INDY HAMFEST SWAG, PART THREE

A "what-is-it?" box.  I was pretty sure I knew...

...Inside, a power transformer, top-hat diode, a lot of R and C and a relay (not visible in this photo) suggested what it might be.  Two pots and a three-circuit jack on the front were a clue.  A pair of insulated screw terminals on the back, marked "S" and "T" were the final giveaway: it's an all-passive-component automatic code keyer!  The two pots are for speed and "weight," the timing ratio between dits (dots) and dahs (dashes).
     For $3.00, it's a good steel minibox and a handful of probably-useful components.

     Elsewhere -- and I should have bought more from the guy, he had a bunch of knobs and dials -- this gadget:

     Just a calibrated knob, you say?  Have a look at the other side!

     It's a nice old vernier dial, made by Crowe up in Chicago.  (You can just read "CRO..." behind the drive at the lower center-left.)  They've been gone for decades but their knobs and dials, etc. still show up.  It's good stuff.  Paid a couple of bucks.  (The one mounting leg that looks broken was made that way.  I don't know why.)  It needs a little fixed scale to be a true "vernier" dial and I may have one the right size.

INDY HAMFEST SWAG, PART TWO

      A low-impedance/high-impedance microphone matching transformer:

     This example arrived with some barbaric ham-radio mic connector on the low-impedance side, a three-pin, threaded-shell item.  ETA: I found the mating connector in my collection!  This is why you save 'em.

       The transformer is a Shure Brothers A86A.

      "Model A86A is a  high-quality cable-type transformer which offers additional versatility when used in conjunction with Shure Models 55,556, and 51 Dynamic Microphones, which employ the impedance matching switch. It solves the frequent problem of installations requiring long  lengths of microphone cables  without the loss of high-frequency response. Model A86A matches 35 to 50 and 150 to 250 ohm microphones to high impedance amplifier input.  Compact, sturdy.
Case diameter 1-5/8", length 2-7/8",  7 foot cable.  Shipping weight 1-1/2 pounds.
Model A86A. Code: RUDEB.  List Price $13.10"

    (Link to a Shure catalog listing it at Preservation Sound.)

     That was pretty a high price, back when this was new.  Frequency response is cited as "within 1 dB, 20 to 20,000 cps," which is darned good.  The various low impedances are selected by removing three screws on the low-Z side, opening the case up and moving a jumper.

     Does this one work?  I don't know yet.  It was priced to move at $3.00 and for that kind of money, worth finding out.

INDY HAMFEST SWAG, PART ONE

     I collect glass antenna insulators.  I'd love to use 'em but I worry about breakage.  In the old days, they were attractive targets for vandals....

     Anyway, I picked up four at the Indy hamfest, including a second "screw thread" pattern (center left) and an unusual large-center one with very rounded curves (lower left).  Here's the entire collection:

     There are at least three different hues of glass: clear, yellow and pale blue-green.  The three large insulators at upper right are supposedly from electric fence service!  (Of course, there's a website.)

A SIMPLE AUDIO SPLITTING/COMBINING PAD

     This is a neat trick for professional balanced audio applications, where a stereo signal needs to be "summed" to mono or a mono one split to two destinations.

     It depends on all the sources and loads being the same impedance, which the resistors match, 600 Ohms in this (typical) example.  Isolation between the two inputs (or outputs) is very good and insertion loss is low.

     Modern practice tends more to quite low-Z sources[1] and very high-impedance loads.  With that, "splitting" one source to several destinations can be a simple (if sloppy) process of paralleling but look out if any one of them gets shorted!  This "Bridge" (as in Wheatstone) pad can be adapted to the low-Z source/High-Z load system by "building out" the pad inputs with pairs of series resistors and adding a fourth 604 Ohm[2] resistor on the load side, but insertion loss will go up as will complexity; a "branching" pad is probably easier--

     --But the isolation between ports is worse.  Still (and especially with a modern very-low-impedance source), a short on any one branch won't silence the others.  (Expanding this to more ports results in increased loss and different resistor values but simple algebra will tell you what they are.)

     This is a little off-topic but analog audio, especially balanced audio, has become more and more "black magic" as the digital age advances.  Perhaps it will be of use to some of the ham operators running AM, who sometimes use older audio mixers and feed multiple transmitters, some of which are former AM broadcast rigs.

     Most sound starts out analog and ends up that way, too.
_________________________________
1. Some "zero-Ohm" sources are more zero than others.  Back when Purdue University's WBAA still used Ralph W. Townsely's amazing homebrew (tube-type!) audio console, the line outputs were very high-fidelity audio power amps set for extremely low output impedance and technically-savvy visitors might be handed a paperclip by the (irreverent) student operators and invited to "short out the Line audio."  All that happened was the paperclip got hot; the audio level never changed.  Please don't try that with the "0 Ohm" analog outputs of most pro audio gear -- your results will be quite different.

2. Why 604 Ohms?  It's the nearest 1% metal-film resistor value to 600.  1% resistors cost pennies these days but you could use the closest from the (even cheaper) 5/10/20% carbon resistor bins, either 560 or 620 Ohms.  If you're feeding something critical like a passive equalizer, you may be able to tell the difference.

TUBE-TYPE MIC MIXER, PART 2

     A reader complimented the schematic I posted yesterday.  Of course, that's the "clean copy."  Here's the one I drew as I traced it:

     You will note several layout errors that are corrected in the later version -- don't know why I omitted the headphone jack wiring in that one, though.  Schematic drawings can nearly always be improved after the first draft gets everything on paper (or on the screen, if you work that way).  The rules are simple, applying them sometimes isn't.

     And here's what I traced:

     The Thordarson "Tru-Fidelity" mic transformers are not original -- the fellow who gave me the mixer salvaged the original UTC transformers.  The Thordarsons are at least as good.  They're also larger.  I'm not happy with the length of unshielded wire in the grid leads, it will want changed.  Looks like I took most of the wire harness lacing out when I traced it, too, so that needs redone as well.  I wonder how those Sprague "Atoms" electrolytics are holding up?  (They're on the terminal board behind the right-hand mic jack.)
    
     Back panel:

     Blank and ready for call letters!  (It's traditional.)

     2" Cannon connectors, with a penny for scale and ancient masking tape stuck on the chassis:

     12-pin Jones plug, holes where a previous user had relocated the output terminals, fuseholder:

     Rannie the cat loves my fancy screwdriver set:

      This made disassembly a little more difficult.

     (Here's another take on a small mic mixer, kind of interesting...if you have the parts.)

TUBE-TYPE MIC MIXER

     It's been awhile since I traced the circuit and got it working:

     Naturally, it was not working when I took it down from the shelf.  Time to get out the signal tracer.

     At some point before I got it, the front panel was covered in wood-grain laminate.  I'm thinking a nice coat of plastic-friendly matte black would do it a world of good.  And perhaps some RCA-type knobs that had not been painted, too.

     (A word about the '2" Cannon' notation for the input connectors: that's what they are, great big old-fashioned microphone sockets.  A mic-connector roundup post is in the works to provide examples.)

THE FIRST PLAY-BY-PLAY MAN

     In fact, he probably invented the idea.  And he was a Hoosier.

    ...It was The Big Game -- and not a home game for his school's team.  The year was 1903.  University of Michigan student,  Indiana resident (and part-time telegrapher) Floyd J. "Jack" Mattice had already dreamed up (and sold to Western Union!) the notion of having sports-knowledgeable telegraphers transmit moment-by-moment scores and high points directly, instead of waiting on copy from print reporters and now he'd come up with another idea: a long-distance telephone line.  He'd describe the game to the other students back at UM as it happened.

     Bell Telephone installed a phone booth on 40-foot poles along the sidelines at mid-field and a direct line back to the auditorium at UM. Shortly before the game began, Mattice climbed up and placed the call--

     But remember, this is 1903.  Electrical amplification is still several years away.  Backstage, ten more sports-fan students were listening in, each one in turn memorizing a few minutes' play and rushing onstage to relay it to the audience, and then back to his listening post as the next "sportscaster" took over, on after another.

     It was a popular notion.  At UM, the practice continued until radio broadcasting took over in the 1920s.  As for Mattice, he had another career; you see, he'd arrived at college already a member of the bar.  After graduation, he went home to Rochester and practiced law, serving a couple terms as county prosecutor.  Even that wasn't enough -- during WW I, he went to work for the Bureau of Investigation's office in Indianapolis and then at the U. S. District Attorney's office.  He went back into private practice a few years later and then served in various posts with the city, then back to Federal work during WW II, ending up as a prosecutor at the war crimes trials in Japan.

     Still, inventing sportscasting probably made more people happy than any of his legal work.  He retired to Rochester and passed away in 1971.  He maintained a lifelong interest in telegraphy and if you've ever wondered about the curious number of attorney/sportscasters (Howard Cosell, for instance), while I don't think Mr. Mattice can be blamed for it, he was certainly one of the first examples.

     (Crossposted from my personal/political blog.)