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!