The GRA-39 can be used in four
ways:
- Radio transmission and reception from the remote control
unit.
- Radio transmission and reception from a local battery
switchboard and the telephones connected to the
switchboard.
- Telephone communication between local and remote control
unit operators.
- Radio transmission and reception from the local control
unit.
In order to control a radio from a remote position that's
fairly close to the radio, like in the same vehicle or from a
vehicle to a nearby tent, an audio extension cord can be
used. This cord needs to have good RF shielding and have
a short circuit DC resistance that's considerably lower
than what the radio requires to key the PTT circuit.
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 keyed this tone is sent that's outside the normal
300 to 3,000 Hz audio range and the local control unit hears
this tone and activates a relay closing pins "A" and "C" on
the radio's AUDIO cable thus keying the radio. The local
control unit also filters the control tone from the audio so
it will not be heard by the local control operator or the
radio.
The remote unit also amplifies the audio level so any wire
losses will not result in too weak a signal for the radio
input.
There is also Radio Frequency Interference (RFI) filtering in
the remote control units to prevent problems caused by the
nearby radio transmitters.
The GRA-39 can be used with any radio that uses the standard
5 pin AUDIO connector, this includes
the
PRC-68 Family,
PRC-25,
PRC-77,
VRC-12 Series Radios,
PRC-104, etc. Fair Radio lists:
VRC-12, VRC-43, VRC-49, VRC-53, VRC-64; GRC-125, GRC-160;
PRC-25,
PRC-74B,
PRC-77.
The GRA-39 can be used with the
SB-22
local battery switchboard. When this is done there needs
to be a connection to one of the line cards both by means of
the remote unit line terminals and by means of a special audio
cable that connects pins "A" and "C" from the remote units
audio jack to the same line card. The switchboard
operator needs to act and the remote control operator for the
radio circuit, but once a call has been established to one of
the other line cards the person on that line controls the
radio PTT using their telephone PTT switch.
The GRA-39 does NOT provide Radio Wire Integration
(RWI). Because this requires that an in band audio tone,
typically 1,600 Hz, be sent over the radio to act as a ringing
signal. The 3,900 Hz tone used 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
encrypted 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
remote unit and the C-2329 local unit that connects to the
radio. The
HYX-57 serves a similar purpose,
but the exact same unit is used at both ends of the remote
control circuit. One of the advantages of the HYX-57 is
that a pair of them can be used back to back (with a special
cable between them) to act as a line extender. Thus for
each pair of extra units the length of the field wire can be
extended. Also they can be used front to front connected
by a few feet of field wire and connected to two radios to
make a
retransmission
system. The GRA-39 can not be used either as a wire line
extender or as the 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 above units 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
can be separated from the battery box and extended with DC
power applied by 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 since RFI 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
Repair Parts & Special Tools List
Page created 15 Nov. 2004.