[ARC5] Pilot Headset Audio Control In A Typical AN/ARC-5 System

[Mike Morrow]

Ken wrote of the "command set" receivers":

> Well, depending on the model (early vs late) the AF output is either .5 watts 
> or .75 watts. I have found that if the impedances are properly matched and a 
> speaker is connected, even 0.5 watt output is enough to rattle the windows

In most of the WWII era, when there was an aircraft interphone system that distributed audio from the command, liaison, compass, and interphone systems to the various call stations, the AF amplifier in the interphone system amplified **only** the carbon mic inputs...no amplification of AF output from command, liaison, or compass systems by the interphone system occurred.  So that 12A6 in a command set receiver could wind up feeding 10 headsets in parallel at various interphone stations of a B-17 or similar.  The ambient noise was very high, yet the 12A6 produced more than enough AF for all those headsets in a combat aircraft that were selected to COMMAND.  The same thing is true of the liaison and compass AF output stages.  As Ken states, AF from any of these aircraft sets operated within specifications should be much more than adequate for any likely hobbyist application.

>> so they just ran it wide open and vary the the RF gain which determines how
>> much audio will appear at the detector.  This was a old time legacy design
>> that was shared by many radios in the pre-war era.
>
> Actually, as I understand it, there is an AF gain control in the external controls 
> or rack somewhere. Mike would know. (Either one). Maybe that was on the intercom
> though.

There are about 37 Mikes on the list.  :-)

The only "command set" system that uses an AF VOLUME control in some installations is the AN/ARC-5, in particular those using the most common receiver control box, the C-38/ARC-5. 

In these systems, the AF output of the MF/HF comm receiver (R-25, 26, or 27...labeled MHF REC. C) passes through a simple AF-voltage dividing VOLUME potentiometer in the control box before it gets sent to the headset jack box (like J-16/ARC-5).

Likewise the AF output of the VHF comm receiver (R-28...labeled VHF REC. A) passes through the SAME VOLUME potentiometer in the control box that controls the MHF REC. C AF sent to the headset jack box.

Thus, AF level control for both VHF REC. A and MHF REC. C is performed by one common VOLUME control on C-38.

MHF REC. C RF gain also has LIMITED adjustment on C-38/ARC-5...in a way that is likely very unfamiliar to most here.  The control box has a limited-range RF gain control pot of 1.8K ohms max resistance (compared to a full-range gain control of 50K ohms in other control boxes).  Even at minimum RF gain setting (max resistance), the RF gain remains rather high.  This pot is set by a small thumb-wheel with label telling the pilot to "SET FOR MAXIMUM TOLERABLE NOISE".  There is also a MHF REC. C **switch** that when placed in its OFF position inserts a 100K resistance to effectively kill RF stage gain.

The same circuitry and controls exist on C-38 for VHF REC. A.  However, the limited-range RF gain pot is internally bypassed by a jumper that produces maximum RF gain (minimum resistance) in VHF REC. A.  VHF REC. A functions at max RF gain at all times unless the VHF REC. A **switch** is placed in its OFF position, inserting a 100K resistance to kill RF stage gain.

>From these descriptions, it can be seen that the pilot's principle control of the AF in his headset from both VHF REC. A and MHF REC. C is the shared single VOLUME control on C-38.  It is generally necessary to balance the output of REC. C compared to REC. A, and that is done by using the thumb-wheel for REC. C's limited RF gain control to alter its RF gain and AF output compared to REC. A...whose RF gain is not adjustable.

Should the pilot wish to disable the AF he hears in his headset from one of the two receivers, he does so by placing the RF gain **switch** for the unwanted receiver to OFF.  This switch has nothing to do with operating power to the receiver.

So...that's the details of how the most common AN/ARC-5 installations controlled the AF levels from the two communications receivers in the pilot's headset.  It's very different than that used in ARA and SCR-274-N three-receiver command sets.

I just said "three-receiver", but I've only discussed VHF REC. A (the R-28/ARC-5) and MHF REC. C (a R-25, 26, or 27/ARC-5).  The third receiver in the late-war modern AN/ARC-5 receiver rack would be the R-4*/ARR-2 VHF homing receiver.  The right one-third of the C-38 has controls for that.  The R-4 is controlled by a wide-range 50K RF gain pot labeled SENSITIVITY.  The AF output of the R-4 is sent to the headset jack box without going through any AF level potentiometer like REC. A's and REC. C's common VOLUME control.

So...the pilot balances the RF gain of Rec. C against REC. A using REC. C's RF gain thumb-wheel, then afterwards controls the AF level heard from both receivers with their common VOLUME control.  Then, when using the R-4 homing receiver the pilot employs its SENSITIVITY (RF gain) control to get the desired AF level in his headset from that receiver.

It may be of interest that the C-38 has no communications receiver TUNING controls.  VHF REC. A is four-channel crystal-controlled, but those channels are selected on the C-30A/ARC-5 MF/HF/VHF transmitter control box.  MHF REC. C is a frequency-stabilized pre-set locked-tuned receiver and requires no tuning control for the pilot.

VHF REC. A is VOICE mode only.  System interconnection wiring sets MHF REC. C for VOICE (MCW) only.  No communications receiver mode controls are needed.  The only communications receiver controls on C-38 are the RF gain and AF level controls discussed earlier.

And what about the C-38's REC. B?  Its circuitry inside C-38 is very similar to REC. C's.  But it is used only when C-38 is part of a liaison-style installation with an ARB as REC. B.  So that's NA to this three-receiver AN/ARC-5 and AN/ARR-2 discussion.  REC. B's controls would be hidden under metal covers on the C-38.

The C-38 controls for R-4/ARR-2 are more complete.  In addition to the SENSITIVITY control discussed earlier, there is a CW-VOICE mode switch, a CW PITCH control, and a mechanical six-channel selector for the homing receiver's MF modulation frequency section.

FWIW, in addition to AN/ARC-5 systems using the C-38/ARC-5 receiver control, any system using the C-27, C-39, and/or C-48/ARC-5 controls have AF level pots in the system used as described earlier.  AN/ARC-5 receivers using the C-26 and C-125/ARC-5 controls did not.

AN/ARC-5 installations are generally quite different from earlier ARA/ATA and SCR-274-N systems in system capability, complexity, flexibility, and versatility.  It can be very non-obvious how components are configured, if all that one knows about are the two earlier command set systems.

Mike / KK5F