Hello John,
Since I last contacted you I have finalized things with the 712. The following is what I have done:

1.                   I use a dynamic microphone/headset arrangement from Roanwell Headsets.
2.                   I have made a case for the control panel (714-E3) which I purchased from Fair Radio as a control panel for an ARC-95 system.
3.                   Inside the case I have mounted a 317 three terminal adjustable regulator to provide 12 volts from the 32 volts which
                 is supplied via the main cable from the 712.
4.                   This regulator powers an 8 pin IC pre-amplifier for the microphone and a 5 terminal 3 watt amplifier for headphones and speaker.
5.                   A provision for PTT is also provided for the headset and a separate front panel jack for a foot switch.
6.                   A TUNE ENABLE miniature switch is mounted on a panel at the rear (actually the front) of the 712 for tune-up.
7.                   From the front panel on the 712 a cable runs to my case which supplies switching for the main antenna/receive
                  antenna connectors on the 712. This cable also takes voltages to jacks on my case so as to enable metering important voltages from the 712.
8.                   The original main antenna connector on the 712 has been changed to a type “N”. 

I have today had QSO’s with two of the Route 66 special event stations. Conditions were deplorable and other than the signal being very readable it was too difficult to comment on signal quality. I run about 400 watts PEP with no problems.

The 3 phase 400 Hz. 110 volt supply is a Pacific Power Systems ASX-315 laboratory supply. This is overkill but I obtained it rather inexpensively.

I also have a PP-2352/UR solid state supply (I think made for the ARC-75 system) which is a 2 kilowatt, 3 phase, 400 Hz, 110 volt supply which runs off 28 (24) volts.

OK heres an update  Dated Jan 10, 2018

 

 Collins RT-712
Part of ARC-105 A pressurized communications system especially designed for
tactical jet aircraft with performance characteristics comparable to the 618-T.

 

I purchased my RT-712 (712) quite a few years ago and although I used it to receive on from time to time I never seriously set about putting it on the air.

During the period soon after I purchased it I spent much time, money and effort attempting to provide the three phase, 400 Hertz, 110 volt
power required. Initially I purchased three 110 volt single phase 400 Hertz units which operated from a 28 volt source. Attempting to
maintain the necessary 120 degree phase difference between the three units proved an impossibility. 

Next I had a 24 volt high current automobile alternator rewound such that it produced the requisite 110 volt three phase power
when coupled to a 240 volt synchronous electric motor; whilst this was successful the heat rise over time precluded this method from being
used over a long period of time. 

Next I purchased three military aircraft power supplies, the PP-2352 UR, which were designed to provide three phase, 400 Hertz,
110 volt power and were used, I believe, in the ARC-58 system which appears to be a precursor to the ARC-105 system.

These units however produced only square wave power other than for a one phase filtered sine wave for use
with low power waveform conscious devices.

Whilst I now had what in the future would be a totally suitable supply for the 712, I was somewhat loathe
 to place its square wave output into my 712 so I hunted further for what I considered, at that time, something more suitable.

This I eventually found through a friend who knew of someone who was selling a Pacific Power Source 315-ASX
programmable power supply capable of more than satisfying my requirements. This supply is a beautiful thing
and is capable of one, two or three phase output of programmable output voltages and output frequencies and can
provide up to two kilowatts of power. It is all solid state. Its input requirements are 120 /240 volts at 50/60 Hertz.

 I now set about preparing the 712 for airtime operation. 

When I initially acquired the 712 I purchased a second one as spares plus a separate power supply module and
at that time I purchased a 714-E3 cockpit control unit which, whilst not designed specifically for the 712,
works perfectly without any modification whatsoever (contrary to what some would have you believe).
It should further be noted that some modules are interchangeable between the 712 and the 618T series,
again contrary to what some claim.

The female connector which formed part of the original rack mount was impossible to obtain thus some other means
needed to be found to connect the 712 mounted male to external devices such as the control head, power supply and audio systems.

To this end I purchased a multitude of Amphenol 18-12S connectors which I could purchase without the shell.
Thus I had enough female pins, of the correct size, to place over the male pins in the ITT Cannon connector on the 712.

 These female pins are perfect for the job.

 The connector needed to mate with the connector at the rear of the Control Head is an Amphenol PT06A20-30S(SR)
and is readilly available from various suppliers.

 For the three phase power to the 712 I used  Amphenol PT06A14-5S(SR)  connectors, male mounted on the rear
of the 712 and female to the Pacific Power Systems unit. For mic, speaker and PTT connections I used a Cannon 6 pin
connector and mate but anything suitable could have been used. 

Although my installation is now such that the control head and the 712 are within a few feet of each other I had originally
envisaged a much greater distance separating the units thus my cabling is much longer than necessary. 

The pressurized system requires that cooling air be passed through the unit so as to remove the warmed internal air.
To this end I used a surplus Sony fan model No. SFF22C. Attached pictures show the fan but as is obvious any suitable alternative could be used.

I was unable to purchase schematics for the RT-712 but was able to, for an outrageous price, purchase copies of the 618-T1 manual
which references the ARC-105 and has the schematics necessary for maintenance and wiring of and for the 712. I am led to
believe that more up to date data is available from KØIP at a very reasonable price. I paid USD250 for my manual and I
think that KØIP (pocatelloarc.org/618t)  gets a $5 donation. 

I then set about fabricating the housing for the control head and mic and speaker controls, sockets etc. 

Within my control module I have placed a 12 volt three terminal regulator (supplied by a 32 volt source
coming in to the connector on the control head), a five watt audio amplifier integrated circuit and an LM380
preamp integrated circuit for the dynamic microphone which I use. This microphone by the way is part of a
Roanwell military headset and the insert has a response which is flat from 600 to 3000 Hertz. All reports that
I have received note than the audio quality is excellent. 

When the unit is powered up and completely enclosed in its housing very little noise emanates from it, this
however is in stark contrast to the fan noise generated when the finned cover is removed. This little fan turns
at 20,000 RPM and although small is very noisy. The fan from the PP-2352 is also very noisy.

My unit puts out a very comfortable 300 watts PEP and is really just idling along. I could turn the mic gain up
but things are fine at the 300 watt level. On AM 100 watts carrier is the norm. I am not a CW person so have
never used it on CW although provision is there for this option. Both upper and lower sideband are supported. 

Initially I completely stripped the unit from its chassis and sand blasted and painted the base and case and at that
time I replaced the main antenna connector with an “N” type. This was accomplished by fabricating a circular
fitting drilled such that the type “N” mounted in it and I attached it to the 712 lower case via three machine screws. 

On the front of the unit are two antenna connectors, the type “N” mentioned earlier and a “BNC” connector which
can be used in installations where a separate receive antenna is desired. This connection is enabled by connecting to
ground, pins of either the unit front connector (16-26P) or the main rear connector.

Further to this is that all important voltages can be monitored via the front 16-26P connector. I found the mate
to this connector a PT026A16-26P(SR) difficult to obtain but did eventually manage to find one.

There were two more “modifications” which I performed. The first was the change from 100 microfarads “AGC Capacitor”
in the AM module to 4.7 microfarads as the AGC release rate was too slow (for me) and the grounding at the “Sidetone”
potentiometer of the “Audio Cold” point. Neither of these modifications were vitally necessary but for me they made sense.

Finally, this is a beautiful unit, very typical of Collins and rock solid in stability. I find it a joy to operate and my only wish
is that I was able to tune to the half KC points, but that is a very small complaint and it is far outweighed by everything else about the machine.

 Pictures are attached so as to show how I solved various mechanical issues.

 Phillip Grimison VE6ABW

 Langdon Alberta Canada  e-mail: phillip.grimison@yahoo.com