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.