[Milsurplus] ARC-5 Dynamotor

[Mike Morrow]

>I recently acquired the MD-7 Plate modulator unit. Up to now, I have been
>using the screen modulator unit.  Are these interchangeable ?  I currently
>use the 274N units, the unpainted aluminium ones. Should I use my ARC-5 ,
>the black crinkle ones instead with the Md-7 or does it make a difference ?

Simply stated...the answer is YES, it does make a difference.  See a short
discussion of these command set differences at http://en.wikipedia.org/wiki/ARC-5 .

The original set was the USN's ATA.  The USAAF's SCR-274-N was essentially
a superficially-modified and simplified set of components which are almost
directly interchangable with the USN's ATA components.  The most significant
electrical difference between the ATA and the SCR-274-N is the impedance
of the audio circuits (low-Z in the ATA, and high-Z/low-Z selectable in
late SCR-274-N).  The USN's replacement for the ATA was the improved but
non-interchangable ATA system which is the transmitter part of the AN/ARC-5.

The ONLY components in the AN/ARC-5 transmitter system that are electrically
interchangeable with similar components in the ATA or SCR-274-N systems are
the dynamotor on the modulator, and the antenna relay.  That's it.

Because of these differences, connectors for the AN/ARC-5 were chosen to
prevent, say, an ATA transmitter to be installed in an AN/ARC-5 rack,
or an ATA control box to be used in place of the correct AN/ARC-5 box, or
the ATA modulator to be connected into an AN/ARC-5 installation.  The
several unique AN/ARC-5 connectors can be the most difficult components
to find today, when re-creating an accurate installation.

Of course, one can ham-up a MD-7 to a SCR-274-N transmitter, but the
connection diagrams and schematics of both systems will need to be
closely studied.  The proper plugs for the 12- and 18-pin connectors on
the MD-7 are MUCH harder to find than the ATA or SCR-274-N equivalents.

>I am  afraid I have been guilty of assuming that all of these units were
>Arc-5.

As has nearly every ham radio article published in the common ham rags for
the past 65 years.

Mike / KK5F