm in the mood, I've got a jig that I made to drill mounting holes in the corner of PCB with some degree of precision.  This isn't one of those times, so I laid the board where I wanted it to go, drilled the hole for one corner and fastened it with a screw and nut.
The single screw/nut hold the board in place while I drill the other three, and then all four corners get fastened.


Next, I fastened the front and rear panels to the bottom with pop-rivets.  Unfortunately, I didn't capture the excitement photographically, but trust me, it happened.

Now for the only "exotic" piece of hardware in this entire project: Rivnuts! 
Fastening the top of the enclosure to the box means that I'm not going to be able to use screws with nuts, and if I use rivets, then I'll invariably have to drill them out to fix something.  I could use sheet-metal screws, but they eventually get sloppy after being undone-redone a few times.  Rivnuts are cool; they're threaded inserts that attach like a rivet - the tool looks like a pop-rivet tool that, rather than having a hole for the rivet "lead", has a threaded stud.  Installation is a snap - drill the hole (for #6-32 inserts, drill a 9/16" hole), screw the Rivnut onto the tool, insert, squeeze the handle and bingo!


And that's it.  The finished product is certainly "good enough"; with a little body-work (filing the edges smooth and massaging out any dents) and paint, nobody will know that it was whipped together in about 45 minutes from a piece of scrap.


73! 

Monday, July 31, 2017

Discreet Transistor Keyer, Part 3 - Final design, Up and Running!


In Part 1, I mentioned that this wasn't going to be a clone of an older design.  Well, I lied... Kind of.

For the past few nights, I've been trying different circuits and deep-diving the design of Jim Ricks' (W9TO) original tube keyer, the Hallicrafters HA-1 and HA-4 variants, Heath's HD-10 and W2YM's keyer from the Spring, 1964 edition of RCA's Ham Tips.  I gotta tell you, that Jim Ricks was a brilliant son of a gun!  While, in my various prototypes, I certainly succeeded in making the keyer more complicated, I failed to make it better!  So, rather than reinvent the wheel, I merely adapted the design to use the modern parts that I have on-hand.

The schematic diagram below shows the sum of my efforts:


Those familiar with the circuits I mentioned earlier will immediately find mine familiar, the main difference is that I've "scaled" the circuit to operate from a single-ended 9 volt supply, while the earlier designs required "split" positive and negative supplies; those circuits used "negative logic" PNP transistors and the positive supply created cut-off bias for the bi-stable "DAH" flip-flop. 

I could go into the Theory of Operation, but that would mean a lot of typing and I haven't taken any photos to break it up yet, so I'll suggest anyone interested either drop me a line or check-out the Hallicrafters HA-4 manual (available on the BAMA site.)

So, what's it like to use?  In one word: humbling.  I cut my teeth on a HA-1, but it's been years since I used a non-iambic keyer and let me tell you: modern (Curtis A or B) keyers can masquerade your lack of rhythm.  You have to sync yourself up with the timing of a "TO" keyer, but once you get in the zone, it's not a problem - and the guy at the other end will certainly appreciate your perfectly timed fist.  I'm not ashamed to admit that it's going to take some practice before I'm ready to put this thing on the air!

So, my next entry will detail fabricating the chassis and enclosure from sheet metal.  If you follow N6QW's blog, he posted his method for doing this about a month ago, and frankly, mine's not much different.  Pete's corners are probably much more square than mine, but I don't think anyone's awarding style-points.  Seriously, though, the $40 18" Harbor Freight bending brake is a worthwhile investment - have you priced pre-fab boxes lately? Ouch!

Time for me to practice my sending...

73 - Steve N8NM 

Tuesday, July 25, 2017

Discreet Transistor Keyer - Part 2 - Refinements.


After building a prototype of the first circuit, I found a few things that weren't very good:  First, the speed and weight pots interacted significantly due to a flaw in my design.  I'll call it a brain fart.  The adjustment range was pretty much unusable as well, and the output waveform was pretty funky.

 Since it kind of worked, I did some further noodling and came up with the tweaks shown in the circuit above.  While I haven't built it in the physical world yet, I've modeled it in LT Spice and it looks pretty good.  The adjustment range for the speed control doesn't go as slowly as I'd like (about 13 WPM), but I seldom send any slower than that anyway, and the mid-scale speed of about 25 WPM suits me just fine. If I find that I need it to go slower, I can always add a "range" switch to add more R or C to the circuit - not a big deal.

With any luck, I'll have time to build this tomorrow and see if it works as designed.  Stay tuned!

Sunday, July 23, 2017

Mini-Project: Discreet Transistor CW Keyer; Part 1


Summertime in SE Michigan is painfully short, so I try to make the most of it while it lasts, reserving my hibernation-in-the-shack time for the seemingly endless winter months.  There are days, though, when the humidity and, here in swamp country, the mosquitoes make outdoor activities somewhat unpleasant.  So, I like to keep a few short-term projects in the queue for those occasions where I need to spend some time in the air conditioning.

My favorite mode is CW, and even though I'm a proud member of the SKCC (#3173), I don't mind admitting that I prefer using an electronic keyer.  And, while I love my old AEA "Morse Machine", there's something inappropriate about using it with my homebrew rigs.  I mean, if I can design and build my own SSB/CW transceivers, I damned well ought to be able to build a decent keyer, right?

Actually, I've done it before, but that one uses a microcontroller.  It works fine, but I want to do something "Old School", using no ICs.

When I first started "noodling", my thought was to, basically, build a solid-state version of the old Hallicrafters HA-1 "TO Keyer", so I immersed myself in the manual until I understood what made it tick.

The concept is incredibly simple: The DITs are formed by an astable multivibrator, and the DAH's formed by "ORing" the output of the DIT circuit with that of a bistable multivibrator.  Cool.  That gives me a starting point, now to make it happen in silicon.

Actually, it's been done - Hallicrafters had their HA-4 and Heath had their HD-10, but I want to come up with my own circuit rather than copy someone else's.  I also want to run this thing from a 9V battery, and both of the aforementioned designs require "split" positive and negative supplies.  So, while there is going to be some similarity to these earlier designs -  there's only so many ways to make a transistor multivibrator - this project won't be a clone.

I'm going to do something a bit different than I have in the past in that I'll post to this blog as I go, rather than waiting until the project is complete to do a "wrap-up" series.  Because of this, there are bound to be some mistakes along the way, but that's part of the fun, right?

Anyway, might as well start with the easy part: the DIT circuit:


Transistors Q1 and Q2 form the astable multivibrator, which is essentially a free running oscillator that produces square waves by feeding the output of each transistor to the input of the other.  The frequency and duty cycle are determined by C1, C2 and R1, R2 and R3.  By using variable resistors for R2 and R3, we're able to vary the speed and weight (element to space ratio) from the front panel.

When the DIT key is open, transistor Q3 is turned-on through R8, which forces Q2 off by forcing it's base low.  When the key is closed, Q3 is turned off, allowing Q2 to be turned on by Q1 and allowing the multivibrator to multivibrate.

R7's purpose in life is to create the "self-completing" dits by creating something of a "Wire OR" with the key input: If either the key OR the collector of Q2 are LOW, Q3 is turned on.  So, if the key is opened halfway through a dit, Q2 will keep Q3 on until the dit is complete.  Cool stuff, no?

This is what's running on the solderless breadboard pictured earlier - I'll be tweaking the timing resistor values as I move along, but this is a start...

73!

Thursday, July 20, 2017

Knowing Enough to be Dangerous

In my last post, I mentioned that, even though I'm not wild about computers, they do sometimes make for decent tools, so I try to know enough about using them to be dangerous.

Now that I'm "officially" finished with the SR-16 project, I'm working on ideas for the next one, which will be an even more ambitious attempt at a "contest grade" multi-band HF transceiver.

In building the 30m rig, I initially made a bad choice of IF frequency - 13.51 Mhz - which placed the 2nd harmonic of the VFO smack in the middle of the band.  The easy fix was to move the IF to 13.56 so that the harmonic of the VFO fell outside of the band, but that also required additional bandpass filtering to manage the now out-of-band spur.  Moving forward, I'm determined to avoid that by analyzing my choice of frequencies and avoiding those that interact unpleasantly.

This requires repeated calculations, something I hate doing but also something computers are pretty good at.  So, knowing enough to be dangerous, I wrote a little C++ program to crunch the numbers:

#include <iostream>
using namespace std;
int main ()
{
  float lo;
  float lowend;
  float highend;
  float step;
  float rf;
  int harmonic= 1;
  float spur;
  float range;
  int harmonicNumber;
  cout << "Please enter the LOWEST receive/transmit frequency in MHz: ";
  cin >> lowend;
  cout << "Please enter the HIGHEST receive/transmit frequency in MHz: ";
  cin >> highend;
  cout << "Please enter the STEP in MHz ";
  cin >> step;
  cout << "Please enter the IF frequency in MHz: ";
  cin >> rf;
  cout << "Please enter the number of harmonics to extend to ";
  cin >> harmonicNumber;
  cout << "The tuning range you entered is " << lowend <<" to"<<" MHz.\n";
  cout << "The IF you entered is " << rf <<".\n";
  cout << "Analyzing to the " <<harmonicNumber << "th Harmonics... \n";
  for(range = lowend; range <= highend; range = range + step)
  {
    //lo = rf - range; // low side injection
    lo = rf + range; //high side injection
    for(harmonic=1;harmonic <=harmonicNumber;harmonic ++)
    {
        spur = lo * harmonic;
        if ((harmonic > 1) && (spur > (lowend*.9)) && (spur < (highend*1.1)))
        {
            cout<<"Warning!!! When tuned to " << range <<"; the " << harmonic  <<" harmonic of the LO ("<<lo<<") is: " << spur << ".\n";
        }
        spur = rf * harmonic;
        if ((harmonic > 1) && (spur > (lowend * .9)) && (spur < (highend * 1.1)))
        {
            cout<<"Warning!!! When tuned to " << range <<"; the " << harmonic  <<" harmonic of the IF ("<<rf<<") is: " << spur << ".\n";
        }
        spur=lo*harmonic+rf;
        if ((harmonic > 1) && (spur > (lowend*.9)) && (spur < (highend*1.1)))
        {
            cout<<"Warning!!! When tuned to " << range <<"; the " << harmonic  <<" harmonic of the LO ("<<lo<<") + the IF ("<<rf<<") is: " << spur << ".\n";
        }
        spur = lo*harmonic - rf;

       if ((harmonic > 1) && (spur > (lowend*.9)) && (spur < (highend*1.1)))
        {
            if (spur>0)
            {
                cout<<"Warning!!! When tuned to " << range <<"; the "<<harmonic <<" harmonic of the LO ("<<lo<<")- the IF ("<<rf<<") is: "<<spur<<".\n";
            }
        }
        spur=rf*harmonic+lo;
       if ((harmonic > 1) && (spur > (lowend*.9)) && (spur < (highend*1.1)))
        {
            cout<<"Warning!!! When tuned to " << range <<"; the "<<harmonic <<" harmonic of the IF ("<<rf<<") + the LO ("<<lo<<") is: "<<spur<<".\n";
        }
        spur=rf*harmonic-lo;
      if ((harmonic > 1) && (spur > (lowend*.9)) && (spur < (highend*1.1)))
        {
            if (spur>0)
            {
                cout<<"Warning!!! When tuned to " << range <<"; the "<<harmonic <<" harmonic of the IF ("<<rf<<") - the LO ("<<lo<<") is: "<<spur<<".\n";
            }
        }
    }
  }
  main();
  return 0;
}
If this looks like gibberish, here's something that I hope will be encouraging: I'm not a programmer, not terribly bright, and figured out how to do this in a couple of hours.  If I can do it, anyone can!

Anyway, this program runs in the Linux terminal and will prompt you for the low and high ends of the band of interest, how many points within the band to analyse, the IF (BFO) frequency and the number of harmonics that you want the analysis to look at.  Then it crunches the numbers, and displays any "hits" that occur within 10% of the band edges.  If there are no hits, it dumps you back to the beginning (exit with "control-C".)

Here's a screen shot of running an analysis of the 40m band (in 1 Hz increments) and an IF of 6.144 MHz, running out to the 100th harmonics.  This took about 2 seconds for the computer to crunch the numbers and give an "all clear".

Here's one where I chose a bad IF frequency so that it'll show some "hits":


I literally had this written within a couple of hours of writing my first "Hello World" in C++.  It helped that I understood the math and some basic programming concepts, but yeah - this is something anyone can do, so there's no reason to be intimidated by it.

I promise, I'll get back to the Real Radio Stuff in my next post; I just wanted to write this one to show an example of how a computer and **very little** knowledge of programming can be a useful weapon in the homebrewer's arsenal.

73 Steve N8NM


Tuesday, July 11, 2017

SR-16 Arduino Sketch "Module" - Simple Interrupt Service Routine to Generate CW Side Tone.

I'm going to start this post with a proclamation: I am not a programmer.  Truth be told, I'm not all that wild about computers, but, like any other tool, they've got their purpose, so I try to get along with them when it suits me.

I've got some friends for whom the computer is the means and the end - it's the computer itself that provides the entertainment.  Me, not so much, but, being a guy who spends his free time building ham radios in his basement, I don't think it's my place to judge.  Different strokes.

That being said, computers are usually good at doing things that are mind numbingly routine and stupidly boring, and they do them at lightning speed and with consistent results - something I couldn't do if I wanted to. But, enough about my personality quirks, let's get on to the good stuff.

The Arduino has the tone command that Farhan and others (including me) use to generate the CW sidetone that is (after filtering - it's a square wave) sent to the audio chain for monitoring and the balanced modulator for transmission. 
So, in the code, you're monitoring the input connected to your key and, when it's closed, sending a tone to the appropriate output.  Once you've defined the input and output pins, the code to do this is alarmingly simple:

int buttonState = digitalRead(KEY_IN);  // read "KEY_IN", save value in "buttonState"
  if (buttonState==LOW)                         // is the key closed?
    {
      tone(TONE_OUT,750);    }                // if yes, send 750 HZ tone on "TONE_OUT"
    else                                                     // Otherwise
    {
      noTone(TONE_OUT);                      // send no tone on "Tone_OUT"
    }

Looking at the Raduino sketch, I see where Farhan's got similar code already written to accomplish this, but it looks like he never calls it - actually, he's commented out the lines in the loop() that would call this function - and I think I know why:  The transmitted CW would sound like my fist after a few too many visits to the Guinness low-gravity draught head. 

The loop() function executes it's instructions sequentially, over and over until the cows come home.  So, before it gets to the instruction telling it to check and see if the key's closed, it's gotta finish all the prior instructions.  If you have a very short loop(), this is probably OK - the Arduino is pretty speedy.  On the other hand, if you're doing a lot of things in the loop(), it can be a problem.

A better way to do it would be to use a pin change interrupt. Then, the moment that the key input goes from high to low, the Arduino will stop whatever it was doing (loop())  and execute the commands in the interrupt service routine (ISR).

I've been in the habit of avoiding using interrupts because, truth be told, I didn't know how.  Then, I found this website (https://thewanderingengineer.com/2014/08/11/arduino-pin-change-interrupts/). Print it, study it, learn it, it'll change your life; it's certainly changed mine!

Basically, I just followed the three steps outlined in that blog:

Step 1 - Turn on pin change interrupts:

OK, I'm using input D11.  The Arduino data sheet shows that  will be one of    the inputs on "Port B", so I'll turn that on:
PCICR |= 0b00000001;

Step 2 - Choose which pins to interrupt:

Again referring to the data sheet, D11 is Pin 3 on Port B, so I'll create a "mask" so that we only look at that pin:
PCMSK0 |=0b00001000;

Step 3 - Write the ISR:


/***************************************/
/* Interrupt service routine to       */
/* generate CW tone on keydown  */
/* Version 1                                  */
/**************************************/

ISR(PCINT0_vect) {
//Generate CW:
     int buttonState = digitalRead(KEY_IN);
  if (buttonState==LOW)
    {
      tone(TONE_OUT,750);    }
    else
    {
      noTone(TONE_OUT);
    }
}


And, by golly, that's it!  Loaded it into my rig and it works like nobody's business.

Now, if you cut and paste this into your sketch, I guarantee it's not going to work because I've deliberately left out a few things.  I'm silly that way. But, my intent isn't to write a how-to, but to encourage anyone who, like me, was intimidated by the concept of using interrupts to just go out there and grab 'em by the cajones.  Like most things technical, it's not hard to understand once you find an explanation that "clicks" with you.

73 - Steve N8NM

Friday, July 7, 2017

SR-16: Let's Get Modular

In my last post, I mentioned that the only way to eat an elephant is one bite at a time.  Building anything with any degree of complexity is the same way; you have to break it into manageable chunks, otherwise you're invariably going to find yourself overwhelmed.  In an electronics project containing more than a single stage, I will divide the overall project into a series of modules, each with a defined function (eg: IF amplifier stage) and parameters (input/output levels, impedances, gain).  Modularity allows each element to be developed, tested and refined individually, which is important because, realistically, at the amateur level (where I proudly reside) things seldom work right the first time.  It's much less frustrating to have to scrap a single module than it is an entire project.

The photo above shows the modules on the underside of the SR-16. Each module is on it's own circuit board and wired such that it can be removed independent from all other modules (with the exception of the power and TR switching, for obvious reasons.) 

The large, shielded enclosure to the left contains the Hayward/Damm HyCas IF/AGC circuit, which I'd built some time before conceiving of this project.  Since I knew the function and I/O parameters of HyCas module, designing and building a rig around it was straightforward:  I drew-up the block diagram to use as a road map and went to town.

Like Master Homebrewer Pete, N6QW, I have a number of favorite circuit modules that I use in my rigs.  An example of this is the mic and audio preamp module; I've got a design based around a single LM324 that works well, so rather than reinvent the wheel, I've used the same circuit in my last three rigs.  But, since it's built as a module, if I find something that I like better, swapping it out will be a snap.

Modularity isn't limited to the hardware side of the project, but extends also into the Arduino sketch that runs the bloody thing.   Since I'm using an SI5351 to provide the LO and BFO signals, much of the work has already been done for me byJason Mildrum, NT7S, Przemek Sadowski, SQ9NJE and Tom Hall, AK2B, so I used their code as a starting point.  Not being a programmer, it took a lot of studying before I understood how their sketch worked and was able to modify it to perform the additional functions required by this project. 

In the same manner that the block diagram served as a road map for the rig's hardware, I sketched (pencil and paper - too ugly to post!) a flow-chart for the sketch and wrote modular "functions" for each, well, function of the software. Each function is analogous to a hardware module - they can be developed independent of the main program, and subsequently refined individually without disrupting other parts of the code.

While I don't want to share my ugly flow-chart for the entire sketch, here's how I keep the functions organized on my hard drive:

Here's a drill-down into the folder containing the function to load the VFO frequencies and status from EEProm when the rig's fired up.  Notice that it took me several attempts to get it right!

Notice that I keep to a standard that I've developed for myself to maintain version control - this becomes increasingly important as the code grows longer.  I think that'll be the next entry into this crazy blog - it's absolutely essential to keep track of where you've been in order to get where you want to go.

I can't imagine successfully homebrewing anything more complicated than perhaps a Michigan Mighty Mite without using the modular approach. 

73 de N8NM

Top view of the SR-16 outside of it's cabinet.  The circuit modules occupy space on four subchassis.  Each subchassis is also modular in that it can be removed, tested and refined independent of the others.  Modules containing modules. It's module madness!