[ARC5] WWII aircraft microphone and audio circuit wiring question

[Mike Hanz]

Well, there was a valid reason that I equivocated in my answer, since I 
didn't want to get everyone mired in the minutia.  Again, from the 
documents I have here, it all depends on both the aircraft and the 
specific interphone system involved.  The earlier USAAC systems up 
through RC-34 have a note that states "If audio howl or feedback occurs 
in any installation, the microphone and headphone circuits may require 
shielded wire"...thus leaving it up to the aircraft manufacturer.  The 
later RC-36 installation instructions say what Mike described below.  
That policy clearly evolved over time, and by the time they hit the 
replacement of all the BC-347 interphone amplifiers with the AN/AIC-2 in 
early 1945, the note read, "The wire attached to terminal 7 [input 
terminal] of the interphone amplifier and terminal 4 [ring of the mic 
plug] of each [BC-1366] jack box shall be shielded."  The later AN/AIC-4 
and following interphone systems followed the example I linked in my 
previous e-mail, and added radio receiver outputs to the list, as in the 
RC-36.  The Navy seems to have preferred shielded wiring only on the 
microphone lead, and included shielded wire in their multi-conductor 
cables used for interphone interconnection for that purpose.  The note 
in the RL-7 manual, for example, says "Particular notice should be taken 
when examining Figure 14 regarding the two single shielded wires in this 
cable, which connect to terminals #1 and #10 on the cable receptacle.  
These are microphone circuits and are shielded to prevent crosstalk 
between the talking circuits and the balance of the system."

So, now that everyone's eyes are glazed over - like everything in this 
business, "it all depends..." :-)

73,
Mike

On 9/30/2011 10:35 PM, Mike Morrow wrote:
> 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
>
>
>
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