[ARC5] WWII aircraft microphone and audio circuit wiring question

[jcoward5452 at aol.com]

Thanks for the replies Gentlemen,
 Next question: How was the shielding "dressed" at the connectors? What I've seen is a wire soldered to the shield and then backed out of the shell and secured to a screw of the cable clamp useing a crimped lug. How might this be done with "A.R.C." type connectors with the wire loom?
 Thanks,
             Jay

-----Original Message-----
From: Mike Hanz <aaf-radio-1 at aafradio.org>
To: arc5 <arc5 at mailman.qth.net>
Sent: Fri, Sep 30, 2011 8:19 pm
Subject: Re: [ARC5] WWII aircraft microphone and audio circuit wiring question


Well, there was a valid reason that I equivocated in my answer, since I 
idn't want to get everyone mired in the minutia.  Again, from the 
ocuments I have here, it all depends on both the aircraft and the 
pecific interphone system involved.  The earlier USAAC systems up 
hrough RC-34 have a note that states "If audio howl or feedback occurs 
n any installation, the microphone and headphone circuits may require 
hielded wire"...thus leaving it up to the aircraft manufacturer.  The 
ater RC-36 installation instructions say what Mike described below.  
hat policy clearly evolved over time, and by the time they hit the 
eplacement of all the BC-347 interphone amplifiers with the AN/AIC-2 in 
arly 1945, the note read, "The wire attached to terminal 7 [input 
erminal] of the interphone amplifier and terminal 4 [ring of the mic 
lug] of each [BC-1366] jack box shall be shielded."  The later AN/AIC-4 
nd following interphone systems followed the example I linked in my 
revious e-mail, and added radio receiver outputs to the list, as in the 
C-36.  The Navy seems to have preferred shielded wiring only on the 
icrophone lead, and included shielded wire in their multi-conductor 
ables used for interphone interconnection for that purpose.  The note 
n the RL-7 manual, for example, says "Particular notice should be taken 
hen examining Figure 14 regarding the two single shielded wires in this 
able, which connect to terminals #1 and #10 on the cable receptacle.  
hese are microphone circuits and are shielded to prevent crosstalk 
etween the talking circuits and the balance of the system."
So, now that everyone's eyes are glazed over - like everything in this 
usiness, "it all depends..." :-)
73,
ike
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) 
ere
 shielded.  I guess it wasn't necessary with the low impedance carbon 
icrophones
 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 
nvironment
 made that necessary.  But shielding was only required if there was more than 
en
 feet of separation between the receiver and the interphone jack boxes.

 Mike / KK5F



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