[ARC5] ATA, SCR-274-N, and AR/ARC-5 Transmitter (and Receiver) Rack Differences

[Mike Morrow]

Mac asked:

> ...what is the difference between the AN/ARC-5 MT-series and
> SCR-274-N FT-series racks? 

The ATA and SCR-274-N transmitter racks are identical electrically
and mechanically.  The ATA and transmitter racks lack the local
transmitter test keys of the AN/ARC-5 racks.

The AN/ARC-5 transmitter racks differ significantly electrically and
mechanically.

The orientation and the center location of the transmitter rear connectors
are identical, and the connectors consist of six unevenly-spaced pins and
one center pin.  But that is as far as the similarity goes.

ATA and SCR-274-N transmitter connector:  The centers of the six concentric
pins are arranged in a circle 9/16th inch in diameter.  The top two pins
(1, 6) are separated by a gap of 7/16th inch.

AN/ARC-5 transmitter connector:  The centers of the six concentric pins
are arranged in a circle 11/16th inch in diameter.  The top two pins (1, 6)
are separated by a gap of 9/16th inch.

The wiring connected to the seven pins are very different:

Pin     ATA and SCR-274-N   AN/ARC-5
1       Ground              HV (pin not used by transmitter)
2       No connection       Oscillator HV
3       Oscillator HV       Transmitter select (ground to key)
4       PA screen HV        Ground
5       Transmitter select  28 vdc supply
6       28 vdc supply       PA screen HV
7       PA plate HV         PA plate HV

The AN/ARC-5 racks contain a selector jumper plate for each rack position
that is used to assign the associated transmitter as number 1, 2, 3, or
4 to the transmitter control box transmitter select switch. 

The AN/ARC-5 racks have a test key for each transmitter position.

The 12-pin rack connector that goes to the modulator unit is even more
altered mechanically and electrically for the AN/ARC-5:

Pin    ATA and SCR-274-N         AN/ARC-5
1      Ant. relay RF meter       28 vdc
2      Ant. relay RF meter       Select transmitter 1
3      Key                       Select transmitter 2
4      28 vdc                    Ground
5      Ground                    Select transmitter 3
6      NC (for 2 position rack)  Select transmitter 4
7      NC (for 2 position rack)  Key
8      Select transmitter 2      PA plate HV
9      Select transmitter 1      HV (not used by transmitters)
10     PA plate HV               Oscillator HV
11     Oscillator HV             Rack test keys
12     PA screen HV              PA screen HV

It is also important to note that this 12-pin connector is very different
mechanically as well as electrically.  The ATA and SCR-274-N racks use
the fairly common PL-154 or PL-154-A connector.  The AN/ARC-5 racks use
the A.R.C. type 9377 or 9589 connectors that can be very hard to find.

In addition, the AN/ARC-5 racks have a separate 2-pin connector J-56:
Pin
1      Ant. relay RF meter
2      Ant. relay RF meter

The connector to the antenna relay is wired identically and is electrically
identical for the racks of all three systems.

> Is the orientation different? 

>From above...NO.

> Is the distance of the connector from the bottom plate different?

>From above...NO.  But the chassis hole for the AN/ARC-5 connector is
larger in diameter than that for the ATA and SCR-274-N transmitters,
and the pins are arranged differently.

> Or is it simply different wiring?

>From above...the wiring is very different and the connectors themselves
are different and mechanically incompatible.

> Can racks from one series be adapted to transmitters of the other 
> series without serious destruction? 

No.  The AN/ARC-5 transmitter system is sufficiently different from that
of the ATA and SCR-274-N that mechanically different connectors were
chosen to prevent the use of the a wrong system (ATA) component in the other
system (AN/ARC-5) and vice-versa.  (Both of these systems served simultaneously
in the US Navy for several years.)  There are ONLY TWO components of the
AN/ARC-5 transmitter system that are electrically compatible and 
interchangeable with the equivalent components of the ATA or SCR-274-N:
(1) The antenna relay and (2) the modulator dynamotor.  That's it!!

FOr what it's worth, the receiver racks for the ARA, SCR-274-N, and
AN/ARC-5 are interchangeable electrically and mechanically.  The small
mechanical enhancements for the AN/ARC-5 receiver racks are anchor points
impressed in the front of the rack to enable lockwiring of the A/B audio
bus select switch in the A or B positions.  Also, the MT-7/ARR-2 single
receiver rack lacks that A/B audio bus select switch...but the later
MT-7A/ARR-2 has one.

For a good on-line reference to details such have been just discussed,
see:

SCR-274-N   http://bama.edebris.com/manuals/military/scr274/
AN/ARC-5    http://bama.edebris.com/manuals/military/an-arc5/

73,
Mike / KK5F