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BC-221, part of SCR-211 Frequency Meter Set – TS-174B/U
- Top, three pictures of a BC-221-Q in metal case; this model was made by Allen D. Cardwell, a
known manufacturer of variable capacitor. Bottom left, a BC-221-N made by Philco Corporation:
this model uses thermally compensated tuning inductors, with a bimetal spring that controls the
coupling of a shorted loop. Bottom right, the TS-174 was a VHF variant of the BC-221; this TS-
174B/U was made in Italy by Stirer srl, Pomezia.
For about half a century BC-221 has been in service as reference generator or frequency meter in
many and many communication and ham labs worldwide. BC-221 has been produced by several
manufacturers in 25 different models. It is a three tubes, two bands heterodyne frequency meter,
covering the range from 125KHz to 20MHz. Internal oscillator covers from 125 to 250KHz, in the
low band, and 2.0 to 4.0MHz, in the high band; fundamental and harmonics are used to get the full
coverage. The dial has 50,000 division; the individual calibration book lists 5,250 points on the low
band and 6,000 points on the high band; exact frequency reading is obtained by interpolation.
Accuracy can be checked at a number of listed points against the harmonics of the internal
1,00MHz Xtal oscillator. Overall accuracy was granted within 0.034%, even at the lowest
temperature of -30ºC; average accuracy was assumed to be within one half of the above value.
Batteries are hosted in a chest into the lower part of the cabinet. For fixed operation, a compact AC
power supply can be fitted into the battery compartment.
The various models, from Bendix, Cardwell, Philco, Rauland and Zenith, can differ from each other
for more or less significative details: metal or wooden case; AM modulation option; type of tubes
and circuit details; spare parts supplied. The complete description of each model can be found in the
TM11-300 technical manual.
Probably the BC-221 is the best example of top design and execution. Compact and rugged, it could
virtually operate in any field environment, still retaining a precision hard to find in the best
laboratory instrumentation. Coils were treated against moisture, LC resonating circuits were
thermally compensated, the variable capacitor was a masterpiece for its smooth anti-backlash worm
gear drive.
The BC-221 was so appreciated to originate a VHF variant, the TS-174. This equipment covered
from 20 to 280MHz. It used the same tubes of many BC-221, one 6K8, one 6SJ7Y and one 6SJ7,
plus one 1.0000 MHz quartz crystal as calibrator.
Wavemeters D. No.1 MKII
The wavemeter shown here can be considered a British equivalent of the BC-221. It is however a
very crude instrument, when compared with the refined execution of its U.S. competitor.
Gen-Rad 805-C Standard Signal Generator
The 805-C may be dated around the mid ‘950s. This huge signal generator measures 16” high, 38”
wide and 12” deep; net weight is some 117 ½ pounds. It covers from 16KHz to 50MHz in seven
ranges. A spare range position can accommodate a special set of coils according to custom
requirements. Accuracy is within 1% of the selected frequency. CW or AM; 400 and 1000Hz
internal modulation. Two meters, for modulation depth and for RF output.
13 tubes: 6C8G, 6L6, 6L6, 6L6, 5U4G, 2A3, 2A3, 6SF5, 0D3, 6AL5, 6H6, 3-4 Amperite.
Mial 145
, early ‘950s, and
Lael 145C
, late ‘950s – early ‘960s
Here two signal generators from Italy. The first one, upper three pictures, was made by MIAL,
which eventually sold its line of instruments to LAEL. Although very similar from outside, the
second one uses one miniature tube and a selenium rectifier, all assembled on a PCB.
Mial 145: E1R or ECH4, RF and AF oscillator, and E1R or EF6, rectifier.  100KHz to 22MHz
in six bands.
Lael 145C: 6U8A, plus selenium rectifier. 140KHz to 42MHz in six bands.
Here is a clean British made signal generator. Carrier ranges from 100 KHz to 80 MHz in six
bands. CW or AM, 400Hz internal modulation or external. Tubes: 6X5GT, ECC91, 6SL7GT,
1N21 equiv, 4 x 1N914 equiv. as AF rectifiers.
Note: the original British coaxial connector has been replaced with a standard BNC.
TS-413C/U, Harvey-Wells
Harvey-Wells, Southbridge MA, was a known manufacturer operating since 1939. TS-413C/U
signal generator was designed for United States Air Forces. The one in the pictures had been in use
by ‘Aeronautica Militare Italiana’ or Italian Air Forces.
The generator covers from 75 KHz to 40 MHz in six bands. Signal may be CW or AM. Typical
accuracy better than 1% may be obtained also using the internal crystal calibrator. Two meters give
readings for modulation and RF output.
This set uses 11 tubes: 9002, 6AG7, 6AL5W, 6J6, 6SA7, 5Y3GT, 0A3, 0D3, 6SN7W/GT, 6C4,
Manual: T.O. 33A1-8-83-2/AN16-35TS413-3
Measurements 65-B Standard Signal Generator
Measurements Corporation, in Boonton NJ, was directed since 1940 by Harry W. Houck, who in
1922 had built the superheterodyne with Major Edwin H. Armstrong. Measurements was
appreciated for its production of very fine equipment. This standard generator shows several unique
features, such as the push-button band selector, the motor-driven RF tuning and the porcelain base
oscillator tube, 6SK7 custom built by RCA for Measurements.
Model 65-B covers from 75KHz to 30MHz in six bands; CW or AM modulation, 0 to 100%, 400 or
1000Hz internal, or external. Output from 0.1 microvolt to 2.2V.
Tubes: 5U4GB, 0D3, 6V6, 6H6, 0D3, 6L6, 6SJ7, 6SJ7, 6SK7 special base, 6AG7, 6H6, plus
3A10B ballast.
Military R.F. Signal Generator Set AN/URM-25*
The AN/URM-25* was a family of military signal generators in production from 1953 to about
1966 for U.S. Navy. Frequency coverage from 10KHz to 50MHz in eight bands. 0.5% tuning scale
frequency accuracy, 0.25% using the internal Xtal calibrator. Extremely rugged construction.
Pictures show a model AN/URM-25D. Tubes used are: 6AH6, 6AH6, 6AG7, 6AL5W, 6BE6W,
5814A, 6AH6, 0A2WA, 6X4WA.
Military R.F. Signal Generator Set AN/URM-26*
The URM26 was the VHF companion of the URM25, with the same shape and using a similar
cabinet. It covered the range from 4 to 405 MHz in six bands; output pulse modulated, continuous
and AM.
Several versions were produced through the years: the equipment shown is the version SG-
117/URM-26B, built in 1975 by the contractor Twenty-First Century Electronics Inc., Riverside,
7 tubes: 5675, 5814A, 6005, 5726, 6X4, 0B2, 0B2
The SG-24/TRM-3 AM, CW and FM sweep marker generator covers from 15 to 400 MHz in six
bands. The marker can be set at one of four crystal controlled frequencies: 200 KHz, 1, 5, 20 MHz.
35 tubes, plus one silicon diode are used into this massive generator. The RF unit is fully shielded
inside a die cast aluminum box. The FM deviation can be as high as 20%, the FM modulation being
achieved through a magneto-dynamic unit driving the movable part of the tuning capacitor. Tubes:
6841, 5UP1, 2X2A, 0A2, 5R4GA,  6X4,  0B2,  RTF4,  6005, 12AT7, 12AX7, 5814A, 5654, 6481,
5UP1, 2X2A, 0A2, 5R4WGA,  6X4W,  0B2,  3TF4,  6005, 12AT7WA, 12AX7, 5814A, 5654.
Frequency Meter Set I-129-B
This set can be considered as the simplest frequency meter available: each module is a simple LC
resonating circuit and the resonance causes a current change in the monitored RF source. The set is
supplied by Hickok, as indicated on the label, but the modules and probably also the carrying case
were made by James Millen. The three modules cover from 1.5 to 18 MHz.
Signal Generator 8616A Hewlett Packard
Here a late vacuum tube generator from Hewlett Packard. 8616A covers from 1.8 to 4.5 Ghz in a
single band. The RF PIN attenuator and the klystron are inside the two aluminum castings visible in
the lower left picture. The low frequency circuits and part of the power supply are mounted on two
printed circuit boards.
TS-147* X-band Signal Generator
TS-147*, as its equivalent TS-35, was one of the early generators for checking and servicing X-
band radars during WWII. This unit was still in use in 1986 by Italian Aeronautica Militare.
TS-148/UP X-Band Spectrum Analyzer
Even if this is not a signal generator it could be used as frequency meter. It could be considered as
the first X-band spectrum analyzer, used to check and service radar sets since WWII. Only 14 tubes
were used by this masterpiece of ingenuity and of engineering. This unit has been kept in service by
Italian Aeronautica Militare until 1984.
More to come:
Signal Generator 1108AF Polarad
FM Signal Generator I-208, Detrola
I-72 Signal Generator