SCR= Signal Corps Radio

The American SCR-274N "Command" radio system consisted of three receivers and two transmitters in a configuration specific to the Lancaster MR/MP. It was used for short range aircraft to aircraft connunications.

An FT-220A chassis connects three receivers and the BC-450 remote control box. Frequency coverage is 190 to 550 KHz, 3 to 6 MHz and 6 to 9 MHz. A remote control permits independent selection of frequencies on each receiver. A FT-226 rack supports  two transmitters which are pretuned to their required operating frequency which can be in the bands 4 to 5.3 MHz or 5.3 to 7 MHz and 7 to 9.1 MHz .  Also connected is a voice modulator unit  (BC-456B), remote control box (BC-451) and an antenna changeover unit (BC-442). Selection of either CW, voice or MCW operation is facilitated by the remote control box. A fixed, wire antenna mounted on the starboard side of the Lancaster aircraft is shared between receiving and transmitting via the antenna changeover relay. Audio from all three receivers can be applied simultaneously to the station boxes via JBR No.1 and adjusted individually from the remote control box. .

In the US Army,  SCR-274N series radios were available in black wrinkle paint, or natural aluminum finish. The equivalent US Navy equipment (referred to as ARC-5 series) were shipped with black wrinkle paint only. In the Lancaster, the radios were normally bare aluminum but on occassion both types could be found.

Transmitter Power Output: 24 watts
Transmitter Input Power (each) : 28 VDC @ 9 amps at full power and 2.5 amps when in standby.
Receiver Input Power (each) : 28VDC @1.6 amps

Three receivers in FT-220 rack. (Photo courtesy Wikipedia)
Typical black crinkle finish receiver. (Photo by Jerry Proc)
FT-220 rack and FT221 mounting details for three transmitters The rack for the two transmitter configuration was similar. (Signal Corps photo)
Two transmitters in an FT226 rack. Two of the standard frequencies used were 3105 and 3023.5 KHz. (Photo courtesy Wikipedia)
Typical black crinkle finish transmitter. (Photo by Jerry Proc)
SCR-274N accessories. 1- BC-456 Modulator   2-Antenna Relay BC-442   3-Remote Control BC-450   4-Control Box BC-451     (Photo courtesy Wikipedia) 
Close up of the BC-450 remote control box for the receivers. Left dial for 3 to 6 MHz. Centre dial for 190 to 550 KHz. Right dial for 6 to 9.1 MHz.  (E-bay photo)

lanc_scr274_cording_diagram_s.jpg This is the cording diagram for a 3 receiver, two transmitter SCR-274N radio system.  Click to enlarge.  (Diagram courtesy Consolidated Aircraft B-24 Radio Systems Handbook, 1943)

Credits and References:

1) SCR-274 manual TO 12R2-3 SCR274-2 dated December 1943
2) Lancaster engineering manual EO 05-25A-2 dated 30 August 1957
4) B-24 Radio Systems Manual  - Consolidated Aircraft, 1943

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Oct 22/09