Non Sequitur: Parts Scrounging and 6L6 Transmitter

MOPA rig with N3ZI VFO

A recent post on the SolderSmoke Blog got me thinking about the time-honored amateur tradition of building things out of parts obtained curbside.

I think I was probably eight or nine years old when I began lugging home and systematically dismantling junked radio and TV sets, something that became almost an obsession with me in my pre-teen years.

People threw away some cool stuff in the early 1970s, when one could count on harvesting at least one old black and white television set from among the household rubbish set out for collection, and I got good at retrieving them.

Garbage night was something of a weekly holiday; I'd rush home from school, grab my bike and begin hovering the neighborhood, hoping to catch someone in the process of dragging one of the old behemoths to the curb.  If my reconnaissance mission were successful, the next challenge faced was getting the thing home.  With smaller sets, I found that I could dismount my steed and precariously balance my bounty on the seat and handlebars of my Sting-Ray bicycle.  Larger, console sets posed a greater challenge, because I'd have to zip home, grab a wheelbarrow and zip back before someone else claimed my prize.  Much to my parent's dismay, my success rate was pretty decent.

With the derelict safely on the workbench, I'd begin the laborious disassembly process.  Every part would be identified, carefully unsoldered (no lead clipping for this kid!) and added to my ever expanding inventory.   Tubes were carefully sorted and stored in egg cartons, resistors, capacitors and small inductors were taped to index cards and categorically filed for easy retrieval, while the crown jewels: transformers, would be stashed under my bed in old shoe boxes.

Meanwhile, I'd be pouring over the various books and magazines, dreaming of building some of the featured projects.  Unfortunately, I wasn't quite there yet.

I wonder whatever happened to all that junk?  I should ask my mother.

Fast forward another 35 years.  While I don't practiced "Alley Picking" like I did as a kid, I had somehow amassed another hoard of salvaged parts.  But, this time, thanks to skills acquired over time and the endless resources available on the web, I knew I'd be able to do something with them.

In the search for candidate projects, I came across an article from the September 1964 Electronics Illustrated on KH6SKY's web page that described a simple 6V6 CW transmitter built from junkbox parts, appropriately named "The Scrounger".

The Scrounger was built on an inverted cake pan, which, enjoying cake as I do, appealed to me.  So, I built my first Scrounger prototype, which was reasonably faithful to that described in the EI article.  The damned thing actually worked, and I made quite a few contacts until disaster struck:  I accidentally set some papers down on my J-38 without a crystal in the rig and zorched the transformer.

That unfortunate incident taught me of the importance of protective bias, and armed with this knowledge, I set about building the improved version of the transmitter pictured at the top of this entry.

Unlike the original Scrounger, this version employs a 6AG7 and 6L6 in a Master-Oscillator-Power-Amplifier (MOPA) circuit, complete with bandswitching and a PI output network.  Of course, I added protective bias and changed from cathode to grid keying. 

Having addressed those "shortcomings" of the original Scrounger, I became dissatisfied with being limited to crystal control, so I added a synthesized VFO based around one of N3ZI's AD9850 boards.  Since the AD9850 doesn't have enough "oomph" to drive the transmitter, I added another 6AG7, making the rig a three tube affair.

It's actually a pretty decent little rig now, but has a much different character than my first version, mainly because it uses mostly new and Eastern Bloc surplus parts rather than those scrounged from the junk.  It's still got soul, but it's of a different nature.

I've still got the remnants of my original Scrounger, I should rebuild it someday.

Back to the SDR rRig: Band and Low-Pass Filter Boards

I doubt anyone really likes winding toroids, and I'm no exception.  But, unless you're in a position to hire someone to do it for you, you gotta suck it up and get it done, which is what I've been doing for the last three nights.

Below is a pic of the nearly completed bandpass filter and low-pass filter boards for the SDR-2017.   Nothing high-tech about 'em, just capacitors, coils and relays.

I think I mentioned in a prior entry that I'd basically stolen the basic design of the filters from those used in the IC-735, though I did massage them a bit using LT-Spice.

If you look carefully at the boards, you'll see that I'm using Manhattan style construction using small round pads.  I kind of stumbled onto the idea of making these pads after ordering some el-cheapo PCB stock that doesn't like being cut with shears, which is how I've always made Manhattan pads in the past. 

Amongst my seldom used tools is the hole-punch set pictured below.  I found that, by inserting the PCB stock into the punch upside-down, I was able to consistently knock-out perfect circles in various sizes.  I've used Rex Harper's ME Pads and like them, so to pay tribute to Rex, I call these "My Pads".

Hole punch used to create Manhattan Pads. 1/8" pad shown near the face of the ball-peen hammer.

Inverted cheapo PCB stock in punch. I found it necessary to punch the stock from the backside, otherwise the phenolic tended to shatter.

Y723-2017 Schematic

Here it is:

Pretty straightforward.  The "front end" variable caps and inductors were salvaged from the Zenith chassis.  4.7pF caps were added across L3 & L5 to get the dial to track properly on FM.

The variable inductor in series with the 10.7 MHz resonator was added to get the resonator onto the same frequency as the ceramic filter.

The two SPST switches are sections of the Zenith's rotary AM/FM switch.  One section is open on AM and closes to the 5 Volt line on FM. This is applied to the TA2003 IC to switch it over from AM to FM and back.

The other switches in about .05 uF of capacitance on the audio output of the TA2003 (FM De-Emphasis.)

One LM324 section is used as a Baxandall tone circuit that can cut or boost the high and low frequencies by about 8 dB.

The transformer was salvaged from an old Motorola battery charger.

Spontaneous Construction: Y723-2017

When you work on "antique" radios, you can't exactly ring-up the manufacturer and order replacement parts, so you tend to hoard things that most people would consider rubbish.  It may seem like odd behavior for otherwise reasonable people, but it is completely rational; you never know when you or a friend might need the epicyclic reduction drive off of the tuning condenser for a 1938 Philco.  It's always nice to have a few on-hand.

But, left unchecked, the hoard will eventually grow to an unmanageable size.  If you forget that you've got that 1938 Philco carcass in the junk pile, what good is having it at all?  None. Like it or not, you have to occasionally thin the herd. Unfortunately, that requires making some hard, cold-blooded choices.

Such was the case with this Zenith Y-723 with the cracked case and partially disassembled chassis.  It had already given up a number of parts so that other sets could live on, but I was still having a hard time with the idea of pitching it.

Then the thought occurred to me: The tuning mechanism and it's associated LC components are still there, Why not build a "modern" radio around them?

Normal people don't think of things like that, and only a real whack job would actually do it.

And with that thought, I present to you: The Zenith Y723-2017!

The salvaged tuning section from the defunct Y723 found itself sharing a chassis (fabricated from scrap ductwork) with a power supply built around the transformer from an old Motorola desk charger.  Below the chassis is the RF/AF circuitry; the radio is based on the TA2003 IC, and the audio amplifier uses a Sanyo LA4425A.

The speaker is a 3 1/2" model of unknown origin, another junkbox treasure.  I'm a big fan of these mid-50's Zeniths, but have to admit that their tone sounds a lot like the plastic case.  So, to get away from that, I built an MDF enclosure for the speaker, hoping that it absorbs some of that cabinet resonance. The new chassis and speaker box fit snuggly into the original cabinet, which remains cracked, albeit reinforced internally with lots of hot-glue.

How's it work?  Great!  AM reception is on par with the original set, but FM is significantly better: It's far more sensitive and just plain sounds better.  The LA4425A delivers enough audio to drive the orphan speaker loud enough to fill a room, and the audio doesn't sound like it's originating from a plastic cave.

That said, the low-frequency response of the orphan speaker was a bit limited, so I retroactively cobbled-in a Baxandall tone circuit built around one section of a LM324 quad op-amp.  This is adjusted internally; I didn't bring the controls out because I envision it as serving only to equalize the audio response, not as something I'd be playing with.

I'm still not sure what I'll do with this, but I won't be throwing it out anytime soon.

Wow!

Holy cow!  It's already close to a month since I've posted anything!  Where does the time go?

Truth is, I've gotten side-tracked by a couple other projects while waiting on some parts for the SDR-2017 project.

The first is an old Zenith 9-S-262 "Shutter Dial" that I electronically serviced for a friend.  Wow, what a great old radio!  The thing sounds fantastic and, in spite of needing multiple repairs, has been super easy to work on.

When I picked-up the set, I had a bad feeling about the power transformer, which ended up being justified because it was toast.  Not just toast, but toast burnt crispy black.  Bummer.

 Fortunately, my habit of hanging on to crusty old radio carcasses came in handy and I had a suitable replacement. Cool! 

After swapping the transformer, the rest of the repairs were routine:  Replacing first the electrolytic filter caps and then the old wax-paper coupling and bypass capacitors. 

Once this was done and new drive belts installed, I was able to give the set a "shake-down" test to evaluate any other problems that might be lurking.  I was happily surprised to find that, other than dirty/noisy controls, the only other problems were a dead "eye" tube and sluggish "Robot Dial" motor.  

Rebuilding the motor was trivial: Take it apart, clean out 80 years worth of crud, lubricate the bearing surfaces and reassemble.  About half an hour's work had it working like new. 

They eye tube required a little creativity.  Zenith originally used a 6T5, a tube that is very scarce, with good examples selling for hundreds of dollars.  That tube was long-gone from this set, having been replaced with the fairly common 6U5.  Electrically, the tubes are interchangeable, but the eye pattern is different.  I swapped the 6U5 for one that I had in my junk collection, but, alas, this tube was also very weak.  So, off to Ebay.

6E5 tubes are reasonably plentiful and affordable, and are pin compatible with the 6U5, but have different gain characteristics; they're a bit "hotter", which means that the eye will be partially closed during no-signal conditions, and will close beyond 100% on strong signals.  But, that's an easy fix: knock down the AVC voltage to the grid with a simple resistor divider and it'll work just fine.  In this case, I used 100K in series with the AVC line and 1MEG to ground.  Perfecto. Finis.

With that project out of the way but still waiting on parts to get back to work on the SDR-2017, I jumped into another "spontaneous construction" project:  This one being an AM/FM broadcast receiver using a $0.23 TA2003 IC.  Unlike many IC based radios, this one actually works well - more to come.

 

SDR-2017: Progress and Modularity

If you've read my earlier posts, you know that I'm a big advocate of the modular approach to design and construction, and that it all starts with "Noodling" - thinking about what I want the rig to do and laying-out how I want to accomplish it in a block diagram, which will serve as the project's "road map". 

I used to scratch out these diagrams on paper until I discovered the Yed program, which is now my tool of choice.  It takes a little more time, mainly to get all the boxes uniform in size and all the lines straight, but I find that process also helps me think in a little more depth than I do when drawing on paper, and it's certainly easier on trees.

Anyway, above is the SDR-2017 diagram that I "finalized" back in August, and I've "X" marked the modules as I've completed them. Notice that I haven't deviated from the original design concept; the diagram keeps me on track and prevents succumbing to "feature creep"!

 At this point, I've got a pretty respectable software defined receiver that covers from below the AM broadcast band up to the lower VHF spectrum, where it begins to run out of steam around 100 MHz.  Good bones for what'll ultimately become a 160 to six meter transceiver.

 My primary use for this rig will be on the amateur bands, but I also want the ability to occasionally use it to receive shortwave broadcasts, which makes designing the bandpass filter networks a bit more complicated than it would be otherwise.  Being lazy, I borrowed much of the bandpass filter design from a commercial rig (IC-735), with some tweaks to optimize the circuit based on this application and what I have in my junkbox. 

What I did first is create models of the Icom circuit in LT Spice, so that I could see what the Icom designers came up with.  Below is a model of the filter for the 160 Meter band:

 

 In this case, where I'm shamelessly stealing someone else's proven design, these models wont be used so much to tweak the design as they will to confirm that, once constructed, it's working as it should - more on this in a minute.

My next step was to build the inductors.  Sure, I could just order them from Mouser, but what's the fun in that?  So, I spent a few hours working with a spreadsheet that I put together years ago for this purpose, which spit out the number of turns and type of core for each filter element, then spent a few more hours winding the bloody things.  I don't know of anyone who enjoys that, but if you do, something's wrong with you...

With all that info, I drew the schematic shown below and began putting the filter board together.

 Rather than using diode switching as in the Icom, I'm using relays for a couple of reasons: First, I haven't had much luck homebrewing diode RF switches; I can get 'em to work, but the isolation is always poor.  Second: I got a good deal on the relays.  I started construction with the input switches, and will add the output switches as each band's filter is completed.  Doing it that way makes building the filters easier because I'm not reaching over the relays while working.

Speaking of that, there's been significant noodling involved in determining the physical layout of the filter board.  The board itself is 6" square, and I've divided it into seven "lanes" - one for each band - of about 0.75" each.  The input switches are at one side of the board and the outputs on the other.  Since the RF signals on this board are going to be low (<0dBm), I don't expect that there'll be any crosstalk between the filters, but if so, I'm leaving room to install shielding dividers between each lane.  I'll post pictures as I get further along in the construction.

As of last night, I've completed the 160m filter and "swept" it using my spectrum analyzer (Rohde & Schwarz FS-315.)  The response matches the LT Spice model so closely that it's almost spooky!  The curve is virtually identical, the only differences are in the loss (actual filter slightly better than model) and frequency of the peak (actual filter peaks slightly lower in frequency, but still FB in the amateur band.)  Excellent!

Once I have this board completed, I'll circle back and complete the T/R switching and mic audio circuits; much of this is incorporated into the Softrock hardware and Quisk software already, so that *should* be fairly simple... We shall see.

This is the point in any project where it becomes exciting and I have to fight the temptation to "pull an all-nighter" to get things done.  I'm getting close to retirement when that'll become practical, and man, am I ever looking forward to that!

SDR-2017: Assembly, First Light and Audio Tweaking

Receiving AM Broadcast station - note the IBOC "sidebands".

After lots of "noodling", I'm finally getting to the fun part: Putting it all together. 

Touching a bit on the mechanical aspects: I didn't want to fall in to my usual trap of trying to squeeze 10 pounds of stuff into a 5 pound bag, so this rig has a large form factor when compared to my others.  Dimensionally, it's 16" wide, 12" deep and 6" high, so with any luck, I won't be building each module 3 times in effort to make it fit into the smallest available space.  We'll see how that goes.

Because of my limited metal working skills and facilities, I've built the chassis from 26 gauge galvanized steel.  Getting this rather flimsy material formed into a rigid housing took a little thought and some trial and error, but it's working out reasonably well.  Basically, it's a lot like modern cars in that it's a box that gets its strength from being made from smaller boxes.  So, while it's very light in weight, it's structurally rigid, and very easy to work with - except for the bleeding part; some of the edges are pretty sharp.

Electronically, I haven't deviated from the topology that I laid-out in my initial block diagram:  The touchscreen equipped Raspberry Pi, running Quisk in "kiosk mode" talks to the Arduino that handles all the hardware switching and programming of the synthesizer, while a modified "Softrock" connected to a USB soundcard handles the modulation/demodulation.  Ahead of the Softrock is a diode double-balanced mixer that converts the received signals up or down to 9 MHz.  This is all working fairly well, though there's still a lot of refinement work to be done.

An example of this is in getting the receive audio sounding decent.  My original intention was to take the line audio out of the pi and feed it, through a 10K pot, to the LA4425A power amplifier.  This works, but picks up a ton of stray computer noise unless I significantly load the input to the power amp, which reduces gain more than I'd like.

The LA4425A has a rather high input impedance, somewhere in the 10s of K Ohms, while the output of the on-board soundcard is fairly low Z, and I think this mismatch is causing the problem.  So, I'm going to try a simple common-base transistor amplifier between the soundcard and volume control. 

The common base amplifier's characteristics are: Low input impedance, less than unity current gain, moderate output impedance and relatively high voltage gain; exactly what I think is needed.

I've built enough transistor amplifiers that it's almost become second nature, but I still like to go through "the design process" first, then model the circuit in LTSpice before melting any solder or frapping any silicon.

When it comes to designing a common-base amplifier, I tend to approach it in much the same way as I would a common emitter circuit.  R1 and R2 form a voltage divider that's "stiff" enough, current-wise, to keep the base voltage at about 2.1 volts.  This biases the circuit so that the drop across R4 will be about 1.5 volts.  Dividing 1.5V by 100 Ohms gives us the quiescent current flowing through the device, about 15 mA. Next, I want the collector to be able to "swing" about a volt, so I chose a resistor of 560 Ohms, which has the collector resting at about 3.25 volts.  Cool.  

Plugging these values into LTSpice, I was able to see that the model confirmed the numbers that I'd come up with.  Yes, they're not exactly the same, due largely to my assumption that the B-E Voltage drop was 0.6V, but still well within in the ballpark.  

 

Initially, I ran the model without C3 in place, and the predicted gain was just under 18 dB from a few hundred Hz to well beyond the audio spectrum.  Since this isn't a hi-fi, I added C3 so that the gain rolls-off above about 3-4 KHz.  Since I had the circuit already "running" in LTSpice, I simply plugged-in different capacitance values until I got the response curve I was looking for, but this could be determined algebraically with just a little more effort.  

 

This response shaping cost  around 4 dB in overall gain, which is insignificant in this case because I'll still have more than enough signal to push the LA4425A to it's maximum.  Actually, I think I'm probably going to have to reduce the gain a bit... Maybe not, we'll see.

The next step will be building the circuit, plugging it into the rig and seeing if it works as intended.  Stand by!

73 de N8NM