The BC348-O Communications Receiver

This is a receiver I've always wanted since seeing them advertised as government surplus 40 years ago. Prices were never low as there always seemed to be a strong export demand with as many people advertising to buy rather than sell. Originally designed by RCA in the mid-30s this marque was to be built by various manufacturers and supplied mainly for airborne use, such as in the B17, and some mobile use in vehicles such as Jeeps. Earlier examples carried the number BC224.

You can tell who made a particular receiver by the letter which follows the number.

Stromberg-Carlson: E, M & P

Belmont Radio Corp: H, K, L R & S

RCA: B, C & O

Wells Gardner: J, N & Q

It is said that versions "A, D, F, G & I" do not exist.

The model "O" shown here is made by RCA; however all was not so clear cut because many receivers were recalled or subsequently rebuilt after first manufacture to add an extra MF waveband. These usually carry internal labels showing the progression from one type to another.

The valves used depended on who made the set and as the original line-up was invariably military coded, both these and their commercial equivalents, are quoted. The "O" type uses:

VT86 (6K7), 1st RF Amplifier; VT86 (6K7), 2nd RF Amplifier; VT91 (6J7), Frequency Changer; VT65 (6C5), Oscillator;

VT86 (6K7), 1st IF Amplifier; VT70 (6F7), 2nd IF Amplifier & CW Oscillator; VT93 (6B8), 3rd IF Amplifier, detector & AVC;

VT48 (41), AF Amplifier; RCA991, Voltage Stabilizer

The sets six wavebands are 200-500 KHz, 1.5-3.5 MHz, 3.5-6.0 MHz, 6.0-9.5 MHz, 9.5-13.5 MHz, 13.5-18.0 MHz

Early sets didn't have the lower band and had slightly different tuning ranges to the later models. To help with dial reading, only the band in use is visible through a window coupled to the wavechange switch. This was presumably a useful feature in a darkened aircraft when the only illumination was in the tuning window of the receiver.

The BC348 has a few interesting features. The volume control is actually a pair of potentiometers in tandem. One operates as an RF gain control and the other an AF gain control. When the AVC switch is set to manual or "MVC", the RF potentiometer is in circuit. When AVC is selected the AF pot is operational. In an ordinary RF circuit, internal noise increases as the frequency is raised so in the BC348, in order to keep the audio output flat, there is a compensating circuit governed by a potentiometer driven from the end of the tuning condenser. Pretty unusual?

Because the set was originally designed for use in a 14 or 28 volt environment in an aircraft the set uses a built-in dynamotor (rotary transformer) to develop the HT line. In my example this has been removed and a small mains transformer fitted for this purpose. To simplify the heater supply requirement a standard 6.3 volt heater transformer has also been fitted and the valve heaters rewired for parallel instead of series/parallel operation.

In operation the tuning control is very smooth with no backlash and resolution of SSB extremely good. Getting from one end of a band to the other is quite a chore involving some 100 revolutions of the knob. Tuning resolution is superb, to the extent that even though Droitwich is only 2KHz lower than the lowest dial reading, it is too far for the receiver to tune it properly. Stability is excellent as the chassis and general construction is very solid being based on welded aluminium castings. A crystal filter may be switched into circuit to improve selectivity and the BFO is adjustable. The Intermediate Frequency is unusual being 915KHz and fault-finding is possible via test points accessible under a detachable plate which can be seen on the right of the front panel.

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