©Brooke Clarke, N6GCE

Background

Racal developed the PRC-139 family of military squad radios that are the next generation after the PRC-68 series of radios.  The Air Force Scope Shield II program uses the PRC-139 radio family.  The Scope Shield I program was based on the magnavox PRC-128 radio family.
These use a replaceable module concept to allow three different frequency bands of operation.

T suffix for Tactical band of 30 to 88 MHz - 12.5 kHz steps

H suffix for High band 136-174 MHz - 12.5 kHz steps

U suffix for UHF band 403-470 MHz - 12.5 kHz steps

Note 1: CTCSS tones are: 100.0, 103.5, 107.2, 110.9, 114.8, 118.8, 123.0, 127.3, 131.8, 136.5, 141.3, 146.2, 150.0 (30 - 88 MHz only), 151.4, 156.7, 162.2, 167.9, no tone.  Lower frequencies like 94.8 or higher frequencies like 107.2 are NOT available.

Note 2: the frequency step size is 12.5 kHz for all bands.  So not all repeater frequencies can be programmed.

Note 3: Carrier squelch can be set for 6, 8, 10, 12, 14 or 16 dB.

For each band a different antenna is used.  Other options are:

• G suffix for gray color
• SYS for a system, radio, antenna and battery

There are also a number of variations, for example radios sold with just a single band module, different colors, US Type 1, VINSON Compatible or Exportable Digital Encryption, etc.  The PRC-6725E is a single band (one of the above three) radio with Keypad, Graphics Display (better than an alphanumeric LCD) and built in Speaker-Microphone. It can be fitted with US Type 1 COMSEC, Exportable Digital Encryption or unencrypted.

In addition to the radios themselves there are accessories like vehicle adapters, and repeater packages in addition to the smaller accessories.

Company Name

Warning

There are a number of Racal PRC-139 and PRC-6725 radios out there that have no keyboard, display, built-in microphone or speaker.
These are not of much use without a computer programming adapter and external handset. Maybe they were made for repeater use?
They are not very human friendly.

Photos

Front - This PRC-6725 has neither keypad nor LCD display, it can only be programed using an accessory.
            There appears to be no Speaker/Microphone built in, like this was a radio designed for a non human use like a repeater.
Back - just the label
Right - PPT switch, ? switch and instructions?
Left - interface connector used for a true "jerk and run" vehicular mount and other tasks like use in a repeater
Top - Scan switch to watch all the programmed channels, standard 6 pin U-229 style military audio connector, ? antenna connector, Zeroize, A, B, C or D crypto key and Clear switch, 1 through 14 channel selector, OFF-Volume control.
Bottom - NiCAD battery

Analysis

The front cover is held on using Torx-7 screws (American Vermont 15349 bit) while the back/RF module is held by Pozi Drive #1 screws.
On the front side of the main PCB there is a Max232EWE 9 pin IC.  This is a TTL to RS-232 level shifter.  But why is it here because the PC programming cable has a small PCB inside the DB-9(f) connector that goes to the computer.  I would expect that the cable both shifts the computers RS-232 levels to TTL and also has some pull up and/or down resistors controlled by some transistors.  If the cable is supplying TTL to the radio then why does the radio have a level shifter chip, unless it's to provide ESD protection?

Digital I/O

With the radio off Audio pin F is at 2.38 Volts and goes to + 10.4 V when the radio is on.  This pin has no digital data.

AUDIO pin E has digital data when the radio is turned on that lasts for 13 ms and during operations with the MA6941. The levels are between 0 and 9.8 Volts.
Timing: 50 uS@0V, 180 uS @ +10, 180 uS @0V, 50 uStrying to get to +10 but only gets to +8.4 (this may be due to the use of the audio test adapter between the MA6941 cable and the PRC-6725).

If a standard 5 wire audio test adapter is used between the MA6941 cable and the radio, the digital up/download software hangs.  The "F" pin must be some kind of signal and/or handshake signifying it's OK to use digital coms.  The 6 wire audio test adapter is needed.

Antenna Matching

Note that the external components used for the antenna system will vary depending on which RF module (frequency band) is installed in the radio.  This 6725 has the 30 - 88 MHz module insalled.

• The radio has a TNC(f) connector on the top.
• Into this a short matching circuit/adapter is connected.  The adapter for 30 - 88 MHz measures a short from center to ground.  It probably has some type of inductor and/or matching network inside.  The female threads on the top of the matching adapter are the same 1/4-28 as are on the PRC-68B radio
• There is a 6.5" long flex with male 1/4-28 on one end and female 1/4-28 on the other end
• The 1 meter antenna is a tape measure type, except instead of just using metal tape this appears to have two plastic tapes joined in such a way that there is a hollow space between them.

Battery

Manuals

Operator's manual for the radio (TM(O) PRC6725)
Depot level manual (TO31R2-2PRC139-3)

"Scope Shield II Radio System Notes
Technical Orders (TOs)
Scope Shield II Radio System does not come with Operational and Maintenance Instructions.  The TOs are available and should be ordered by following the normal TO ordering procedures outlined in TO 00-5-2 (Technical OrderDistribution System), Chapter 5. 
The Scope Shield TOs consist of the following:

TO 31R2-2PRC139-1:  Operational and Maintenance Instructions with Illustrated Parts Breakout (IPB),
    Radio Set AN/PRC-139(C) Hand Held Radio, including Receiver-Transmitter Set,Radio RT-1695/TRC-199,
    Receiver-Transmitter Set, Radio RT-1696/U(C),
    Programmer Unit, Frequency Radio MX-11531/U (Frequency FillDevice),
    Battery Chargers, and Battery Reconditioner. 
    Changes 1 and 2 have been issued and are dated 10 Aug 94 and 15 Mar 95, respectively.

TO 31R2-4-1085-1:  Operation and Maintenance Instructions with IPB,
    Adapter Group, Radio OF-228/U (Vehicle Adapter).  Change 1 was issued on10 Aug 94.

TO 31R2-2TRC199-1:  Operation and Maintenance Instructions with IPB, Radio Set AN/TRC-199 (Tactical Repeater).  Change 1 wasissued on 10 Aug 94. 

TO 31R2-2GRC238-1:  Operation and Maintenance Instructions with IPB, Radio Set, AN/GRC-238 (Base Station).  Change 1 was issued on 10 Aug 94.

TO 31R2-2PRC139-06:  Work Unit Code Manual

Training Aids (Tapes)

Three Scope Shield II training tapes (PIN Number 611901) covering operator and maintenance tasks are available.  These training tapes werere produced and distributed by the Air Force Media Center.  The training tapes may be ordered by faxing a request to the Joint Visual Information Center:"

Parts List

Accessories :

• OZ-69 Mobile amplifier

• Cloning Cable - for radio to radio cloning side multipin connector on one end and U-229 type on other end.
  
• MX-11531/U Frequency Fill Device - a keyboard is part of this device

MA 6941 PC programming cable - allows a laptop to program the frequencies and other variables such as Receive Frequency, Transmit Frequency, CTCSS Squelch Tones, Data/Crypto Clock Speed, Bandwidth, Transmit Time-out, Radio Zeroize, Scan, Internal Speaker Off, Squelch Level, Cloning Mode, Display Timer, Display Intensity

• Crypto Fill Cable - looks exactly like the Cloning cable - maybe has a microcontroller with read and fill buttons?
• Ear Microphone Headset - the Telex EM200 got a very good review and I expect this one would also
• Handset H250/U -
• Microphone M-80C/U - only usable with the vehicular adapter that supplies the speaker
• Vehicle Adapters - OZ 6727 & MA 6940 - Specs -

The cloning and crypto fill cables look the same.  They have a U-183 (6 pin U-229) on one end and a 19 pin connector to interface with the connector on the left side of the radio. (they may be the same cable or just be wired differently).  For cloning one radio sends out it's programming data via either the 19 pin connector or the U-183 and the other radio receives the data on the other connector.  For crypto fill the U-229 or U-183 connector goes to the fill device and the 19 pin connector feeds the key data to the radio.  When a radio is installed in a base station the frequency programming is fed to the radio through the 19 pin connector.  The implication is that for cloning the data is output from the U-182 connector on the top of the radio.

Links

Thales- Thales Communications (formerly Racal)- PRC-139 - Specs - PRC-6725E - specs

This appears to be a Racal radio that can operate at VHF Low (30-88 MHz), VHF Hi (136-174 MHz) and UHF (403-470 MHz) with a module change and was part of the Scope Shield II program.  Jan 2007 the PRC-139 no longer is on the Thales site??
Aeronautical Expeditionary Airfield Equipment -

Over $125 Million in Sales for Racal's Advanced Tactical Radios  -
MIL-STD-188-220A - Tacter-21 - TCP/IP - Solider Command and Control -  AN/PRC-139(c) radio with GPS (Global Positioning System); helmet mounted display (HMD) (32x24 mm, VGA, 640x480, 8 shades of grey), camera (w/8mm recorder), microphone and earphone; and a ruggedized menu control glove.

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