[ARC5] Re: [Milsurplus] Taming the BC-375 (Interphones)

[Mike Morrow]

>If you have an old T-17 microphone in original condition,
>forget it.  Were it capable of driving the rig, it will sound awful.
>I have a of dozen and none are suitable.  The elements 
>have not aged well, and were intended to be used with 
>interphone amplification before being applied to a rig in any case.

Dave,

It's a shame when a NOS T-17 is found to be almost inert.  But common.

Regarding the usage of aircraft interphone systems, I am not aware of any WWII-era US military aircraft interphone system that amplified mic audio prior to sending it to a transmitter input.  This includes the very common RC-36 that likely served as the interphone on the great majority of USAAF WWII aircraft, and this also includes the USN RL-series of interphones.  The mic was switched directly to the transmitter's mic input, which supplied carbon mic element excitation derived from 24 vdc.

There is also a common notion among many that the AF outputs of the several receivers (command, liason, compass, etc.) connected to the interphone system were amplified by the interphone amp.  They weren't.  The receiver audio heard at any control station came directly from that receiver's final AF stage.

The only signal amplified by WWII-era interphone amplifiers is mic audio from a calling station that was being sent to other interphone stations.  

If the interphone amp failed (for example, tube failure in the single-tube BC-347 amp of the RC-36), interphone stations still have fully functional radio (mic, receiver AF, sidetone AF) connections.  There were some limited-function backups to loss of the BC-347 amp.  For the SCR-274-N system, the BC-451-A transmitter control box can be selected to a non-existent transmitter.  Then when a mic whose interphone switch is selected to COMMAND is keyed, modulator AF sidetone will be applied to other interphone stations without transmitting RF.

Mike / KK5F