Last update

  7  August 2010





C-6287 / ARC control panel                                        RT 742/ ARC51  transceiver

                                                                              (click to open the units)


This UHF transceiver was widely used in military aircraft and on the ground throughout the  seventies. This remote controlled, very compact set, operates with AM  in the  225-400 MHz military communications band., and was made by Rockwell-Collins.

The unit is powered by 28Vdc, and with 20W RF output, the dc input current is 10A.

A control panel is needed to set the frequency, and stores 20 preset frequencies.

The modular unit contains 11 (planar) tubes of 6 different types and 71 transistors.



The  original ARC-51X  had 1750 channels with 100kHz spacing,  soon followed by the  ARC-51A, B, AX or BX , all with  3500 channels at 50kHz spacing. The A stands for  a telephone impedance of 150 ohm, B is 600 ohm.

There is one fan inside ( sounds like a jet engine), and another outside the pressurized unit.

The "X" indicates that both fans are powered from the internal dc/ac/dc inverter.  Without the “X”, an external 400Hz source is required  only for the fans.  Inside the unit are straps to select :   carbon or dynamic microphone, 150 or 600 ohm telephone, silence or tone during  channel changing, and 4 or 6 kHz audio bandwidth.


Removable parts fixed to  outside of case:

ID-1003/ARC   Standing wave ratio indicator ( mounted on front, see above)

HD-615/ARC-51   Air cooler and filter, mounted on rear of  RT742(*)

The PRC-41 is a portable version of the ARC51 with  1750 channels, and 3W output.



Frequency control

Three rotary knobs on the radio control select the tens of MHz (18 positions), units MHz  (10 positions) and  kHz (20 positions, in 50kHz steps). Each rotary switch position is transferred by 5 or 6 contacts to the ARC51-BX. 

The receiver is a triple superhet, the first knob controls the first  local oscillator (LO) ,  etc.

There are 18 + 10 + 20 = 48  Xtals, selecting  18 x 10 x 20 = 3600 channels. The first 100 are below 225MHz and may not function well.

During transmit (key down), the oscillator frequencies are added backward from third to first LO using the same IF filters as in the receive mode. This requires a lot of relays.

The intermediate frequencies are 20-30 MHz ( tuned), 2.9-3.9 MHz (tuned) and 500kHz (fixed). More details  here

The first local oscillator ( 200-370 MHz in 10MHz steps) has 18 crystals. Its predecessor, the ARC27 had  only one crystal (10MHz) , followed by a  harmonics spectrum generator. Still, the first Local Oscillator in the ARC51 is called “the spectrum generator” as well.

The radio control C-6287/ ARC-51 allows  either manual frequency selection , or one of  20 preset channels. Presets are programmed by shifting 8 pegs over  19 positions on a rotating drum.  For details and circuit diagram look here


Guard receiver

A  separate 243MHz guard receiver is inside the ARC51,  operating in the T/R+G position of the mode switch. When the frequency mode switch is in “GD XMIT“, the unit not only receives on this guard frequency, but transmits there as well.


Automatic Direction Finder

The ARC51 can be connected to an automatic  UHF Direction Finding Antenna.

This is the  AN/ARA-25  with tubes, or the solid state version AN/ARA-50.

An external coax relay switches the  ARC51BX antenna cable from the normal communications ( blade) antenna to the directional antenna  when  “ADF” is selected on the radio control, except during transmit.  More on its function here.


AN/ARC51BX   Circuit diagrams

             External wiring  to control, headset and power

             External wiring by Collins

             Control head C-6287

             Module circuit diagrams


Special versions

The AN/ASQ-19 and AN/ASQ-88  nav/comm bundles had a modified ARC51BX as the comm part . This was the RT-793/ ASQ  ( 1964)

This  version  accepts frequency control  in  BCD format  thanks to a built-in  BCD  into 2-out-of-5 code translator.  Further, the power supply has no inverter transistors , instead  the transformer is fed directly with 400Hz, like the fans. The external fan is not  placed on the rear, but on the side of the unit.   You can find  pictures and more about the ASQ-19  here.


The PRC-41 is a portable version of the ARC51 with  1750 channels, and 3W output.

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