[ARC5] Need Information

[Jack Antonio]

On 3/28/2013 7:31 PM, Floyd Petri wrote:
> Gentlemen I need some help with understanding a few things.
> Is it true that ARC5 Transmitters and Receivers are not interchangeable with
> SCR-274N Transmitters and Receivers?

The receivers will interchange for basic functions. However the AN/ARC-5
"navigation receivers", the R-23 and R-24, had some internal wiring 
changes to support external accessories that the 274N sets did not have.
If you don't have the accessories, then there is no issue. The receiver
mounting racks, and control boxes will also interchange. There are some
minor functional differences, such as the -274N receiver control boxes 
have a power on/off, the AN/ARC-5 receiver control boxes do not.

Also be aware that the AN/ARC-5 receivers had a slightly different
tube line up, the 2nd IF is a 12SF7 instead of a 12SK7. This was done
to implement an AVC circuit that the -274N did not have. But the
receivers would still work if swapped.

> Is it true that this non-compatibility if any is due to the plug on the back
> of the radios being different?

The rear panel plugs on the transmitter units have a larger spacing than
the 274N sets, plus the pinout is different.  I have heard that this was
done to prevent a receiver from being plugged into a transmitter rack,
but I really don't believe that. The AN/ARC-5 transmitters were plate
modulated, and I would venture the larger spacing was to handle the
higher peak voltages.

The interconnect cable plugs from the modulator to transmitter racks, 
and modulator to transmitter control boxes are different between the 
-274N and ARC-5.  In fact, for the AN/ARC-5, the connector to make the 
cable from the modulator to transmitter rack is *very* scarce.

> Is it true that ARC5 equipment is always black and SCR-274N equipment is
> always unpainted aluminum?

The early SCR-274N sets were finished in black wrinkle, but the later
production was bare aluminum. I would guess that maybe about 20% of the
-274Ns were black wrinkle.

> Is it true that the differences in rear plugs on the equipment is due to
> different voltage requirements?

I think there were two reasons, One is as I mentioned, the higher peak
voltages involved in plate modulation.  The other reason?

You have to remember the SCR-274N was the Army's version of the Navy's
ATA and ARA. With a few very minor differences, the components of these 
sets were completely interchangeable.  I think when the Navy went to 
plate modulation, they wanted to make sure that an ATA (or -274N) 
modulator did not get put where an ARC-5 modulator was supposed to go, 
and vice versa.

Hope this helps.

Jack Antonio WA7DIA/4