[ARC5] WWII aircraft microphone and audio circuit wiring question

[Mike Morrow]

Mike Hanz wrote:

> Yup.  The earlier systems usually described the usage in notes on the 
> drawings, rather than showing each input that was shielded.

The RC-36 (with the BC-347-* interphone microphone amplifier) generally
utilized shielded cables on three of the ten circuits that typically came
into the BC-366 crew jack box.  I have several BC-366 boxes that were cut
out of aircraft with cable stubs still remaining, and they reflect what's
listed below.  

Those three shielded cables carried:

BC-366              Signal
Pin 1  AF output from the SCR-269 ADF receiver (BC-433).
Pin 6  AF output from the SCR-287 liaison receiver (BC-348) and
                 from the liaison transmitter (BC-375) sidetone.
Pin 10 AF output from the SCR-274-N command receivers (BC-453, 454, 455) and
                 from the command transmitter modulator (BC-456) sidetone.

None of the microphone audio input circuits (interphone amp, command, liaison) were
shielded.  I guess it wasn't necessary with the low impedance carbon microphones
and the high DC excitation currents flowing through them.

It surprises me that the AF outputs of the receivers required shielding, since
none of these outputs were amplified by the interphone system.  Perhaps the
relatively high impedance (even when "low") in an electrically noisy environment
made that necessary.  But shielding was only required if there was more than ten
feet of separation between the receiver and the interphone jack boxes. 

Mike / KK5F