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Radio direction finder
BC-792-A was a portable covert
radio direction finder,
first manufactured during WWII – in 1943 –
by Andrea Radio Corporation on Long Island (New York, USA).
The device was used by military intelligence for finding clandestine radio stations (spies)
during WWII
and during the first part of the Cold War.
It covers 100 kHz to 60 MHz — LW,
MW
and part of the SW bands —
in AM
and CW.
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The device is housed in a pig skin leather suitcase – typical for the era –
that was constructed in such a way, that it could be used as a
radio direction finder whilst carying it inconspicuously.
The device is powered by rechargeable batteries and has two antennas: a
directional loop that is hidden inside the suitcase, and a vertical
telescope reference antenna that protrudes the case at the top.
Before starting a direction finding job, the receiver must be set to the
desired frequency band, after which the TUNING knob should be used to tune in
to the station under surveillance.
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The suitcase is then closed, after which volume, BFO and sensing
can be adjusted from a small control panel that is
hidden under a leather flap
below the carrying handle. The control panel has a socket for connection
of a miniature earpiece
of which the wiring should be guided through the
sleeve of the operator's coat, allowing inconspicuous deployment.
The complete set – including
suitcase and
accessories – was known as
SCR-504-A.
Its first user was the OSS
(now: CIA).
It is unknown how many devices were made, but given the surviving
serial numbers, it is likely that it was less than 1000. The unit shown
here has serial number 10 and was used after WWII by the
Radio Monitoring Service (RCD)
of the Netherlands. Another one – with serial number 330 –
was used by the Radio Monitoring Service of the British
Department of Trade and Industry (DTI).
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The diagram below provides an overview of the features of the BC-792-A.
The device is disguised as a regular leather travel suitcase, which is shown
here with its lid open (towards the front). The actual receiver is visible
at the centre, with its 8 valves (tubes) aligned along the lower case edge.
There are two antennas: (1) a directional loop – hidden behind a leather
flap at the top – and (2) a telescopic reference antenna which protrudes the
case at the top, close to the rightmost lock.
At the centre is an 8-position band selector, with the frequency scale above it.
The frequency to be monitored, should be selected with the TUNING knob before
closing the suitcase. A miniature earpiece is then connected to the small
control panel that is hidden under a leather flap below the carrying handle,
with its cable guided through the operator's sleeve. When walking on the street,
the direction to an illegal transmitter can be determined by rotating the
body — and hence the suitcase — and looking for the highest signal strength.
Audio volume, Beat Frequency Oscillator (BFO) — used for morse signals —
and sensing, are all adjustable from the
small control panel.
The telescopic reference antenna serves two purposes: (1) it can be used as
a non-directional pickup antenna – by pressing the SENSE button on the small
control panel – and (2) it resolves the 180° ambiguity that is inherent to
loop antennas. The proper use of the antennas is explained in
the manual by
means of the Donald Duck cartoon shown above [A p.30]. It shows that the
highest sensitivity is at the short side of the receiver that is closest
to the telescopic antenna.
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The receiver is housed in a pig-skin leather suitcase that
was common for the era, allowing it to be carried around
inconspicuously. The case measures 550 x 380 x 17 mm, and weights
11.6 kg (receiver and batteries included).
Considering its age, the leather case shown here is in excellent condition,
with only minor wear to the stitching. Of most surviving sets, the leather is
in much worse condition, and on some it has even (partially) decomposed.
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The actual receiver is housed in a metal – black wrinkle paint coated –
enclosure that is usually fitted inside the leather suitcase, although it
can also be used on its own. It is shown here with the small control panel
– usually hidden under the suitcase's carrying handle – fitted at the top.
When the receiver is removed from the leather suitcase, the
TRANSFER switch
at the top left is disengaged, which means that the audio volume should
be controlled with the large volume knob on the large front panel.
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To allow inconspicuous operation, the set came with a miniature earpiece,
of which the wiring could be guided through the sleeve of the operator's
coat, onto the control panel
under the grip of the suitcase.
The earpiece consists of an R-27-A receiver (speaker), a molded ear insert,
and a CD-655-A cord with two miniature plugs at the end.
The molded ear insert is missing here.
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The BC-792-A is powered by three rechargeable lead-acid
batteries: one BB-51
6V battery for the filaments of the valves (LT) and two series connected
BB-52 batteries
that supply 36V each (72V together) for the anodes of the valves (HT).
The batteries are
installed behind a hinged lid
at the front right of the receiver.
A syringe was supplied for filling the
batteries with acid, along with
special instructions.
The image on the right shows the two battery types side by side.
The blue tape was applied to avoid leaking acid.
➤ Extract from the manual
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The complete set (SCR-504-A) came with a charger for the LT and HT
batteries. It allows all three batteries to be recharged simultaneously,
from a 6V or 12V source, such as a car battery.
The image on the right shows the original battery charger. At the top
is a lid behind which three batteries can be charged simultaneously:
one BB-51 (6V) and two BB-52 (36V) batteries.
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Technical manual
TM-11-862
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The receiver is housed in a metal black enclosure that is permanently
fitted inside the leather suitcase. In order to remove it from the
suitcase, four flat-head screws have to be removed: two at the bottom
and one at either side of the case, at the edge that is covered by
the case lid.
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Remove the brass end from the telescopic antenna, and collapse the antenna
completely (inside the case). Remove the 4-pin and 5-pin plugs from the
control panel, and the two single-pin ones from the top of the receiver.
The receiver may now be removed from the case.
Next, the interior of the receiver can be accessed by removing the rear case
shell, which is held in place by 10 screws. The interior – shown in
the image on the right – consists of 6 compartments that are separated by
(orange) metal dividers, to which some of the components are mounted.
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The right half contains the Radio Frequency (RF) sections, and has no less
than 28 adjustable capacitors. The left half contains the IF section –
with five filter units – and the audio stage. Note that in the unit shown here,
a makeshift output transformer has been inserted
before the headphones socket, to provide a galvanic separation between the
receiver and a recording device.
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The BC-792-A (SCR-504-A) was released in 1943 and was initially intended for
finding illegal (spy) radio transmitters during the latter part of WWII.
But as the Cold War
broke out as soon as WWII had ended, the device remained
in use for many years in several countries, including the USA, the UK and
The Netherlands. As a result, many devices had to be modified over time,
as – for example – supply of batteries dried up. So far, we have registered
the following modifications:
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As the original batteries were hard to obtain in post-war United Kingdom,
and possibly also because the original battery charger had been lost,
the British radio moitoring service (DTI) modified the battery
compartment of this unit, so that it could be used with the batteries
of the MCR-1 receiver
– which were readily available.
Note the 4-pin socket that is
bolted to the side of the lid of the
battery compartment of this former DTI unit.
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Internal power supply unit
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In this unit — that was found without the leather suitcase —
the battery compartment has been replaced by an internal power
supply unit (PSU), allowing the device to be powered by a single
(12V) supply, such as the battery of a car [6].
Note that the lid of the battery compartment was removed completely,
and has been replaced by a pertinax panel with two switches: one
for the LT and one for the HT voltage [6].
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Another modification — spotted on the same device as above — is the
replacement of the regular American valves by narrower
alternatives with protective spring-loaded caps [6].
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In this BC-792 — that was used after WWII by the British radio monitoring
service (DTI) — a 1:1 output transformer has been added. It provides
a galvanic isolation between the circuitry and an external (tape) recorder
that could be connected to the headphones socket.
Note that the transformer is fitted to one of the mounting posts of the
12-way band selector, and is held in place with black isolation tape.
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This video was taken several years ago by Steve Ellington (N4LQ),
and shows the BC-792-A (SCR-504-A) with serial number ??? — at the time part of the
collection of Brian Harrison (KN4R) — in full working order.
Footage via YouTube (retrieved April 2020) [4].
Note the big stamped letters MFP which indicate that it has had an
anti-fungus treatment (MFP = Moisture Fungus Proofing).
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Frequency 100 kHz - 60 MHz Bands 8 (see below) IF 455 kHz (band 1, 3, 4 and 5)
910 kHz (band 2, 6, 7 and 8) 1 BFO 454 kHz Valves 8 (see below) Batteries 1 × BB-51 (6V, 100mA) 2
2 × BB-52 (total 72V, 17mA) 2 Case Pig skin leather Dimensions 500 × 400 × 150 mm Weight 9.5 kg
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- 1 x 1LB4
1 x 1LH4 (VT-177) 1 x 1LC6 (VT-178) 5 x 1LN5 (VT-179}
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- 100 - 205 kHz
- 240 - 470 kHz
- 540 kHz - 1.1 MHz
- 1.2 - 2.4 MHz
- 2.7 - 5.6 MHz
- 6 - 13 MHz
- 14 - 29 MHz 1
- 29 - 60 MHz 1
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These bands band are not provided with a sensing circuit.
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Power supply 6V/1.2A or 12V/600mA
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Formerly used by OSS (CIA).
Obtained from the collection of Museum Jan Corver [1].
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Without leather suitcase. With different valves.
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850-MPD-43 S/N 10 635-SCGDL-43 All other S/N listed above
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Document scanned by and obtained from Google.
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Document kindly supplied by Cor Moerman [1].
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- Cor Moerman, BC-792-A in suitcase, S/N 10 - THANKS !
Museum Jan Corver. Received March 2020.
- Louis Meulstee, RDF Receiver SCR-504-A
Wireless for the Warrior - Volume 4.
ISBN 0952063 36 0. 2004.
- Radio Museum, Direction Finding Set SCR-504-A BC-792-A
Visited March 2020.
- Steve Ellington (N4LQ), WWII spy receiver
YouTube. Retrieved 18 April 2017.
- H. Keith Melton, Ultimate Spy
ISBN 978-0-2411-8991-7. Page 139.
- Richard Brisson, Photographs of different BC-792 units and manuals
Received April 2020.
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© Crypto Museum. Created: Wednesday 18 March 2020. Last changed: Saturday, 25 September 2021 - 07:20 CET.
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!['Donald Duck Finds the Sense' -- cartoon taken from the manual [A p.30].](img/bc792a_dduck.png)
![Three different manuals for the BC-792-A. Photograph kindly provided by Richard Brisson [5].](img/303383/044/full.jpg)
![Modified BC-792-A with internal PSU. Photograph by Richard Brisson [6].](img/303383/048/full.jpg)
