The BC-348 receiver is a general purpose 8-tube superheterodyne radio receiver. It covers longwave (200 KC to 500 KC) and shortwave broadcast bands (1.5 MC. To 18 MC.), excluding the standard AM band of 500 KC. to 1500 KC. These frequency ranges are divided into 6 selectable bands, which are changed from the front panel. In all 6 bands, it is capable of voice, tone, and c-w reception using either manual or automatic voltage control (MVC or AVC, often also referred to as volume control). They are also capable of receiving SSB transmissions, however they are received by using the C.W. functionality of the receiver, and by manually adjusting the beat frequency. Unlike the command radio, the BC-348 was only operable from the radio operator's station, and not remotely from any other station in the aircraft.
These radios consume 28 volts DC at 56 watts. The internal circuits convert 28 volt 1.25 amp DC input to 240 volt .07 amp DC output, which is then filtered and converted to nominal voltages to power the radio circuits. Output is distributed to jackboxes throughout the plane, and can be heard on the "VHF LIAISON" switch position. Output is also provided to two headphone jacks on the receiver itself, labeled as "TEL." on the front panel. In order to hear the output from the receiver, a headset with a 1/4” jack must be supplied, and plugged into either one the two jacks on the front, or a jackbox switched to "VHF LIAISON."
Output for this receiver is particularly interesting. In a standard radio, output is achieved by placing a headset between the plate (-) of the output tube and the B voltage (A high + voltage, usually in excess of 90 VDC). This means that a standard receiver outputs a negative voltage. However, the BC-348 "outputs" a positive voltage, because it is safer and necessary for all electronic systems on the aircraft to have a common ground, being the aircraft's airframe and skin. The theory being, that the electrons in the negative source (the airframe) will travel through the headset to a positive voltage in the receiver. In order for the receiver to “output” a positive voltage, the receiver employs an iron-core transformer, capacitor, and ground connection.
To understand how adding the transformer is able to switch the positive and negative poles of the output, it's necessary to understand the basic principles of an inductance transformer, and inductance itself. To spare several paragraphs of writing, it will be explained in a simplified form: Inductance can be pictured as a magnetic force, which only occurs when electrons change velocity. This, in itself, is false. However, it is an easy way to explain the way in which a transformer works. If a constant DC voltage is supplied through the primary of a transformer, such as that supplied by a battery, the secondary will give a voltage of zero volts, regardless of number of windings. Apply the same voltage in AC, and the secondary will give a voltage proportional to the ratio of the number of windings on the primary to the number of windings on the secondary. This, again, is an over-simplified example.
The reason for this behavior, is that the electrons in the example of the DC source do not induce an electromagnetic force (commonly referred to simply as EMF) in the secondary, because they are not oscillating ("changing velocity") as they would with AC.
In the receiver, the output from the output tube is DC, however it is oscillating at the same frequency as the signal which was input. For this example, let's assume the input signal is sound in the form of a voice. The voice itself is made up of varying frequencies, which electrically, are simply higher and lower voltages. These varying voltages, when placed in the transformer's primary, are enough to induce an EMF across the secondary, therefor creating an output voltage which is proportional to the ratio of the number of windings on the primary to the number of windings on the secondary. The secondary simply outputs a polarized signal, with a positive and negative side (similar to a battery, however the voltages fluctuate at the same frequencies as the input, as opposed to a constant voltage current).
The core of the transformer and a ground are connected to one side of the transformer, grounding the transformer as well as providing a permanent connection to the common ground of the aircraft. A capacitor is then placed between the two sides of the secondary, to boost output and provide a filtering effect. The negative side of the output can then be wired to the common ground, and the positive output wire can be strung out to the various jackboxes.
Page 70 for schematic diagram
The manner in which output is achieved is particularly important when attempting to use the radio in combination with another audio source, such as a different radio or interphone. This would be achieved inside of a jackbox. While wiring the jackbox, it is particularly important that care is taken to wire the positive and ground wire properly, or else a radio can be shorted, even when the selector switch is not on the designated position. Additionally, while the jackbox selector switch is on “CALL” any audio source which does not use the standard output transformer, will override any other audio sources (for example, a directly driven receiver, or output from a phone, computer, etc) and cause an excessive draw on that source. This is because a directly driven source will route through the output transformers rather than headphones, therefor causing an excessive draw on the source (traveling through a low resistance set of transformers- Low resistance to the overall current because they are wired in parallel relative to the "CALL" switch) and cutting out the signals from said transformers (A direct source of current will override the induced current on the secondary). Never use an excess directly driven voltage for any jackbox function, or the source may exceed operating conditions.
The use of the "CALL" position on the jackbox should only be used before and after flight, or in an emergency. It utilizes the aforementioned nature of the output transformers to broadcast to all switch positions by using the interphone amplifier, overpowering the other signals.