Click for homepage
BC-792-A   SCR-504-A
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.

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.
  
BC-792-A direction finder inside leather suitcase

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).

Leather suitcase with BC-792-A direction finder BC-792-A direction finder inside leather suitcase BC-792-A in open leather suitcase BC-792-A in leather case -- open -- revealing the loop antenna Control hidden under the leather grip One of the batteries partly removed Small control panel under the carrying handle Earpiece
A
×
A
1 / 8
Leather suitcase with BC-792-A direction finder
A
2 / 8
BC-792-A direction finder inside leather suitcase
A
3 / 8
BC-792-A in open leather suitcase
A
4 / 8
BC-792-A in leather case -- open -- revealing the loop antenna
A
5 / 8
Control hidden under the leather grip
A
6 / 8
One of the batteries partly removed
A
7 / 8
Small control panel under the carrying handle
A
8 / 8
Earpiece

Features
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.

Click to see more

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.

'Donald Duck Finds the Sense' -- cartoon taken from the manual [A p.30].

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]. It shows that the highest sensitivity is at the short side of the receiver that is closest to the telescopic antenna.

Battery compartment Opened battery compartment One of the batteries partly removed Band selector and frequency scale Suitcase lid sensing switch Smll control panel hidden under leather flap Small control panel
B
×
B
1 / 7
Battery compartment
B
2 / 7
Opened battery compartment
B
3 / 7
One of the batteries partly removed
B
4 / 7
Band selector and frequency scale
B
5 / 7
Suitcase lid sensing switch
B
6 / 7
Smll control panel hidden under leather flap
B
7 / 7
Small control panel




Parts
Leather suitcase Receiver
RX
Headphones Rechargeable batteries Battery charger Technical manual
Leather suitcase
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.

  
Leather suitcase with BC-792-A direction finder

Receiver   BC-792-A
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.

  
BC-792-A receiver with remote control panel on top

Earpiece   HS-34-A
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.
  
Earpiece

Batteries   BB-52 & BB-52
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 one of the HT batteries (BB-52). The blue tape was applied to avoid leaking acid.

 Extract from the manual

  
Battery - top

Charger   PE-128 — wanted
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.

The charger is currently missing from our set.
  
Click to see more

Technical manual   TM-11-862
Several manuals were available for the BC-792 – such as operating instructions, technical manual, and repair instructions – but very few have survived. Of these manuals, only the Technical Manual (TM-11-862) has been scanned by Google and is available for download below.

 Download TM-11-862
  
Three different manuals for the BC-792-A. Photograph kindly provided by Richard Brisson [5].

Leather suitcase with BC-792-A direction finder BC-792-A direction finder inside leather suitcase Earpiece Earpiece - rear Earpiece - front Earpiece plugs Battery - top Battery - bottom
C
×
C
1 / 8
Leather suitcase with BC-792-A direction finder
C
2 / 8
BC-792-A direction finder inside leather suitcase
C
3 / 8
Earpiece
C
4 / 8
Earpiece - rear
C
5 / 8
Earpiece - front
C
6 / 8
Earpiece plugs
C
7 / 8
Battery - top
C
8 / 8
Battery - bottom




Click to see more

Click to see more

Interior
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.

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.
  
RF section

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.

Serial Number 10 -- Note the order number '850-MPD-43' which is different from most others BC-792-A receiver with remote control panel on top Bare BC-792 receiver (with valves removed) Connections to the control panel External control panel (fitted under carrying handle) Internal view of the control panel Control panel -- sockets for wiring to receiver Control panel fitted to the bare receiver
Interior Interior RF section Many adjustments in the RF section Filters Audio section with makeshift output transformer Battery compartment seen from the interior Tuning mechanism
D
×
D
1 / 16
Serial Number 10 -- Note the order number '850-MPD-43' which is different from most others
D
2 / 16
BC-792-A receiver with remote control panel on top
D
3 / 16
Bare BC-792 receiver (with valves removed)
D
4 / 16
Connections to the control panel
D
5 / 16
External control panel (fitted under carrying handle)
D
6 / 16
Internal view of the control panel
D
7 / 16
Control panel -- sockets for wiring to receiver
D
8 / 16
Control panel fitted to the bare receiver
D
9 / 16
Interior
D
10 / 16
Interior
D
11 / 16
RF section
D
12 / 16
Many adjustments in the RF section
D
13 / 16
Filters
D
14 / 16
Audio section with makeshift output transformer
D
15 / 16
Battery compartment seen from the interior
D
16 / 16
Tuning mechanism

Modifications
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:

Alternative batteries
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.
  
Alternative power socket

Internal power supply unit
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 has been removed, and has been replaced by a pertinax panel with two switches: one for the LT and one for the HT voltage [6].
  
Modified BC-792-A with different valves and internal PSU

Alternative valves
Another modification — found on the same device — is the replacement of the regular American valves by narrower alternatives with protective caps [6].   
Modified BC-792-A with different valves and internal PSU

Output transformer
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 switch, by means of black isolation tape.
  
Makeshift output transformer

Alternative power socket Battery lid with makeshift power socket Modified battery compartment Modified BC-792-A with different valves and internal PSU Makeshift output transformer Audio section
E
×
E
1 / 6
Alternative power socket
E
2 / 6
Battery lid with makeshift power socket
E
3 / 6
Modified battery compartment
E
4 / 6
Modified BC-792-A with different valves and internal PSU
E
5 / 6
Makeshift output transformer
E
6 / 6
Audio section

Video footage
BC-792-A in action

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 anti-fungus treatment (MFP = Moisture Fungus Proofing).


Specifications
Complete set   SCR-504-A
  • BC-792-A
    Radio Receiver (direction finder)
  • PE-128-A
    Portable battery charger
  • CD-658-A
    Cord for PE-128-A
  • ?
    Synchronous 6V vibrator
  • CS-96-A
    Carrying case for PE-128-A battery charger
  • Valves
    Spare set of 8 valves
  • BB-51
    Battery 6V (1x)
  • BB-52
    Battery 36V (2x)
  • HS-34-A
    Headset, receiver R-27-A, cord CD-655-A
  • ?
    Ear inserts (2x)
  • TM-11-862
    Technical manual for SCR-504-A
Receiver   BC-792-A
  • 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
    1 x 1LB4, 1 x 1LH4,VT-177, 1 x 1LC6/VT-178, 5 x 1LN5/VT-179
  • Battery
    1 × BB-51 (6V, 100mA), 2 × Battery BB-52 (total 72V, 17mA) 2
  • Case
    Pig skin leather
  • Dimensions
    500 × 400 × 150 mm
  • Weight
    9.5 kg
  1. Using 2nd harmonic.
  2. Rechargeable by means of PE-128-A charger.

Frequency bands
  1. 100 - 205 kHz
  2. 240 - 470 kHz
  3. 540 kHz - 1.1 MHz
  4. 1.2 - 2.4 MHz
  5. 2.7 - 5.6 MHz
  6. 6 - 13 MHz
  7. 14 - 29 MHz 1
  8. 29 - 60 MHz 1
  1. This band is not provided with a sensing circuit.

Charger   PE-128-A
  • Power supply
    6V/1.2A or 12V/600mA
Known serial numbers
  1. Formerly used by OSS (CIA). Obtained from the collection of Museum Jan Corver [1].
  2. Without leather suitcase. With different valves.

Known order numbers
  • 850-MPD-43
    S/N 10
  • 635-SCGDL-43
    All other S/N listed above
Documentation
  1. Radio set SCR-504-A, Technical Manual
    TM-11-862. War Department (USA), 22 November 1945. 1

  2. Radio Receiver BC-792-A Repair Instructions — wanted
    TM-11-4060. October 1945.

  3. Le Poste SCR-504 (Radiogoniomètre) — wanted
    TM-11-862 (French). 1952.

  4. SCR-504 Datasheet
    War Department (USA). Date unknown.
  1. Document scanned by and obtained from Google.

References
  1. Cor Moerman, BC-792-A in suitcase, S/N 10 - THANKS !
    Museum Jan Corver. Received March 2020.

  2. Louis Meulstee, RDF Receiver SCR-504-A
    Wireless for the Warrior - Volume 4. ISBN 0952063 36 0. 2004.

  3. Radio Museum, Direction Finding Set SCR-504-A BC-792-A
    Retrieved March 2020.

  4. Steve Ellington (N4LQ), WWII spy receiver
    YouTube, 18 April 2017.

  5. H. Keith Melton, Ultimate Spy
    ISBN 978-0-2411-8991-7. Page 139.

  6. Richard Brisson, Photographs of different BC-792 units and manuals
    Received April 2020.
Further information
Any links shown in red are currently unavailable. If you like the information on this website, why not make a donation?
Crypto Museum. Created: Wednesday 18 March 2020. Last changed: Saturday, 25 April 2020 - 07:14 CET.
Click for homepage