[Milsurplus] Interphone question

[Mike Hanz]

Mike Morrow wrote:

>Mike wrote:
>
>>..he was looking at BC-347 usage in a B-29...
>>
>Was the lowly and simple BC-347 still being used in B-29s?  
>

Up until late 1944, it seems.  I have a UR (Unsat Report) here dated May 
1944 that cites problems with the amplifier:

Interphone     Interphone Equipment        Activity believes that       
  Due to the numerous failures reported on
Amplifier       becomes Intermittent and    the use of Vacuum Tube   
this equipment, ARL Wright Field, has
BC-347 in      at times the Amplifier         VT-99 is the cause of   
    developed a new Interphone Equipment AN/
B-24             goes out completely at         this 
trouble.                    AIC-2 which corrects defects inherent in
                    altitudes above 15,000 ft.  
                                        the RC-36. When sufficient 
quantities of
                               
                                                                   the 
new equipment is available, interphone
            
                                                                                      
equipment in B-26-(  ), B-17-(  ), B-24-(  ),
                                                                         
                         and B-29-(  ) airplanes now in service will
            
                                                                                      
be changed with this new item. In the
                                                                         
                         interim, frequent exchange of Vacuum Tube
                                                                         
                         VT-99 is suggested.


>When did the AN/AIC-1 come into service?
>

I don't have much info on the AIC-1 here, Mike - it was probably one of 
those many prototypes that didn't see any significant deployment.  The 
replacement for the BC-347 used in the scads of USAAF RC-** interphone 
systems was the AM-26/AIC-2, and judging by the URs it must have begun 
to see widespread deployment in the summer of 1944.  The Enola Gay has 
it installed, as you can see at 
http://members.cox.net/aaf-radio-2/Enola_Avionics_Descriptions_-_Radio_Op.htm.

>I've always wondered how reliable the BC-347 amp was.  If just one small glass tube or the PE-86 dyno dies, then the bomber's interphone system is gone.  I think there's some mention somewhere that if the aircraft had a SCR-274-N, an unused transmitter position could be selected on the BC-451 and interphone comms could continue by selecting the COMMAND position on the BC-(1)366 individual jack boxes.  But then, only those BC-366 boxes that could key the command transmitter could speak over this backup.
>

Yeh, that was the dilemma...which jack boxes to enable.  It was 
frequently an issue, apparently.  The Sept '45 ARMN I have here mentions 
a difficulty with the AN/AIA-2 that the Navy used in a variety of 
aircraft, the complaint being that the gunners were keying their mikes 
on the ARC-5 VHF transmitter and sending on the ARC-5 HF channel as 
well.  The use of a common modulator kinda forced that outcome.  The 
resolution wasn't very satisfactory from an operator's perspective - the 
engineers looked at the problem, said 'too hard and too expensive to 
fix...live with it and make the pilot responsible'...then called it case 
closed.  :-)  I wonder if that would fly today?  Heh, heh....

73,
Mike