From: "Loek d'Hont"
To: kgreene
Date: Fri, Mar 25 2005 3:35:03 pm

Kevin, more interesting stuff - Attached pictures of the VRC-81. This is a 
military land-mobile version of the 718U-2 that I once owned.
It was in use in various foreign armies, the Netherlands army had it in use 
until the mid-nineties (it was replaced there with a Harris radio)

The VRC-81 has most of the modules that you describe on your site:

 - 671U-4D HF receiver exciter 2..30 MHz
 - 635U-1 Pre/Post selector
 - 636X-2A internal power conditioner (28V)
 - C-4526 Radio Set Control
 - PP-4415 27.5 VDC 50 AMP power supply (not shown)
 - RT-4402/MT-4405 equipment case and mount
 - 548T-1A PWR AMPL

The C-4526 radio set control (see pictures) uses a serial FSK protocol over 
a single twisted pair connection. This was done to allow remote control over 
several miles in the battlefield, through a field-telephone hookup. In this 
way, if the enemy homed-in on the radio with a nasty missile, the radio 
would be destroyed but not the operator!

That is also the reason why there is a 671U-4D version: To facilitate this 
slower (but more robust) FSK control scheme. The modulo-2 control method is 
much faster but does not fit in a 3 kHz phone bandwidth. So that protocol is 
mainly used in aircraft where bandwidth is not an issue for the control 
signal, as the link between radio and control unit is relatively short.
The difference for the 671U between the FSK method and modulo-2 method is 
therefore only the control card.

However, there is another feature that the 671U-4D has (as used in the 
VRC-81) that the A version has not: It has additional SMC connectors to 
link-in the 635U-1 pre-post selector which would then be switched-in before 
the 671U RF stages

This PPselector allows the use of multiple VRC-81 radios operating in close 
frequency range on the battlefield, thus increasing large-signal handling 
for the radio system as a whole. For aircraft this was not so much a 
requirement (you don't want those things to come too close to each other for 
obvious reasons, hi) so that is why a 635U PPselector is rare in aircraft 
installations.

A 671U-4A unit will work in a VRC-81 (assuming the FSK control card is 
swapped for the modulo-2 in it) but the PPselector will not be in use in 
that case, even when it is installed in the mounting rack. I usually used 
the VRC-81 without the PPselector anyway (if you pull it a relay will 
automatically bridge the input-output RF link for the 671U-4D so the radio 
will still work) because if you had the 635U installed, it takes the radio 
much longer to tune when you change frequency, even for a 100Hz step!

In the pictures you also see the 490T-1 ATU. The VRC-81 normally used the 
490T-3 ATU (which has a cooling fan added) which was also mounted inside a 
(separate) olive drab protective case, but I never had that one. Actually, 
the whole family of Collins 490T tuners work with the 718U without a 
problem, they all have the same radio interface.

I bought this installation many years ago from a Rockwell-Collins technician 
located in Cedar Rapids. It was in poor condition but very restorable. I 
spent several years restoring it to new condition (getting spare parts was a 
real bear) . Later, when it was all completed, I sold it to a Japanese ham 
(real nice fellow) who also is an enthusiastic Collins collector. I had to 
sell it because we moved to a smaller house. Somehow I now regret that sale, 
but that's the way it goes.

Feel free to place this on your site

Cheers,

Loek