Tuesday, January 16, 2018

Learning Python

In my quest to learn Python, I came across a great (and free: My favorite price) interactive tutorial at This Site.  I'm impressed - quite a bit has somehow penetrated my thick skull in just the few hours that I've spent working the lessons thus far.

What is working for me are the exercises at the end of each lesson; they require you to creatively apply the concepts presented in the lesson, and, if you make a mistake, will show you the part of the code that's broken, but leave it to you to fix it. 

I wanted to share this because I reckon that I'm not the only one who learns best by doing, and this site is working for me.

Sorry, no radio related images today, just a picture of a very irritated cat.  You've heard of Grumpy Cat?  Meet Psycho Kitty.


Saturday, January 13, 2018

Flashback: RME-45

It's been quite a couple of weeks!  On New Years Day, I had the pleasure of being interviewed by Eric, 4Z1UG, for an episode of his excellent QSO Today podcast, which subsequently "aired" on January 7.  I imagine Eric's listeners responded in one of two ways:

1 - Who's this clown?
2 - N8NM? He still owes me money!

But, seriously, it was an honor.  To look back to some of those who've been featured in past episodes, many of whom are people who've had a marked influence on me and my amateur "career", it's nothing less than humbling.

Then, this morning, I hear my friends at SolderSmoke mention me during the same segment as Jeff Damm - wow!

And then I sit down and try to figure out why the SDR-2017/18 won't do what I want it to.  Python.  I suppose it's a great language, but I don't speak it, and, after nearly a month of hacking away at it, I've determined that I need to step back and start learning it from the ground up.  I'm a slow learner, this could take a while.

But, in the meantime, enough about me; let's talk about an old receiver: the RME-45.

Why you don't combine Ambien and Ebay.

I first became aware of this model about 25 years ago, when I was purchasing another old radio from a local collector and saw one that he had in his hoard.  What a dial!  But, since I'd already given over all my cash, I didn't ask if it were for sale, just filed the information with the intent to own one some day.  That came about five years ago, when I made the mistake of combining prescription drugs and Ebay.  Drugs are bad, just say no.  I really don't know what prompted me to pull the trigger on this deal, but I did, and a week later unpacked the wreck shown above.  At least I didn't pay much for it, at least not on the initial purchase.

Oh, but that dial!

Realistically, this should have been scrapped for parts; it had a lot of parts missing (and a few added), was rusted and dented, and, worst of all, had taken a significant "whack" to the tuning shaft, which destroyed the ball-drive and damaged the multi-section tuning cap. Ouch.

So, I'm looking at this thing, wondering what the hell I'm going to do with it, when I get this brilliant idea: Let's strip it down completely and rebuild it Heathkit style.

I'll let the pictures do the talking, but a few points of interest:
  • The ball-drive dial mechanism was replaced with one from a junked Philco 38-8 chassis, surprisingly little machining was necessary.
  • I filled the extra hole in the panel with JB Weld.
  • The wrinkled finish was simulated by spraying on a coat of Rust-O-Leum textured paint, then spraying a thin, blue-grey color coat over the top of it.
  • Yeah, I painted the chassis to cover up significant mouse-pee damage.
  • I had to make a new cover panel for the gear drive out of 22 GA steel.  This was bent-up old-school, in a vise.
  • During disassembly, I took approximately 200 pictures so that I'd know how every part was installed and every wire routed.
  • Repairing bent and broken four gang variable capacitors is hard.
  • Loctal tubes are cool!
Top view of chassis "before".  It looks better in the pic than it did in person. Mice had been busy using it as a toilet.

The stripped and painted bare chassis.

Starting to come together.

A little more progress.

The crystal filter module, ready for installation.

 First received signals were of my friend Don, K8BB, who's only a couple miles from me and was booming in at 60+ over!

As it is today.  Someday, I swear I'll get to silk-screening the labels, but as one of RME's founders said when asked why the controls on a prior model weren't labeled: "If you don't know what they do, you've got no business buying one."

So, how does it work?  Truthfully, it's outstandingly mediocre.  The performance is on-par with something like an NC-57 or S-40, the only real advantage it has over those rigs is the crystal filter, which works like that on an SX-42 but isn't quite as effective.  But it's got that great dial, and the tuning rate is adequate.  This is an early production model and has the dual-rate tuning system - the center of the the tuning knob is a slow rate, the outer concentric much quicker. 

The rig's OK, but for the same kind of money (and real-estate, it's a large box!) something like an NC-183 would have been a better value. 


Tuesday, January 2, 2018

Non Sequitur #2 - Sweep generator for FM Alignment

Happy 2018!

Work on the SDR-2017 is continuing, though progress has been slow and fraught with frustration.  It pains me to say this, but it's mainly due to that "black box" stuff that N2CQR warns about - Linux and Quisk are fighting with my sound devices.  I'll get 'em whipped into shape, but in the meantime figured I'd share a project from a few years ago:

I've worked on enough old radios to where I can usually align the AM circuitry by-ear; I know where the local stations are on the dial and what to expect for sensitivity and selectivity.  AM is fairly easy because, for the large part, you're adjusting for maximum smoke.  FM is a little more tricky, though.  Yeah, it's possible to align by ear, but I find it actually faster and easier to align the IF and discriminator stages with a sweep generator, RF probe and scope.   But, my sweep generator was old, large and kind of a pain to use, so I built a small, stripped-down unit that didn't take up more bench space than the radio I was working on.

For those who may be new to the hobby, a sweep generator is an oscillator that repeatedly "drifts" from one frequency to another over a certain period of time, then abruptly "tunes" back to the frequency where it started.  When used in conjunction with an oscilloscope and detector probe, it allows one to view the shape, or bandpass, of a tuned circuit.  There are some great YouTube videos demonstrating the procedure, so rather than go into the procedure here, I'll focus on the circuit I threw together.

The heart of the circuit is a voltage-controlled-oscillator (VCO) based around Q4, that, at the center of it's control voltage range, runs at 10.7 MHz.  The 555 Timer is configured as a sawtooth-wave generator that serves as a common timebase for the VCO and "X" axis of an oscilloscope (via buffer U1B); the gain of U1C determines the sweep width, or differential between the high and low frequencies of the generator, and U1D adds a bias voltage to the ramp to scale the signal to tune the oscillator as desired. 

The circuitry around U1A, Q1 and Q2 is, I think, kind of cute.  I wanted to be able to indicate, on the screen of the scope, where 10.7 MHz would be on the X-axis, and came up with the rather crude but effective thought that, if I paused the timebase for just a couple milliseconds at any given point, it would light the screen phosphors a little brighter than the others - in other words, create a "blip" on the screen.  So, U1A is a comparator that, when the ramp waveform reaches a certain threshold (set by R2), stalls the ramp generator just enough to increase the intensity at that point.  Unfortunately, it doesn't photograph well, but you can almost, kind of see it at the peak of the waveform in the picture below:

The picture above and the one at the top of the page show the frequency response curves of properly adjusted IF stages.  This is done by applying the sweep generator signal at the input to the stage and monitoring the output with a simple, diode detector probe (connected to the scope's Y axis.)  The stages are "tweaked" for maximum amplitude at the marker (blip) frequency (10.7 MHz.)

The next shot shows what's called the "S-Curve" response of a properly aligned discriminator (detector) stage.  Again, the sweep generator is connected upstream, but this time the scope is connected, using a regular scope probe, to the output of the discriminator.
Again, the marked didn't photograph well, but you can almost see it right in the middle of the "S".  When the IF and discriminator are adjusted correctly, the positive and negative portions of the signal will be at their peak amplitudes, indicating that the IF's are adjusted for max.  They'll also be of equal (but, obviously, opposite) amplitude with their zero-crossing  point right at 10.7 MHz, indicating that the discriminator is in harmony with the IF passband.  If you have the volume of the set turned up, it'll sound like a motorboat or a long, impressive fart, depending on how your mind works.

The finished product is shown above.  The knobs are pretty self explanatory - marker is adjusted to put a "blip" right at 10.7 MHz, Width controls the difference between the lowest and highest frequencies of the oscillator (+/- a couple hundred KHz max) and level adjusts the output of the generator to suit the circuit under test.  Even at it's minimum setting, it may be higher than needed for some measurements, hence the stack of in-line attenuators.

The enclosure is a from a defunct PC power supply; I stripped it of it's guts, made front and rear panels from #22 aluminum stock and sprayed it with some Rust-Oleum hammertone paint.  Almost looks professional - close enough for me!

73 - Steve N8NM

Tuesday, December 19, 2017

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.

Friday, December 15, 2017

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.

Thursday, December 14, 2017

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.

Sunday, December 10, 2017

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.