Collins RT-712/ARC-105 Restoration

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RT-712 400 Watt PEP USB, LSB, AM

I believe the ARC-105 system was deployed on the EA-6B Prowler, F-4C, F-105, RF4C, F-101 Voodoo, and early F-111. The ARC-105 was also used on the Sikorsky H3 Helicopters, tail numbers 777, 711 and  801 (from a Navy technician). The high altitude O-Ring seal would sure keep out any corrosive ocean air, good idea.

There is not much information floating around the Internet. I have received some feedback from Air Force personnel to confim a few of these. It is a pressurized version of the famous Collins R-618T-2 that is operational up to 75,000 feet. I decided to give this model a go because I could count on no internal corrosion from laying around in warehouses.  The ARC-105 system consisted of the RT-712, 2 mounting racks, 1 cockpit control head,  1 antenna tuner controller and 1 antenna tuner. See the brochure for the model numbers. Phil at Fair Radio was very helpful getting this madness started.

Letter from Naval Avionics Technician.

The RT-712 is very similar to the 618T-2 but no internal modules are plug compatible. Fortunately this model can still use the passive cockpit control head (rotary switches and pots). The ARC-119 looks similar, has another frequency select digit, and  requires digital communication to the control head - not recommended for ham fun. 

The 26 pin Amphenol front panel connector is available at Newark and Mouser. The rear connector is of the unobtanium flavor. I am going to try Amphenol, gold plated individual pin sockets with shrink wrap and labels.  A first RFQ pass through all the surplus aircraft parts suppliers has turned up a used  MT-3094/ARC-105 aluminum mount for $4,400- no way! The high pressure cooling air connectors will be simple to adapt PVC pipes.

W7CPA ARC-105 RT-712        W7CPA RT712 Back
Collins RT-712/ARC-105 Receiver-Transmitter USB, LSB, AM, 400 Watts PEP (High Altitude RT-618T-2)


W7CPA ARC-105 RT-712        W7CPA RT712 Back
RT-712 Rack Connections (Elecrical and Air) for PA and PA Connector Side

W7CPA ARC-105 RT-712        W7CPA RT712 Back
RT-712 PA Access Lid, 4CX-250F Vents and Beautiful, Compact, Collins PA Inside View


W7CPA ARC-105 RT-712
RT-712 Cover Off - Nice Modules with Many Test Points and Annotations

Cockpit Control Head

I found this in Israel via eBay. It was used with the RT 618T-2 so there are some differences I can live with. The correct control head did not have CW and Data selection, had a Self Test push button and a panel light to show RF output. I'll either drill some holes, ignore or just remote the changes in the box where I'll mount the unit in. The ARC-105 control head had a test switch, CW/Data,  select, RF/Test indicator, and band select lines for the coupler controller only. These are not (hopefully) required for ham usage.

W7CPA ARC-105 Control Head
Collins 714E-3 Control Head - Will Need Some Adaption Since not Exactly the ARC-105 Model

Aviation Headset - OK, So I'm Getting Carried Away!

Thiis is high quality, normally very expensive, military helicopter headset made by the famous Dave Clark Company. I found a NOS unit for $50 - sweet!

W7CPA ARC-105 Headset
Miltary Helicopter Headset by Dave Clark with 6 Pin U-229 Connector and PTT Box

Heat Exchanger Blower

The RT-712 cooling design does not lend itself to cheap hammie Rontron fan placement. Cold air must be circulated through the heat exchanger as designed. An internal blower in the sealed heat exchanger assembly distributes cool air to all of the plug-in modules via the vertical 1" thick channel that all modules plug into. I found a 400Hz blower from a retired radar system. A heavy duty food storage box from Bed and Breakfast (heavy duty lid not shown) did the trick nicely.


W7CPA ARC-105 Blower
400Hz Blower Assembler

115VAC 400 Hz 3 Phase, 10KW  Power Supply

The Collins brochure shows that 400Hz 3 Phase, 115VAC, 1.2KW is required. This is not something that one buy can buy from Astron or HRO. Aircraft equipment runs on 400Hz to reduce weight.  I found a beautiful NOS 10KW part out of a miliary helicopter. I believe it was a ground maintance capability without having to drag/drive out a power unit out to the aircraft. It operates with 220VAC 60Hz, single or 3 phase input. It originally cost DoD $50,000 so I had to have it. Its 10KW capability gives me good headroom <grin>. I tested it and it is truly new. Mind you, the horrendous  fan noise is something I need to fix if I plan to operate in the same room or the same house!

The left unit does power factor processing and generates 400 VDC. The unit on the right is a sophisticated 3 phase inverter that makes 3 beautiful 115vac, 400Hz waveforms with 10KW of ju ju!

Stephen Pastore (stephenpastore@dc.rr.com) is a good source for surplus military power supplies. He has a few of these beauties left.

W7CPA ARC-105 Power 1
Power Supply - Front

W7CPA ARC-105 Power 2
Power Supply - Mounting Unit Connectors and Damn Fans

W7CPA ARC-105 Power 3
Power Supply - Back

W7CPA ARC-105 Power 4
Power Supply - Beautiful, Overkill Cables and Connectors