[ARC5] Distribution of ARC-5 Audio

[Mike Morrow]

Without building upon or referencing material from earlier postings, here is the story on receiver audio routing within the three command set systems.

There are significant differences between the ARA or SCR-274-N systems and the AN/ARC-5 system with respect to audio routing.  For a common base of comparison, I'll compare a typical three-receiver installation of each system.

(1) ARA: The receiver rack CBY-46149 normally contains a CBY-46105 (HF Comms), CBY-46145 (Broadcast Band, for use with a ZB-* VHF honing adapter), and CBY-46106 (HF Comms).  The receiver control box is CBY-23251.

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(2) SCR-274-N:  The receiver rack FT-220-A normally contains the BC-454-B (HF Comms), BC-453-B (Beacon Band), and BC-455-B (HF Comms).  The receiver control box is the BC-450-A.


The SCR-274-N is electrically identical except for AF impedance to the ARA.  The following description applies exactly to both systems:

There is a switch below each receiver in the three-receiver rack that MAY be used to assign that receiver's AF to AUDIO BUS A (left), to NO AUDIO BUS (center), or to AUDIO BUS B (right).  This switch is normally maintained in the center "NO AUDIO BUS" position for all three receivers.  There is a screwed-on brass protector installed on the switch to maintain it in that position.

There is also a switch on each of the three sections of the receiver remote control box that may be used to assign that receiver's AF to AUDIO BUS A, to NO AUDIO BUS, or to AUDIO BUS B.  This is the only switch that is normally used to align a receiver's AF to the desired audio bus.

Typically, the communications receivers would be assigned to one audio bus and the navigation receiver to the other in multi-pilot aircraft where separate use of the communication and homing receiver outputs is useful. 

Regardless of where (rack or control box) the AF has been switched, the affected receiver's AF will appear at all phone jacks for that audio bus at BOTH the rack AND the control box.  If the rack is set to one bus and the control box is set to the other for one particular receiver, then both AF bus A and B will be tied together at all phone jacks for ALL receivers that have been connected to a bus.  This is not a desirable configuration.  Preventing this is the purpose of the rack's select switch protectors which maintain the rack switches for all receivers in the NO AUDIO BUS position.


(3) AN/ARC-5:  The receiver rack MT-65/ARC-5 normally contains a R-4A/ARR-2 (VHF Homing), R-26/ARC-5 (HF Comms), and R-28/ARC-5 (VHF Comms).  The receiver control box is the C-38/ARC-5.  This was a common carrier-based single-seat fighter configuration.

The AN/ARC-5 allows selection of the audio bus for each receiver's AF audio ONLY AT THE RACK.  There are no control boxes that have that capability.  In the AN/ARC-5 the rack audio bus switches are ALWAYS used, *lock-wired* in the proper position to small front-rack attachment points that are found on AN/ARC-5 receiver racks only.

In this system, the communications receivers R-26/ARC-5 and R-28/ARC-5 are selected to AUDIO BUS B at the rack.  The homing receiver R-4A/ARR-2 is selected to AUDIO BUS A at the rack.

The AN/ARC-5 eliminated all AF phone and mic jacks on all control boxes except the rarely-employed C-39 and C-48/ARC-5 auxiliary control boxes.  Even on those, there are no phone jacks for bus A versus bus B audio.  The audio buses are used at the rack as a way to route the audio from two receivers separately through control boxes from the AF of the third receiver.

The C-38/ARC-5 is the only multi-receiver control box in the AN/ARC-5.  It has only an RF sensitivity "set and forget" thumb-wheel control for the R-26, one gross RF sensitivity-dropping ON-OFF switch for the R-26, and one for the R-28.  There is a common VOLUME control for R-26 and R-28 audio.  The C-38 connects to the combined audio of the R-26 and R-28 through rack audio bus B.

On the right third of the C-38 are all the controls for the R-4A/ARR-2 VHF homing receiver, including an RF sensitivity control.  The C-38 connects to the audio of the R-4A through rack audio bus A.

The C-38 passes the audio bus B (communications) signal through the VOLUME control on the C-38.  The C-38 then ties that variable-level AF to the audio bus A (homing) signal (whose volume is controlled by the R-4A sensitivity control on the C-38).  As a result, audio bus A contains the R-4A audio (adjusted by the R-4A SENSITIVITY control on the C-38) plus the combined audio from the R-26 and R-28 (adjusted by the VOLUME control on the C-38).  Audio bus A is then sent from the receiver rack to the MD-7/ARC-5 modulator to the J-16/ARC-5 audio jack box, into which the phones are plugged.  That completes the path from receiver to headphones!


What happens to the audio output from each of the three receivers is considerably more involved for the AN/ARC-5 system than what happens in the simple straight-forward ARA and SCR-274-N systems.

Only after all that comes discussion of the aircraft interphone system...if any.  :-)


Over the years I have collected complete sets as described above of all three systems...lacking only some ARA and ATA racks and mounts.

The versatility of the AN/ARC-5 with AN/ARR-2 is much greater than that of the earlier systems.  One can not properly understand how the AN/ARC-5 system is configured if one has referred only to a manual for the SCR-274-N.  :-)

Mike / KK5F