![\"DSC_0057\"](\"http:\/\/paulcarbone.com\/blog\/wp-content\/uploads\/2014\/04\/DSC_0057.jpg\")
<\/a><\/p>\nIt came with it’s own canvas bag, stenciled BG-81B, it’s strap stenciled ST-19-A.\u00a0 This is the military,\u00a0 EVERYTHING has a designation.<\/p>\n
Out of the bag, it’s not much more interesting, but in remarkably good shape for something that’s over 70 years old<\/strong>.<\/p>\n Lets crack this puppy open.<\/p>\n Ahhh, that old electronics smell.\u00a0 Its like light machine oil meets warm electrons.\u00a0 If anyone has a better description of that smell, I’m all ears. The first thing I notice is just how spotless this thing is.\u00a0 Not a hint of dirt or grime, no chipped paint, and the paper, white stenciling or acrylic hasn’t yellowed.\u00a0\u00a0 I suspect that this WW2 era machine never saw service, and lived out its years on some forgotten shelf in a warehouse until it was unearthed, decades past it’s prime.<\/p>\n Most of the controls operate freely, with the exception of the gain potentiometer, which very stiff on the count of having never been spun.\u00a0 The placard proclaims this to be the BC-221-B frequency meter.\u00a0 The manual says SCR-211-B Frequency Meter Set.\u00a0 One thing I’ve noticed with military gear is that a collection of components grouped into a set receives it’s own designation, so maybe that’s what’s going on here.<\/p>\n Also note the serial number.\u00a0 9.\u00a0 That’s pretty awesome.<\/p>\n Made by the Allen D Cardwell Manufacturing Corp. of Brooklyn, NY (back when actual things were made in New York City).\u00a0 A bit of history on the firm can be found here.<\/a><\/p>\n I’ll explain the vernier tuning another time – there’s a good description of it in the manual, which I’ll post shortly.<\/p>\n Everything on this thing is metal.\u00a0 Even those little knobs are machined & anodized aluminum.<\/p>\n A macro lens is on my short wish-list.<\/p>\n I’ve never seen 70 year old acrylic (plexi? polycarb? ) look this good clean.<\/p>\n The book is serial number matched to the unit…<\/p>\n ..because the dial position is hand calibrated for each unit!\u00a0 Holy crap, how long did this take?\u00a0 Over 60 pages, each with dozens of frequencies listed and matched to a position on the dial, which has been meticulously typed into the booklet.\u00a0 Someone must have spent days per device on this.\u00a0 Maybe it was some poor 4F kid who wanted to do something for the war effort, or maybe it was an army of ‘Rosie the Riveter’ gals (Clara the Calibrator?); their ears in headphones, hours on end, listening as screams became howls became beats until finally silence as they slowly turned the knob, noting the exact position on the dial, so our boys on the front could tune their radios.<\/p>\n Let me try to explain how this thing works (and don’t be shy to correct me if I’m talking shit):<\/p>\n The purpose of this device was to aid in calibrating transmitters and receivers.\u00a0 To do that, you need at least one of two things:<\/p>\n Today we have frequency counters – marvelous little devices that count the cycles per second of an incoming signal and display it on a numeric display.\u00a0 Back in the 1940’s this wasn’t so much a thing that existed,\u00a0 so we had to rely on #2, an accurate frequency source.\u00a0 This device is essentially a frequency source with a way to compare this reference frequency with that of the Device Under Test, and a means to listen to the difference.<\/p>\n When two frequencies, f1 & f2 are mixed, two new frequencies are created: The sum of the two: f1+f2, and the difference of the two: f1-f2<\/p>\n this phenomenon is called Heterodyning<\/b><\/em><\/p>\n The frequencies in question are in the hundreds and thousands of kilohertz, far beyond the range of human hearing, but thanks Heterodyning, we can turn this into a problem that our ears can solve.\u00a0 If you mix a frequency that is 600,000 hz (or 600 kilohertzs, in the lower end of the AM band) with lets say, a frequency of\u00a0599,738.374435 hz, you’ll end up with the sum (which is about 1.2 megahertz, closer to the top of the AM band) and the difference, which is 261.625565, the clearly audible Middle C<\/strong>.\u00a0 As those frequencies get closer together the difference approaches zero, at which point we can no longer hear a tone.\u00a0 When this happens, we can be confident that the two frequencies are essentially the same (plus or minus the 20hz that we can’t hear).<\/p>\n The tear-down continues…<\/p>\n after unlocking the two thumb-screw locks, the business end slides out of the chassis.<\/p>\n Spare tubes, waiting faithfully to spring into duty.\u00a0 +A, +B, -A-B are the battery connections.\u00a0 Ever wonder why there isn’t a B battery?\u00a0 There used to be.<\/p>\n The one thing that’s missing (besides the headphones) is a spare reference crystal that would have been in the clip on the left.\u00a0 The active crystal is the one black can that’s oriented horizontally, the rest are metal vacuum tubes.<\/p>\n hand-wired point-to-point goodness.<\/p>\n Lets take a look at the battery compartment.<\/p>\n The schematics under a piece of acrylic, and a battery eliminator (what radio power supplies were often referred to back in the days).\u00a0 I was sure glad not to encounter the rotting, corrosive remains of 50+ year old batteries.<\/p>\n Battery eliminator removed.\u00a0 Some dodgy looking wires – the cotton clad is actually in remarkably good shape, but the rubber is flaking off the battery lead running up into the chassis.\u00a0 Firing this up will have to wait.\u00a0 In order to have a predicable frequency, you need a predicable voltage, thus the gas regulator tube in the upper right corner.<\/p>\n Mmm… 70 year old capacitors, slowly leaking their gooey innards.\u00a0 Yep, this will need some love before I can power her up, and then only very slowly on a Variac.<\/p>\n Welp – hope you’ve enjoyed this trip down memory lane, more to come soon!<\/p>\n","protected":false},"excerpt":{"rendered":" First Post! Over my years of geekery, I’ve amassed a collection of old electronic test equipment, starting with my very first Oscilloscope (a Tektronix 545A w\/ CA plugin) that I acquired at the age of 12 or 13 for a … Continue reading <\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n\n
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