SCR-211-B Frequency Meter

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 sum of $125.  The machined aluminum chassis, the engraved faceplates, the shock-mounted & meticulously labeled vacuum tube sub-assemblies, ceramic & silver terminal strips, and endless yards of hand assembled wire looms;  I was mesmerized.  It was to me, the epitome of a machine, from the golden era of American electronic manufacturing, and I was hooked.

I’m here to combine two of my favorite things – photography and old equipment, with one of my least favorite things – writing.  My goal is to share photos, descriptions, documentation, and history of some of these wonderful pieces, while at the same time exercising my brain & pulling words out of it.  Words are hard.  Off I go…

This first piece I acquired yesterday at a Hamfest in northern NJ (more on hamfests some other time).  After a bit of deliberation, I parted with my $70 (which is a lot, most of the stuff I get is less then $50) in exchange for this beauty.  This is now one of the oldest pieces in my collection, and without a doubt one of the cleanest.



It came with it’s own canvas bag, stenciled BG-81B, it’s strap stenciled ST-19-A.  This is the military,  EVERYTHING has a designation.


Out of the bag, it’s not much more interesting, but in remarkably good shape for something that’s over 70 years old.

Lets crack this puppy open.


Ahhh, that old electronics smell.  Its like light machine oil meets warm electrons.  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.  Not a hint of dirt or grime, no chipped paint, and the paper, white stenciling or acrylic hasn’t yellowed.   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.


Most of the controls operate freely, with the exception of the gain potentiometer, which very stiff on the count of having never been spun.  The placard proclaims this to be the BC-221-B frequency meter.  The manual says SCR-211-B Frequency Meter Set.  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.

Also note the serial number.  9.  That’s pretty awesome.

Made by the Allen D Cardwell Manufacturing Corp. of Brooklyn, NY (back when actual things were made in New York City).  A bit of history on the firm can be found here.


I’ll explain the vernier tuning another time – there’s a good description of it in the manual, which I’ll post shortly.


Everything on this thing is metal.  Even those little knobs are machined & anodized aluminum.


A macro lens is on my short wish-list.


I’ve never seen 70 year old acrylic (plexi? polycarb? ) look this good clean.


The book is serial number matched to the unit…


..because the dial position is hand calibrated for each unit!  Holy crap, how long did this take?  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.  Someone must have spent days per device on this.  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.


Let me try to explain how this thing works (and don’t be shy to correct me if I’m talking shit):

The purpose of this device was to aid in calibrating transmitters and receivers.  To do that, you need at least one of two things:

  1. a way to measure frequency.
  2. an accurate frequency source.

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.  Back in the 1940’s this wasn’t so much a thing that existed,  so we had to rely on #2, an accurate frequency source.  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.

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

this phenomenon is called Heterodyning

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.  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 599,738.374435 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.  As those frequencies get closer together the difference approaches zero, at which point we can no longer hear a tone.  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).

The tear-down continues…


after unlocking the two thumb-screw locks, the business end slides out of the chassis.


Spare tubes, waiting faithfully to spring into duty.  +A, +B, -A-B are the battery connections.  Ever wonder why there isn’t a B battery?  There used to be.


The one thing that’s missing (besides the headphones) is a spare reference crystal that would have been in the clip on the left.  The active crystal is the one black can that’s oriented horizontally, the rest are metal vacuum tubes.


hand-wired point-to-point goodness.

Lets take a look at the battery compartment.


The schematics under a piece of acrylic, and a battery eliminator (what radio power supplies were often referred to back in the days).  I was sure glad not to encounter the rotting, corrosive remains of 50+ year old batteries.


Battery eliminator removed.  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.  Firing this up will have to wait.  In order to have a predicable frequency, you need a predicable voltage, thus the gas regulator tube in the upper right corner.


Mmm… 70 year old capacitors, slowly leaking their gooey innards.  Yep, this will need some love before I can power her up, and then only very slowly on a Variac.

Welp – hope you’ve enjoyed this trip down memory lane, more to come soon!