The GRA-39 can be used in four
- Radio transmission and reception from the remote control
- Radio transmission and reception from a local battery
and the telephones connected to the switchboard.
- Telephone communication between local and remote control
- Radio transmission and reception from the local control
In order to control a radio from a remote position that's
to the radio, like in the same vehicle or from a vehicle to a
tent, an audio extension cord can be used. This cord
have good RF shielding and have a short circuit DC resistance
considerably lower than what the radio requires to key
In order to control a radio from up to 2 miles (3.3 clicks) an
extension cord will no longer work and additional equipment is
needed. To get around the DC resistance of the field
wire an AC
tone at 3,900 Hz is used. When the remote handset is
tone is sent that's outside the normal 300 to 3,000 Hz audio
the local control unit hears this tone and activates a relay
pins "A" and "C" on the radio's AUDIO cable thus keying the
radio. The local control unit also filters the control
the audio so it will not be heard by the local control
operator or the
The remote unit also amplifies the audio level so any wire
not result in too weak a signal for the radio input.
There is also Radio Frequency Interference (RFI) filtering in
remote control units to prevent problems caused by the nearby
The GRA-39 can be used with any radio that uses the standard 5 pin AUDIO connector
, this includes
the PRC-68 Family
, etc. Fair
VRC-12, VRC-43, VRC-49, VRC-53, VRC-64; GRC-125, GRC-160;
The GRA-39 can be used with the SB-22
battery switchboard. When this is done there needs to be
connection to one of the line cards both by means of the
line terminals and by means of a special audio cable that
"A" and "C" from the remote units audio jack to the same line
card. The switchboard operator needs to act and the
control operator for the radio circuit, but once a call has
established to one of the other line cards the person on that
controls the radio PTT using their telephone PTT switch.
The GRA-39 does NOT provide Radio Wire Integration
this requires that an in band audio tone, typically 1,600 Hz,
over the radio to act as a ringing signal. The 3,900 Hz
in the GRA-39 is an out of band PTT keying signal. The
tone generator will be within 25 Hz of 3,900 Hz.
The GRA-39 probably can not handle wide band audio like the
audio on a VHF or UHF radio and so the HYX-57 was developed.
The GRA-39 is made up of two different units, the C-2328
and the C-2329 local unit that connects to the radio.
serves a similar purpose, but the exact same unit is used at
of the remote control circuit. One of the advantages of
HYX-57 is that a pair of them can be used back to back (with a
cable between them) to act as a line extender. Thus for
of extra units the length of the field wire can be
they can be used front to front connected by a few feet of
and connected to two radios to make a retransmission
GRA-39 can not be used either as a wire line extender or as
heart of a retransmission system.
Both GRA-39 units have ring generator plungers so the other
unit can be
called. They have a "clicker" and the version B and
have a call light. This allows the GRA-39 to function
just like a
local battery field phone like the TA-312
To aid in troubleshooting the front panel of the both units
separated from the battery box and extended with DC power
means of a short coiled cord.
The GRA-39 only has a pair of terminals for the field wire.
There is no
ground terminal like on the HYX-57. This is strange
suppression would need a ground.
The mil spec for the GRA-39 mentions the PRC-35 which was a
radio in the VRC-12 family to replace the PRC-6.
TM 11-5820-477-12 Operator's and
Organizational Maint. Manual
TM 11-5820-477-30 Direct Support Maint Manual
TM 11-5820-477-23P Organizational and Direct Support Maint
& Special Tools List
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