History
The BC-224 and BC-348 receivers were designed by RCA for the American Signal Corps in the later half of the 1930´s. More than 100.000 (!) units were manufactured , most of them by Wells Gardner and Belmont Radio, both in Chicago.
The receivers were used in heavy bomber and transport aircraft during the second world war and all through the Korean war, the most wellknown being the B17 "Flying Fortress", the B24 "Liberator" and the B29 "Superfortress". The photos show a B17 on a bomb run and the radio operator with a BC-348 and the J37 key.
The B24J Liberator "Witchcraft" is operated by the Collings Foundation and making summer tours in the USA. The photos below show the aircraft, the cockpit and the radio operators position with a well used BC-348-N made by Wells Gardner. Thanks for the B24 photos to Tom Murphy, San Lorenzo, California. This is the only B24 aircraft still flying!
Receiver data
The BC-348 and its predecessor BC-224 were used in almost all American and also some British and Canadian bombers and transports as well as in some ground vehicles, from the second world war and 15 years beyond. BC-224-A was developed in the mid 1930´s, and after extensive tests an improved version, the BC-224-B, was adopted as a standard by the US Signal Corps. From this B model all BC-224 and BC-348 receivers were built to the same specification and were so similar that most parts were interchangable, except for the built-in dynamotor for voltage conversion from the aircraft electrical system. BC-224 was built for 14 V and BC-348 for 28 V. Eventually there also were differences in the valve complement. The Signal Corps could easily aquire and maintain the receivers due to the tight standardization.
The receivers were very reliable and easy to maintain. Electrically they remind of the ground receiver AR88 also designed by RCA, but the weight is only around one third of the AR88 weight. The BC-224/348 is a superheterodyne with two RF and three IF stages. In the IF part there is a crystal filter with the positions ”Out/In”, that is wide/narrow. The versions BC-224 A to D and BC-348 B to C (BC-348-A does not exist) cover 1.5 to 18 MHz in six bands. In the later models the receiver covers the frequency range 200 to 500 kHz and 1,5 to 18 MHz, still in 6 bands. The IF is 915 kHz. The sensitivity is in average 1.2 uV (min 7 uV) for 500 (MVC) – 800 (AVC) mW out. The bandwidth is 7 kHz (+- 3.5) for @ - 6dB in the higher bands 1.5 – 18 MHz in position wide, 600 (+- 300) Hz @ - 6dB in position narrow.
Most sets built were of the BC-348-R type, all built by Belmont Radio. These have the valve 6K7 in the RF and first IF stages, 6C5 as oscillator and 6J7 as mixer, 6F7 as second IF / beat oscillator and 6B8 as the third IF / detector stage. The final AF stage is a 6K6GT. All valves except the 6F7 and the 6K6GT ar of the metal type. The RF, oscillator and mixer stages are module built with the components in separate shielding boxes.
The models BC-348 J, Q and N were manufactured by Wells Gardner with point to point wiring which had then become an industry standard, They also had the so called "single ended" valve design meaning valves without top connectors. Still the specifications were the same and most parts were still interchangeable with other models.
In the aircraft the receivers were driven by 14 or 28 V over a built in rotary converter, but in many receivers the converter has been changed to a more or less well designed mains driven power supply. The receiver measures 46 cm x 27 cm x 24 cm (w x h x d), and the weight is 16 kg.
My receiver
My receiver is a BC-348-R which I bought on the auction site Tradera. The receiver was sold at a low cost as non-working, when it was tested by the seller a condenser in the filter unit blew. The receiver did not look too good in the photo but I took a chance that it would be fine inside.
My receiver was built by Belmont like all BC-348-R, in 1943 against the order 31414-WF-43 and it has the serial number 612.
The picture shows the receiver in a typical restoration situation, upside down and with the plate covering the RF tube shelf
removed. The receiver had been repainted and was a bit too shiny in some spots, but it was very good inside, almost like new!
It has the older type of tubes with the top connectors and not the later "single ended" design.
The dynamotor had been replaced by a 220 V power supply, which I find to be an acceptable change for practical reasons!
No other modifications were found, but the rotary switch for AVC-OFF-MVC was faulty and replaced with a DPDT switch.
The output tube was a 6Y6G instead of a 6K6GT.As usual the power supply was badly designed, so the first action was to redesign it. It had no safety grounding and no fuses (when will radio amateurs learn about safety???), and when tested with a dummy load of 70 mA it gave 350!!! V out, the poor receiver is designed for around 220 V. The filtering was very bad, no choke or resistor, only a condenser of 16 uF! The transformer was fine and it fed a nice, classical 5Y3GT. The filament chain had been rewired for 6,3 V instead of the original 28 V and worked well.
More on the condition and the restoration of the receiver can be found in the chapter Restoration, and more photos in the chapter Photo Gallery.
More on WWII receivers
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Updated October 23rd 2009