From: Loek d'Hont

As I am someone who likes "weird" HF radios, the Rockwell-Collins 718U-5M avionics 
HF radio sure fits that bill for me. 
This is a vintage avionics radio in all aspects, still in use in aircraft today. 
In military aircraft, the designation for this radio is the AN/ARC174(v).

The military version is virtually identical to the commercial version except for two additional modes 
on the control module to add secure digital voice modes.

The 671U-4A receiver-exciter design used in this installation (the right larger box in the picture) 
stems from the early seventies and was used in many different HF radio configurations by Collins. 
The amplifier-tuner 548S-3 is of later design, as it was preceeded by an earlier tube version driven 
by the same receiver-exciter (those used in the 618 series).

(Editor note: This power amplifier (548T-1 series) used the 4CX600 ceramic tube, good for 400 watts PEP or average.)

The control unit (514A-7) allows the radio installation to be controlled remotely from the cockpit 
dashboard, through a serial data protocol. It is designed around an early Motorola microprocessor.
An older version of this control box was the 514A-4, having a mechanical dial instead of the 
7-segment plasma display that the 514A-7 control unit uses.
The mechanical dial drove BCD switches which were decoded by a TTL card and converted to the same serial protocol.
 
The 718U-5M radio has 10 digital memories which also store selected mode.

In "VFO" mode, toggle switches for each frequency digit allow rapid frequency change acros the band. 
The radio can be tuned anywhere between 2 MHz and 30 MHz in 100 Hz steps. 
It tunes the antenna automatically as soon as the microphone PTT is activated.
The 548S-3 amplifier (larger black-box on the left) is fully solid-state and has a built-in antenna tuner. 
This is not one of those limited range tuners, this thing tunes anything from coat-hangers to 
large dipoles and long wires. It does this by means of a motorized roller inductor and vacuum capacitor.
RF power is 100 watts PEP and average, specified on the antenna-tuner output terminals. 

This in practice means the solid-state amplifier is delivering 150 watts internally to the tuner, 
to make up for the tuner losses of about 2 dB. The design is in concept very close to the Collins 651S receiver.

The receiver-exciter is built as a card-cage concept similar to the 651S receiver, in fact 
it shares with this receiver the same three synthesizer cards, a conceptually identical RF module 
and uses the same I.F. chain concept, meaning it uses the same 500 kHz mechanical filters 
for USB, LSB and AM but in steel case though (the 651S uses plastic cased filters that can 
age due to moisture penetration).

The whole installation runs of a single-DC voltage anywhere between 22 and 32 volts, 
with 27.5V aircraft bus voltage as the nominal. Current consumption is about 16 amps at single-tone modulation.
Construction of this radio is superb. Only the best components are used throughout, 
and everything is coated with a moisture-protective varnish. 
The digital logic is built with early TTL components packaged in an early-style SMT case. 

There are two versions of the 671U-4A receiver-exciter: the Collins round emblem version, and 
the later Rockwell-Collins version. 

The round emblem is actually a bit nicer from the inside, although slightly older in design. 
Rockwell later re-designed the circuitry for the digital cards to facilitate the newer 
available integrated circuits, as it became more and more difficult to find spare 
parts for the older version. But as said, the older round emblem version is somewhat nicer built.
The later version was manufactured up to the late eighties. 

That is almost 20 years of production for this radio!
Those of you that have read my somewhat critical review of the Rockwell-Collins 
KWM-380 now know why: I am kind of spoiled with the "real" Collins stuff. 
Compared to this radio the KWM-380 is a toy. Many of you might not realize it, 
but Rockwell-Collins has continued building the best in HF transceivers up 
to this very day; unfortunately not for the ham community any more...

The 718U-5M is fun to operate on the HF bands. 

The receiver works excellent and is very robust against overloading. 
Modulation sounds different from ham radios I have been told, quite 
penetrating but probably less pleasant to listen to.
It is hard to say what this kind of stuff would be worth, love is in the eye of the beholder. 
All I can say is that it probably does not make much sense to buy the individual 
modules at flea markets as it is difficult to find all the necessary individual 
components, let alone the information how to tie it all together. 
But if you are a serious collector and appreciate the best in Collins gear: go for it! 

Loek d'Hont

And updated 25 March 2005

Some rectifications from Loek d'Hont AC5XP

- The second IF frequency for this radio system is not 500 kHz as I mistakenly wrote 
  but 450 kHz (as also is the case for the Collins 651S receiver on which this 
  transceiver is based; that part was correct)

- The official name used in databooks for what I called "Early style SMT ICs" is 
  in fact "Flat Pack". The "Flat Pack" case style indeed is the precursor of 
  today's SO (Small Outline) SMT packages